Phylum Arthropoda - Definition, Classification, Characteristics, Examples

Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
(unranked):	Tactopoda
Phylum:	Arthropoda

What is Phylum Arthropoda?

Phylum Arthropoda is the largest phylum in the animal kingdom, encompassing a vast number of invertebrate species. The name “arthropoda” derives from the Greek words “arthros” meaning “joint” and “podos” meaning “leg,” referring to the characteristic jointed appendages found in these animals. With an estimated 85 percent of known species belonging to this phylum, arthropods dominate the animal kingdom, and many more species remain undocumented.
All animals within the phylum Arthropoda share two key features: functional segmentation of the body and the presence of jointed appendages. Additionally, arthropods possess an exoskeleton primarily composed of chitin, a tough and waterproof polysaccharide. This exoskeleton provides support and protection for the arthropods’ bodies. Arthropods are classified as eucoelomate, protostomic organisms, meaning they have a true body cavity and their mouth develops before their anus during embryonic development.

Phylum Arthropoda includes animals that have successfully colonized terrestrial, aquatic, and aerial habitats. It is further divided into five subphyla: Trilobitomorpha, Hexapoda, Myriapoda, Crustacea, and Chelicerata. Trilobitomorpha comprises extinct arthropods called trilobites, which existed primarily during the pre-Cambrian Era. Hexapoda includes insects and their relatives, making it the largest class within Phylum Arthropoda. Myriapoda consists of millipedes, centipedes, and related species. Crustaceans encompass crabs, lobsters, crayfish, isopods, barnacles, and certain zooplankton. Finally, Chelicerata comprises horseshoe crabs, arachnids (such as spiders), scorpions, and daddy longlegs.
Animals are eukaryotic, multicellular, and heterotrophic organisms that lack cell walls. Most animals are mobile, with a few exceptions. The multicellular animals are referred to as metazoa, divided into two branches: Parazoa and Eumetazoa. Parazoans have loosely aggregated cells, poorly differentiated tissues, and lack organs or a digestive cavity (e.g., sponges). On the other hand, Eumetazoa have properly arranged cells forming tissues and organs. Their digestive tract typically has a mouth, with or without an anus. Eumetazoa are further classified based on the number of germ layers present in the embryo, the symmetry of the body, and the mode of origin of the mouth. Animals with two germ layers are known as diploblastic, while those with three germ layers are called triploblastic. Animals within the phylum Arthropoda have three germ layers.
Arthropods are characterized by their chitinous exoskeleton, jointed appendages, and the ability to survive in various habitats. They are incredibly diverse, with an estimated 1 million species or more. Arthropods possess an open circulatory system called haemolymph, which is analogous to blood. Haemolymph circulates through a body cavity known as a haemocoel, supplying nutrients and oxygen to the internal organs. The internal organs of arthropods are typically composed of repeated segments, similar to their exteriors. Their nervous system is “ladder-like,” with paired ventral nerve cords running through each segment and forming ganglia. Arthropods have a wide range of sensory systems, including compound eyes, pigment-pit ocelli, and various chemical and mechanical sensors.

Reproduction and development among arthropods vary greatly. Terrestrial species typically use internal fertilization, while aquatic species may utilize internal or external fertilization. Most arthropods lay eggs, but some give birth to live young. Hatchlings can range from miniature adults to larvae that undergo metamorphosis before reaching their adult form. Maternal care for hatchlings also varies, from none to extensive care provided by social insects.
Arthropods have ancient origins dating back to the Cambrian period. They are generally considered monophyletic, and many studies suggest their placement within the superphylum Ecdysozoa alongside cycloneuralians. However, the exact evolutionary relationships between arthropod groups are still debated. Arthropods play essential roles in the human food supply as both direct sources of food and vital pollinators for crops. Some arthropods are known to transmit severe diseases to humans, livestock, and crops, highlighting their significant impact on various ecosystems.

Definition of Arthropoda (Arthropods)

The Arthropods belong to the bilaterally symmetrical, metamerically separated, triploblastic creatures with a coelom that is diminished and altered. Their body is externally covered in an exoskeleton made of chitin that is molted regularly as well as their appendages joined.

Characteristics of Phylum Arthropoda

Phylum Arthropoda exhibits a wide range of characteristics that define this diverse group of animals. Here are the key characteristics of Phylum Arthropoda based on the provided information:

Habitat: Arthropods inhabit various environments, including land, soil, freshwater, saltwater, and as parasites in the bodies of other organisms.
Body Form: The body form of arthropods varies considerably. They possess jointed legs and exhibit external segmentation. The body is typically divided into three main parts: head, thorax, and abdomen.

Body Wall: Arthropods have a thick, tough, and non-living chitinous cuticle that covers their body, forming an exoskeleton. This exoskeleton provides support and protection.
Body Cavity: The true coelom is significantly reduced in adult arthropods and is only present in the reproductive and excretory organs. The body cavity is primarily a hemocoel, a blood-filled cavity.
Digestive Tract: Arthropods have a complete digestive tract, consisting of the stomodaeum (foregut), mesenteron (midgut), and proctodaeum (hindgut).

Blood Vascular System: Arthropods possess an open circulatory system where blood does not flow in definite vessels. Instead, irregular spaces known as lacunae or sinuses are filled with blood.
Respiratory Organs: Aquatic arthropods use gills or book gills for respiration, while terrestrial arthropods employ tracheae or book-lungs. Some arthropods can also respire through their general body surface.
Excretory Organs: Arthropods have various excretory organs, including green glands, Malpighian tubules, and coxal glands, which function to eliminate waste products.

Nervous System: Arthropods possess an annelidian-type nervous system, consisting of a nerve ring and a double ventral nerve cord with ganglia.
Sense Organs: Arthropods have a range of sensory organs, such as antennae, eyes (including compound eyes), and statocysts (balancing organs). Compound eyes provide mosaic vision.
Absence of Cilia: Arthropods completely lack cilia throughout their body.

Muscles: Arthropods have striped muscles capable of rapid contraction, contributing to their agility. Unstriped muscles are also present.
Endocrine Glands: Endocrine glands, which secrete hormones, are present in arthropods. Some insects release pheromones, chemicals used for communication and attracting mates.
Sexes: Arthropods are usually dioecious (having separate sexes), and sexual dimorphism is often observed within the phylum.

Development: Fertilization in arthropods is generally internal. Most arthropods are oviparous, with the development involving either direct or indirect metamorphosis. Some arthropods also exhibit parthenogenesis.
Parental Care: Many arthropods display parental care, indicating various forms of nurturing behaviors towards their offspring.
Parthenogenesis: Parthenogenesis, the development of an egg into a complete individual without fertilization, is observed in certain arthropods. Male honey bees (drones) are an example of parthenogenesis.

Morphology of Phylum Arthropoda

The phylum Arthropoda encompasses a diverse group of animals characterized by their unique morphology. One distinctive feature is their segmented body, which is composed of sets of segments that fuse together to form functional body regions known as tagma. Depending on the specific arthropod group, tagma can take the form of a head, thorax, and abdomen, a cephalothorax and abdomen, or a head and trunk.
Arthropods possess a central cavity called the hemocoel, also known as the blood cavity. This cavity houses their open circulatory system, which is regulated by a tubular or single-chambered heart. The respiratory systems of arthropods vary among different groups. Insects and myriapods have a network of tubes called tracheae that branch throughout their bodies. These tracheae open to the outside through openings called spiracles and allow for direct gas exchange between cells and the air in the tracheae. Aquatic crustaceans use gills for respiration, while terrestrial chelicerates (such as spiders and scorpions) employ book lungs, and aquatic chelicerates use book gills.
Arachnids, including scorpions, spiders, ticks, and mites, possess book lungs. These structures consist of a vertical stack of tissue resembling the pages of a book, with air spaces between each “page.” This arrangement ensures that both sides of the tissue are in constant contact with air, enhancing the efficiency of gas exchange. Crustaceans have filamentous gills that facilitate gas exchange with the surrounding water.

Different groups of arthropods also employ different organs for excretion. Crustaceans possess green glands, while insects use Malpighian tubules, which work in conjunction with the hindgut to reabsorb water while eliminating nitrogenous waste from the body.
The arthropod’s outer covering is known as the cuticle. It consists of two layers: the epicuticle, which is a thin, waxy, water-resistant outer layer devoid of chitin, and the chitinous procuticle beneath it. Chitin is a tough and flexible polysaccharide. In order to grow, arthropods must undergo a process called ecdysis or molting, where they shed their exoskeleton. During this time, they are vulnerable to predation until their new exoskeleton hardens.
Overall, the arthropod phylum showcases a remarkable diversity in morphology, reflecting the adaptations of different groups to various environments and lifestyles.

Segmentation of Phylum Arthropoda

The segmentation of the arthropod phylum is a defining characteristic of these animals. The embryos of all arthropods are built from a series of repeated modules, indicating their segmented nature. The last common ancestor of living arthropods likely consisted of undifferentiated segments, each with a pair of appendages functioning as limbs. However, in both living and fossil arthropods, segments have become grouped into specialized body regions called tagmata, where segments and their limbs have evolved specific adaptations.
The distinct three-part appearance of many insects and the two-part appearance of spiders are a result of this grouping of segments. In mites, there are no external signs of segmentation. Arthropods also possess two body elements that are not part of the serially repeated pattern of segments. These include an ocular somite at the front, where the mouth and eyes originated, and a telson at the rear, located behind the anus.
Originally, each appendage-bearing segment of arthropods had two separate pairs of appendages. These consisted of an upper, unsegmented exite and a lower, segmented endopod. Eventually, these pairs fused into a single pair of biramous appendages joined by a basal segment, known as the protopod or basipod. The upper branch of the appendage acted as a gill, while the lower branch was used for locomotion. Some crustaceans and certain extinct taxa, like trilobites, possess another segmented branch called exopods. However, the origin of these structures remains controversial.

Throughout arthropod evolution, the appendages in different segments have undergone modifications to serve various functions. They may have transformed into gills, mouth-parts, antennae for sensory perception, or claws for grasping. Arthropods can be likened to Swiss Army knives, each equipped with a unique set of specialized tools. In many arthropods, appendages have disappeared or become highly modified in specific regions of the body, particularly in the abdomen.
The head region of arthropods exhibits the most notable specialization of segments. The major arthropod groups—Chelicerata (sea spiders, horseshoe crabs, and arachnids), Myriapoda (symphylans, pauropods, millipedes, and centipedes), Crustacea (oligostracans, copepods, malacostracans, branchiopods, hexapods, etc.), and the extinct Trilobita—possess heads formed from different combinations of segments, with missing or specialized appendages. Myriapods and hexapods have similar head combinations, but hexapods are nested within crustaceans, indicating separate evolutionary origins for these traits. Additionally, some extinct arthropods, like Marrella, exhibit unique combinations of segments and specialized appendages in their heads, not belonging to any of the major arthropod groups.
Understanding the evolutionary stages by which these diverse combinations of head structures evolved has been a challenging problem known as “the arthropod head problem.” Researchers have found it difficult to unravel the solutions to this puzzle, and the complexity of the issue has even been considered a source of fascination and enjoyment in the field of arthropod research.

Exoskeleton of Phylum Arthropoda

The exoskeleton of arthropods, which is their external skeleton, is composed of a substance called cuticle. This cuticle is a non-cellular material secreted by the epidermis, the outer layer of cells. The structure of the cuticle consists of three main layers: the epicuticle, exocuticle, and endocuticle.

The epicuticle is a thin outer layer that has a waxy coating, providing moisture-proofing and protection to the underlying layers. The exocuticle, made up of chitin and chemically hardened proteins, is the middle layer of the cuticle. It provides strength and rigidity to the exoskeleton. The endocuticle, also composed of chitin but with unhardened proteins, is the inner layer. Together, the exocuticle and endocuticle are referred to as the procuticle.

Each body segment and limb section of arthropods is encased in hardened cuticle, which provides support and protection. The joints between body segments and limb sections are covered by flexible cuticle, allowing for movement and flexibility.

Aquatic crustaceans have exoskeletons that are biomineralized with calcium carbonate obtained from the surrounding water. Terrestrial crustaceans have developed mechanisms to store minerals since they cannot rely on a continuous supply of dissolved calcium carbonate on land. Biomineralization primarily affects the exocuticle and the outer part of the endocuticle. Two hypotheses suggest that biomineralization provides stronger defensive armor and allows for larger and stronger body structures. In either case, a mineral-organic composite exoskeleton is more cost-effective to build compared to an all-organic exoskeleton of similar strength.

The cuticle may also have setae, which are bristle-like structures growing from special cells in the epidermis. Setae serve various functions and can be as diverse as the appendages themselves. They act as sensors to detect air or water currents and contact with objects. Aquatic arthropods use feather-like setae to increase the surface area of their swimming appendages and to filter food particles from the water. Aquatic insects, which breathe air, employ thick coats of setae to trap air and extend their time underwater. Some arthropods possess heavy and rigid setae that serve as defensive spines.

While arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some species still rely on hydraulic pressure inherited from their pre-arthropod ancestors to extend their limbs. For example, spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.

Moulting

Moulting, also known as ecdysis, is a crucial process for arthropods as their exoskeletons do not stretch and therefore restrict growth. To accommodate growth, arthropods periodically shed their old exoskeleton and replace it with a new, larger one that is not yet hardened. Moulting cycles occur continuously until the arthropod reaches its full size.
The period between each moult, until sexual maturity is reached, is referred to as an instar. Each instar stage may exhibit differences in body proportions, colors, patterns, the number of body segments, or head width. After moulting, the juvenile arthropods continue in their life cycle until they either undergo pupation or go through another moult.
The moulting process begins when the animal ceases feeding and its epidermis releases moulting fluid. This fluid contains enzymes that digest the endocuticle, causing the old cuticle to detach. Simultaneously, the epidermis secretes a new epicuticle to protect itself from the enzymes, and the new exocuticle begins to form. Once this stage is complete, the animal absorbs a significant amount of water or air, causing its body to swell. The old cuticle then splits along predetermined weak areas where the exocuticle was thinnest. It typically takes several minutes for the animal to struggle out of the old cuticle. At this point, the newly exposed cuticle is soft and wrinkled, making it difficult for the animal to support itself and move. The new endocuticle has not yet formed. To maximize the stretching of the new cuticle, the animal continues to inflate itself before hardening the new exocuticle and eliminating excess air or water. During this phase, the new endocuticle fully develops. Many arthropods consume their discarded cuticle to reclaim its valuable materials.

Moulting poses significant risks for arthropods. They are vulnerable and nearly immobile until the new cuticle hardens. There is a danger of being trapped inside the old cuticle or becoming preyed upon by predators. Moulting is responsible for a substantial portion, estimated to be 80 to 90%, of arthropod deaths.

Respiration and circulation

Arthropods possess open circulatory systems, although some have short, open-ended arteries. In chelicerates and crustaceans, the blood is responsible for carrying oxygen to the tissues. On the other hand, hexapods, which include insects, employ a separate system called tracheae for respiration. While many crustaceans and a few centipedes utilize copper-based hemocyanin as their respiratory pigment, some crustaceans and insects rely on iron-based hemoglobin, which is also found in vertebrates. In arthropods, respiratory pigments are typically dissolved in the blood rather than enclosed in corpuscles as in vertebrates.
The heart in arthropods is typically a muscular tube situated beneath the back and extending for most of the length of the hemocoel, which is the central body cavity. The heart contracts in waves, moving from the rear to the front, thereby propelling the blood forward. Uncompressed sections of the heart are expanded either by elastic ligaments or small muscles, both of which connect the heart to the body wall. Paired ostia, which are non-return valves, are present along the heart, allowing blood to enter but preventing it from leaving before reaching the front.

Arthropods exhibit a diverse range of respiratory systems. Small species often rely on simple diffusion through the body surface due to their high surface area-to-volume ratio, which adequately supplies them with oxygen. Crustaceans typically possess gills, which are modified appendages specialized for respiration. Arachnids, such as spiders, possess book lungs, while tracheae are the respiratory structures in many insects, myriapods, and certain arachnids. Tracheae consist of a system of branching tunnels that deliver oxygen directly to individual cells.

Nervous system

Arthropods possess a paired main nerve cord that runs longitudinally along their bodies below the gut. In each segment, the nerve cord forms a pair of ganglia, which serve as nerve centers. These ganglia give rise to sensory and motor nerves that extend to different parts of the segment. Although the ganglia in each segment may appear fused, they are connected by commissures, which are relatively large bundles of nerves. This interconnected arrangement of ganglia and commissures gives the arthropod nervous system its characteristic “ladder-like” appearance.
The brain, located in the head region, encircles and primarily lies above the esophagus. It consists of fused ganglia from the acron (anterior region) and one or two of the foremost segments that form the head. In most arthropods, there are three pairs of ganglia in the brain, but chelicerates, which lack antennae, have only two pairs of ganglia. The ganglia of other head segments are often in close proximity to the brain and function as part of it. In insects, these additional head ganglia merge to form a pair of subesophageal ganglia situated below and behind the esophagus. Spiders, on the other hand, take this consolidation further, as all segmental ganglia are incorporated into the subesophageal ganglia, which occupy a significant portion of the cephalothorax (the anterior “super-segment”).

Arthropod eye (Optical)

Arthropods possess remarkable visual systems, typically incorporating compound eyes and pigment-cup ocelli, often in combination. The ocelli, also known as “little eyes,” are capable of perceiving the direction of incoming light by detecting the shadows cast within the cup-shaped structure. However, spiders have specialized pigment-cup ocelli that can form images, enabling them to have visual acuity.[56] Some spiders, such as jumping spiders, even have the ability to rotate their main eyes to track their prey.[57]

Compound eyes are composed of numerous independent units called ommatidia. These ommatidia are typically hexagonal in shape and function as individual sensors. Each ommatidium consists of its own set of light-sensitive cells, lens, and cornea.[56] Compound eyes provide arthropods with a wide field of view, allowing them to detect fast movements and, in certain cases, even perceive the polarization of light.[58] However, due to the relatively larger size of the ommatidia, the resulting images produced by compound eyes tend to be coarse, and their visual acuity is lower compared to that of birds and mammals. Nonetheless, this limitation is not a significant disadvantage since most arthropods prioritize objects and events within a range of approximately 20 cm (8 in).[56] Some arthropods exhibit color vision, with insects being extensively studied in this regard. For example, bees possess ommatidia with receptors for both green and ultraviolet light, allowing them to perceive colors beyond human visual capabilities.

Classification of Phylum Arthropoda

Phylum Arthropoda as well as its many categories have been classified in different ways by different researchers. The classification is closely followed by vandal (1949) and Snodgrass (1960) and also strorer (1979). Onychophora does not considered as arthropods but is seen as an autonomous animal group that is segmented according to the current fashions.

Subphylum 1- Trilobitomorpha (Gr., tria=tree+ lobos=lobe+ morphe=form)

Only fossil trilobites are represented.
Marine forms in all their varieties.
All of us are inhabitants of the bottom i.e. benthozoic and lived since Cambrian up to Permian.

The body is composed of three lobes, separated by two furrows along the length of the body.
A distinct head that has only one antennae.
With the exception of the last segment that are biramous, the rest bear biramous appendages.

Examples: Triarthrus, Dalmanites.

Subphylum 2- Chelicerata (Gr., chele=claw+ keros=horn+ ata=group)

Body divided into anterior cephalothorax, or prosoma and posterior abdomen, also known as opisthosoma.
Prosomatic appendages in 6 pairs. The first set of preoral chelicerae that have claws and feeding functions.

The other pair of appendages is postoral pedipalpi. They are then 4 sets of walking legs.
True jaws and no antennae.
Predaceous and terrestrial, mostly.

Class 1. Merostomata (Gr., meros=thigh+ stoma= mouth)

All marine, exclusively aquatic with medial simple and lateral compound eye.
5 to 6 pairs of abdominal appendages that have been modified to function as branchiae or gills for breathing.
Abdomen, which ends in a sharp telson , or spine.

Excessive excretion through coxal glands. No Malpighian tubules.

Subclass 1. Xiphosura (Gr., xiphos=sword+aura=tail)

Prosoma is covered in convexity by the horseshoe-shaped outer carapace.
Prosoma is a sperm-based cell that has six Appendages in pairs.

Unsegmented abdomen, with an extended terminal telson.
Genital openings, paired and with an operculum genital.
Respiration is facilitated by lamelliform or book-gills that are attached to abdominal appendages.

Examples: Limulus (horseshoe or King crab).

Subclass 2. Eurypterida (Gr., eurys=broad+ pteryx=wing)

Extinct marine, huge water scorpions.
Large-sized arthropods.

Small cephalothorax covered with an dorsal carapace.
Cephalothorax is a 12-segmented abdomen that is smaller in the back.
Cephalothorax with six pair of appendages.

Examples: Eurypterus, Pterygotus.

Class 2. Arachnida (Gr., arachne=spider+ oid=like)

Forms of aquatic or terrestrial life.
Simple eyes. No compound eyes.

Prosoma has six pairs of appendages: one pair of chelicerae 1 pair of pedipalpi and four pair of legs for walking.
No abdominal appendages.
Tracheae, book-lungs or book-gills for an organ for breathing.

Excretion through coxal glands and Malpighian tubules.
Sexes separate (dioecious). typically oviparous, and courtship prior to mating.
Development direct.

Order 1. Scorpionida (=scorpiones)

Terrestrial forms that are under the surface of stones in subtropical and tropical areas.
Long-legged fair-shaped true scorpions.
Small prosoma, broadly linked to a large the opisthosoma.

Prosoma is covered dorsally with carapace. It is accompanied by two chelicerae, pedipalpi pair, as well as four walking legs.
Opisthosoma is divided into mesosoma that is broad and anterior, 7-segmented and narrow metasoma that is 5-segmented in the posterior.
Metasoma that ends in an elson and poison sting.

2 pectines that resemble combs on the 2 2nd abdominal segment.
Book-lungs as respiratory organs.
Examples: Buthus, Palamnaeus, Androctonus.

Order 2. Pseudoscorpionida (=Chelonethida)

Tiny fake scorpions.
It was found under the bark of an oak tree.
Prosoma is composed of 6 fused segments that are covered dorsally by the carapace.

Abdomen 11-segmented without the sting or telson.
Chelicerae 2jointed, with comb-like secretions.
Respiration through the trachea.

Examples: Chelifer, Microcreagris.

Order 3. Palpigradi

Micro-sized micro-sized scorpions with micro whip.
No eyes.

Prosomal carapace is composed of larger anterior and smaller lateral parts.
Opisthosoma with 10 segments joined to prosoma with the pedicle.
Telson with the joined flagellum.

Chelicerae include pedipalpi and chelate. leg-like.
Three pairs of book-lungs that function as respiratory organs.
Example: Koenenia.

Order 4. Solifugae (=solifugida)

False spiders. Often referred to as sun spiders and wind-spinning spiders.
The body is composed of prosoma and the opisthosoma.
Prosoma split into a large anterior and smaller posterior portion.

Opisthosoma comprised of between 10 and 11 segments. There are no spinnerets.
Chelicerae are large and chelate. pedipalpi have a leg-like shape and are elongated.
There are no poison glands.

The trachea is the main source of respiration.
A flagellum is located on each male chelicera, to facilitate the transfer of sperm.
Example: Galedodes.

Order 5. Amblypygi (= Phrynichida)

Smaller or flattened scorpion-spiders whip scorpions.
Carapace with a divided carapace. The pedipalps are large, and they rhaptorial.
Moderate size chelicerae.

12-segmented abdomen, without flagellum.
The first pair of walking legs long, like whips in the natural world.
Example: Charinus.

Order 6. Uropygi (=pedipalpi)

Sometimes referred to as whip scorpion.
Consists of a pair eyes.
Prosomal carapace, complete.
Chelicerae 2jointed and medium in size.
Heavy, large and typically with pincers on the terminals of pedipalpi.
12-segmented opisthosoma. The last segment has the long flagellum or Telson.
Examples: Thelyphonus, Mastigoproctus.

Order 7. Araneae

True spiders.
The body is composed of prosoma and the opisthosoma.
Prosoma and opisthosoma with no visible segments, joined by a pedicle that is narrow.
Prosoma bears appendages with 6 pairs.
Chelicerae 2jointed, with an apex of poison in the terminal claw.
Simple pedipalps that resemble legs that are used to transfer male sperms.
Opisthosoma with 3 spinnerets. There is no telephone.
8 eyes are laid out dorsally, in two rows in the prosoma’s carapace.
Book-lung or trachea-related respiratory tract or both.
Examples: Argiope (writing spider), Aranea (house spider), Lycosa (wolf spider), Agelena (funnel-web spider).

Order 8. Ricinulei (=Podogna)

Rare, small, tick-like, heavy-bodied arachnids.
The body is made up of prosoma as well as the opisthosoma.
Prosoma that has an anterior hood-like removable plates (Cucullus).
Opisthosoma 9 segments , connected to prosoma via the pedicle.
Chelicerae and Pedipalpi are both Chelate.
The third set of legs in males are copulatory organs.
The trachea is the main source of respiration.
Examples: Cryptocellus, Ricinoides.

Order 9. Phalangida or Opiliones

Spider-like Harvest-men or Harvest-spiders and daddy-longlegs.
The body is small and oval. body. Extremely long, slender legs.
Unsegmented prosoma, opisthosoma bears 10 segments.
Prosoma is broadly joined to opisthosoma.
No spinning glands.
The trachea is the main source of respiration.
Examples: Phalangium, Leiobunum.

Order 10. Acarina

Commonly called ticks and mites.
The parasite or free-living.
Small, oval and non-segmented body where prosoma is fused to the opisthosoma.
Chelicerae and pedipalpi are both small and are connected to the mouthparts that are designed to piercing, biting and sucking.
Skin or trachea are respiratory organs.
Examples: Chorioptes (Mites), Sarcoptes (Itch-mite), Idodex (Tick).

Class 3. Pycnogonida

Sometimes known as sea spiders.
Very tiny in terms of size.
The body mostly consists of cephalothorax, abdominal reduced.
Pedipalpi with short segments and tiny Chelicerae.
Typically eight pairs of long walking legs.
Mouth placed on long proboscis.
Simple eyes and four in numbers.
There are no respiratory or excretory organs.
Sexes separate (Dioecious). Females sporting a pair of egg-carriers to carry eggs.
Examples: Pycnogonum, Nymphon.

Subphylum 3- Mandibulata (L., mandibula=mandible+ ata=group)

Terrestrial and aqua, both sea and fresh water.
Body divided into either the cephalothorax and abdomen, or the head, thorax and abdomen.
Head appendages are comprised of 2 or more pairs of antennae, one mandible and one or two maxillae.
Usually, compound eyes.
The respiratory system is characterized by the tracheae or the gills.
Malpighian tubules, also known as green glands in the excretory system.
Sexual dimorphism is a way to distinguish genders.
Development involves larval stages.

Class 1. Crustacea (L., crusta= a hard shell)

Most of the time, it is marine, however there are a few freshwaters and a some live-in moist areas.
Most free-living, but a some parasite forms are.
The head is often joined to the thorax , forming a cephalothorax that is which is covered dorsally by the carapace.
Chitinous soft, hard exoskeleton, limy.
Five-segmented head with two antennae pairs one pair of mandibles and two maxillae.
Typically biramous appendages are common.
Inhalation, either through gills or the general body surface.
Reduced coelom is the form of hemocoel.
Blood vascular system includes a dorsal contractile heart, which communicates through the valvular Ostia, which is encircled by a the pericardial sinus.
Excretion is controlled by either the analyte or maxillary (green) glands.
In general, separate sexual dimorphism is typical.
Nauplius stage of development.

Subclass 1. Cephalocarida

The body is divided into a horse-shoe head shape and 19 segments trunk.
Only the anterior 9 trunk has appendages that look triramous.
Eyes and carapace are not present.
Head has two antennae, a pair jaws and two maxillae.
Genital openings are found on the 19 section. Genital openings are seen on the 19 segment.
Forms of bottom dweller and marine.
Example: Hutchinsoniella.

Subclass 2. Branchiopoda

Small-sized freshwater basic form.
It can be bivalved, shield-like, or even without a carapace.
Flattened leaves, lobed, and leaf-like appendages for the thoracic limbs, with bristles.
Appendages are used for breathing (gills) and locomotion and for filter-feeding.
There are no stomach appendages but it does have a pair caudal forms.
Antennules, and the 2 2nd maxillae diminished or absent.
Parthenogenesis very common.
The larva is called nauplius.

Order 1. Anostraca

Often called fairy shrimps. at least 19 trunk segments.
Only the anterior eleven to 19 segments.
A long body. It has a poorly-developed carapace.
Eyes that are swollen.
Uniramous antennae.
Caudal style is unjointed.
Examples: Branchipus, Artemia, Eubranchipus.

Order 2. Notostraca

A long body Carapace wide shield-shaped.
Sessile eyelashes (without the stalk).
Antennae shortened, anterior half with 35-71 pair of appendages.
Caudal stylests have many joined.
They are also known as tadpole shrimps.
Examples: Apus, Lepidurus.

Order 3. Diplostraca

The body is compressed laterally. body.
Typically, transparent bivalved carapace that covers the body with or without head.
Sessile eyes, usually together.
The large, wide antennae of Biramous can be used to swim.
Unjointed, curving, like claws, caudal styles that resemble claws.
They are also known as water-fleas or clam-shrimps.
Examples: Daphnia, Limentis.

Subclass 3. Ostracoda

Small crustaceans are found in freshwater and at sea.
Poorly segmented body completely covered in a bivalved carapace.
2 pairs of stout , and cylindrical appendages to the thorax.
Large antennae and antennules designed for swimming.
Mandibles are supplied with an e-syringe.
Commonly occurring manifestations of parthenogenesis.
Sometimes referred to as the minute mussel, or seed shrimps.

Order 1. Myodocopa

It was found in the seawater.
A carapace that has notches for antennas.
2 2nd antennae biramous. It is enlarged at the base.
2 2nd antennas used in swimming.
Example: Cypridina.

Order 2. Podocopa

Freshwater and marine.
Unnotched carapace with no apertures to allow antennae.
Two pair of appendages to the trunk.
Uniramous 2 2antennae claws that resemble legs at the ends.
Examples: Cypris, Darwinula.

Order 3. Platycopa

It was found in the seawater.
Unnotched carapace with no apertures to allow antennae.
2nd antennae uniramous.
1 . Two trunk appendages.
Antennae are not designed for swimming.
Example: Cytherella.

Order 4. Cladocopa

It was found in the seawater.
Carapace with no apertures, notched and without antennae.
2nd antennae biramous.
The antennae are used in pairs to swim.
Example: Polycope.

Subclass 4. Mystacocarida

Small-sized crustaceans, with an extended body that is divisible into the head, the thorax, five-segmented and a six-segmented abdominal.
The head appendages that are well developed constitute the one 1st Thoracic Pair (Maxillipedes).
Antennules with prominent antennules and antennae.
A single median eye. No compound eyes.
Limbless abdominal.
A pair of stylets for caudal.
The sexes are different.
Metanauplius is one of the earliest identified larval stage.
Example: Derocheilocaris.

Subclass 5. Copepoda

The crustaceans are parasitic or free, and can be can be found both in fresh and saltwater.
Small body that is segmented and split into chest, the thorax and the head. abdomen.
Carapace is absent.
Appendages to the thoracic artery are typically biramous, with the one st that is uniramous.
Limbless abdomen. Telson with two caudal styles.
There are no compound eyes present, only one median eye is present.
Long antennules. Small antennae.
Mandibles typically have a prick.
Eggs are carried around in eggs sac(s) from females.
The larva is known as nauplius.

Order 1. Calanoida

They are found in lake bodies, freshwater ponds as well as in seawater.
The body is constricted to the segment that bears 5 the leg.
Antennas of Biramous.
Examples: Calanus, Diaptomus.

Order 2. Harpacticoida

In freshwater, seawater, as well as brackish water.
The body constriction is minimal between segments with legs 4 4 as well as 5 5 legs.
Antennas with biramous.
Example: Harpacticus.

Order 3. Cyclopoida

It is found in freshwater lakes and ponds, as well as in saltwater.
The body constriction between segments that bear the 4yh as well as 5 the legs is evident.
Uniramous antennae.
Example: Cyclops.

Order 4. Monstrilloida

Larvae that are parasitic to marine polychaete Worms.
No antennae or mouthpieces.
Example: Monstrilla.

Order 5. Caligoida

Ectoparasitic forms found on marine and freshwater fishes.
Body joint between 3 3rd as well as 4 4 thoracic segments is evident in males.
Ectoparasitic is found in the gill chambers of fishes.
It is connected with the body via antennae.
Example: Caligus.

Order 6. Lernaeopodoida

Ectoparasitic forms of Marine and freshwater fishes.
Not apparent body segmentation.
Appendages that are not reduced or removed.
Usually , they are attached to the host body via 2 2 maxillae.
Examples: Lernaea, Salmincola.

Subclass 6. Branchiura

Often, it is referred to as fish lice.
Parasitic crustaceans that live as ectoparasites living on the gills and skin chambers of fishes as well as amphibians.
Dorsoventrally flattened body.
Carapace shield like that which covers the thorax and head.
Suctorial mouth.
Eyes Sessile Compound 1 Pair.
There are no antennae or antennules.
Five pairs of appendages to the thoracic area.
1 1 maxillae changed into suckers.
A stomach that is not segmented, and bilobed.
Caudal claws minute.
Examples: Argulus, Dolops.

Subclass 7. Cirripedia

Only marine and sedentary forms.
Often, they are referred to as barnacles.
Adults that are attached, sessile or parasitic.
Poorly designed body segmentation.
Carapace is a pair of mantle folds that surround the body and is covered on the outside by plates of calcareous.
No antennae or compound eyes in adults.
Six Appendages of the thoracic are usually Biramous as well as cirriform.
The abdomen is rudimentary, usually with two caudal styles.
Antennules are converted into cement glands to attach.
Hermaphrodites (monoecious).
Nauplius larva moves through cypris stage.

Order 1. Thoracica

Non-parasitic forms of sedentary.
Without or with an herbaphrodite stalk.
Body is enclosed by calcareous.
Six pairs of appendages for the thoracic, in cirriform.
The canal is a part of the alimentary tract.
Examples: Lepas (Goose barnacle), Balanus (Acorn barnacle).

Order 2. Acrothoracica

Parasitic forms penetrate the shells of Mollusca and corals.
Unisexual.
Sessile.
Mantle reduced to an attachment disc with chitinous material.
The shell of calcareous is not present.
Six pairs or less of appendages for the thoracic and cirriform.
The canal is a part of the alimentary tract.
Examples: Alcippe, Cryptophialus.

Order 3. Ascothoracica

Parasitic forms of Anthozoa such as Echinodermata.
Saccular or bivalved mantle.
Six pairs of appendages for the thoracic region.
The mouth appendages are transformed into sucking and piercing organs.
There is no alimentary canal.
Examples: Laura, Petrarca.

Order 4. Apoda

Parasitic forms.
There is no mantle or appendages.
Maggot-like body.
There is no Anus.
Hermaphrodites.
Example: Proteolepas.

Order 5. Rhizocephala

Parasitic forms of decapod crustaceans.
Degraded and severely deformed body.
There are no appendages or alimentary canals.
No evidence of segmentation.
The peduncle is a root-like branch which ramify through the tissues of the host.
Example: Sacculina.

Subclass 8. Malacostraca

Large crustaceans.
It is found in fresh and saltwater.
Usually made up of 19 sections.
The head and one or two thoracic segment create the cephalothorax.
Thorax has 8 segments while the abdomen is comprised of 6 times 7 segments.
Formally well-formed, vestigial or without carapace.
Compound eyes that are paired are generally stalked.
The abdomen is finished in an elongated the telson. No caudal styles.
The development process is via Zoaea stages; Nauplius stages seldom occur.

Order 1. Nebaliacea

Marine crustaceans that are primitive.
Bivalved carapace that has muscles of the adductor.
eight pairs of leaf-like, gills in the thorax.
7 segments abdominal in place of the 6 that is an elson.
Eyes piercing.
Telson wearing a pair of caudal-style.
Example: Nebalia.

Order 2. Mysidacea

Marine crustaceans.
Long body.
The carapace was thin and covered nearly the entire the thorax.
Eyes with a tangle.
All biramous appendages of the thoracic.
Uropods make up the tail fin that resembles a fan.
Examples: Mysis, Hemimysis.

Order 3. Isopoda

Freshwater, marine, terrestrial and parasitic crustaceans.
Dorsoventrally flattened body.
Head and one or two segments of thoracic make up the cephalothorax.
There is no carapace. Sessile eyes.
Most often, it is a short abdomen.
Commonly referred to as woodlice.
Examples: Oniscus, Bopyrus.

Order 4. Amphipoda

Most marine crustaceans.
The body is compressed laterally. body.
Carapace not present. Gills in the thoracic.
Sessile, lateral eye.
They are often called Sandhoppers.
Examples: Caprella, Gammarus.

Order 5. Stomatopoda

Marine forms.
Flattened body.
Small carapace that covers three thoracic segments.
The abdomen is larger and wider that cephalothorax.
Eyes that are stray.
The initial five abdominal appendages are equipped with gills.
Heart with a long neck.
2nd Maxillipedes raptorial.
Also known as mantis shrimps.
Example: squilla.

Order 6. Decapoda

The majority of marine forms.
A well-developed carapace that covers the entire thorax.
The first 3 pairs of thoracic legs form Maxillipedes while the rest of the five pairs function as walking legs.
Gills are usually 3 series that are located on the thorax.
Statocyst present.
The most common species of Larva is Zoaea.

Suborder 1. Macrura

Well-developed, elongated and an extended abdomen.
Antennules and antennae both are both large.
Eyes aren’t encased within orbits.
Examples: Palaemon, Astacus.

Suborder 2. Anomura

Fixed or reduced abdomen.
Examples: Hippa, Eupagurus.

Suborder 3. Brachyura

The body is incredibly soft and abdominal muscles folded under the body.
Examples: Cancer, Carcinus.

Class 2. Chilipoda (Gr., cheilos=lip+ pous=foot)

This includes centipedes as well as 100-leggers.
Flattened Dorso-ventrally body and divided between head as well as 15 to 173 trunk segments.
Head is a solitary body with a pair of antennae, a pair mandibles, as well as maxillae.
Legs 1 pair for every trunk section.
The first pair of legs are poison claws.
Carnivorous, active predaceous.

Order 1. Scutigeromorpha

15 pairs of legs , and extremely long.
Long-length antennae.
Median, unpaired, and dorsally located spiracles.
Example: Scutigera.

Order 2. Lithobiomorpha

15 legs in a row and extremely small.
Many antennas with elongated 19-70 segments.
Laterally and laterally placed spiracles.
Example: Lithobius.

Order 3. Geophilomorpha

Forms of burrowing with slim bodies.
Small legs Small legs, 31 to 170 pairs.
Laterally and laterally placed spiracles.
No antennas or eyes have 14 segments.
Example: Geophilus.

Order 4. Scolopendromorpha

Body elongated bears 21 – 23 legs.
Head without or with eyes It has 17-30 jointed antennae.
Laterally placed, paired spiracles that only affix in the trunk segment that is anterior.
Example: Scolopendra.

Class 3. Symphyla (Gr., syn=together+ phylon= tribe)

Includes garden centipedes.
Terrestrial, but often found in damp areas with Hummus.
A slim body divided into the head and 15-22 trunk, with 10-12 legs.
No eyes.
Head is adorned with one pair of antennae 1 mandible pair, and two maxillae.
The head contains a few spiracles only.
Genital apertures are located mid-ventrally between the 4th pair of legs.
Examples: Scutigerella, Scolopendrella.

Class 4. Pauropoda (Gr., pauros=small + Pous=foot)

Minute grubby body split in head segments and between 11-12 trunks. 9-10 legs.
Soft-bodied arthropods are found in dark , damp areas typically under logs, rocks as well as fallen leaves. They can also be found in soil.
Head is comprised of a pair antennae, two mandibles and two maxillae that form the lower lips.
No eyes.
Gonopores are located mid-ventrally on the 3rd segment.
Examples: Pauropus, Decapauropus.

Class 5. Diplopoda (Gr., diplos=double+ pous= foot)

Millipedes, or thousand-leggers.
Long, cylindrical, sub-cylindrical and capable of being rolled over the body.
The head is divisible into the abdomen stomach, thorax and head.
5-segmented head and thorax, four-segmented and 20-100-segmented abdominal.
Head has one pair of antennae, one mandible and one couple of maxillae.
Thoracic segments, with one set of legs.
Legs Two pairs of every abdominal section (Diplopoda).
No poison claws.
Food habits that are herbivores.
In the mid-ventral region, gonopores located on the third abdominal section.

Order 1. Pselaphognatha

Moment, body covered with serrated bristles.
1 or 13 trunk segments.
Gnathochilarium (fused maxillae on the two sides) with the palps of a pair.
Soft inguments.
No stink glands.
Examples: Polyxenus, Lophoproctus.

Order 2. Pentazonia

Body that can be wrapped into an elongated ball.
13-15segments trunk.
Each trunk segment has five sclerites.
Male Gonopods one or two pars i.e. organs that clasp that are present.
No stink glands.
Examples: Glomeris, Onomeris.

Order 3. Nematomorpha

The body segments from 26-60 and trunk segments 26 to 32
Two or three pair with spinning glands.
Two or more male gonopods in a pair within the seven 7segment.
Eyes present.
Often, they are known as silk-spinning millipedes.
Examples: Striaria, Cleidogono.

Order 4. Juliformia

Trunk made up of up to 40 segment is commonly known as snake millipedes.
One or two male gonopods in the seventh segment.
No spinning glands.
Stink glands are present on the majority parts of trunks.
Examples: Julus, Spirobolus.

Order 5. Colobognatha

Trunk that has 30 to 192 segments that are flattened often referred to as suctorial millipedes.
Conical head and small mouthpieces.
Two male gonopods. 1 pair for segments 7th and 8th segments.
Stink glands present.
Examples: Polyzonium, Platydesmus.

Order 6. Polydesmoidea

Trunk made up of 19-22 segments, often referred to as millipedes that have a flat-back.
One or two male gonopods in pairs on 7th segments.
There are no spinning glands. However, there are stink glands are present.
Examples: Polydesmus, Oxidus.

Class 6. Insecta (L., insectum=being cut into)

They breathe air mostly in terrestrial, but rarely aquatic arthropods.
The body is composed of the head (6 segmented fused) and Thorax (3 segments) and abdominal (up up to 11 sections).
Head is an eye that is compound and antennae, as well as mouthparts designed for biting, chewing, siphoning, piercing or sponging.
Thorax that has 3 pairs of joined legs and 1 or two pairs of wings that could be missing.
The abdomen is without appendages.
The salivary glands, which are not livers, are typically present.
A long, tubular heart that is divided into 8 chambers in the abdomen.
Respiration via a the branched Trachea.
Excretion through Malpighian tubules.
Unisexual.
Fertilization internal.
The process of development can be directly, but more often complex due to metamorphosis.

Subclass 1. Apterygota (Ametabola) (Gr., A=without wings, ptera.)

Primitive insects that don’t have wings.
Abdomen with style-like appendages and cerci.
Metamorphosis can be absent or minimal.

Order 1. Protura

Insects with soft bodies that are minute in size.
No antennae or compound eyes.
There is no metamorphosis.
Mouthparts are bite-sized.
11segments abdomen, plus the telson.
No trachea.
Example: Acerentomon.

Order 2. Thysanura

Tiny primitive bugs.
The body is covered in tiny silvery scales.
Mouthparts that bite with long antennae that are well-developed.
11-segmented abdomen, Cerci and the long telson.
Primitive metamorphosis.
Example: Lepisma (Sliver fish).

Order 3. Aptera

Small, white or light blind insects.
Flattened body.
Chewing with the mouth.
Antennas with many segments.
No caudal filaments.
There is no metamorphosis.
Example: Campodea.

Order 4. Collembola

Insects that are minute; body naked or covered with scales or hair.
Eyes, trachea, Malpighian tubules and metamorphosis.
Mundiatric parts chewing and sucking.
4 – to 6-segmented antennae.
Six-segmented abdomens, typically having a ventral tube Tenaculum, and spring.
Examples: Springtails, Snow flies.

Subclass 2. Pterygota (Metabola) (Gr. Ptera=wings)

Wings are present. They are also lost in some.
No abdominal appendages except cerci.
Metamorphosis that is simple or complicated.

Division1. Exopterygota (Heterometabola)

Wings are developed externally in buds.
Metamorphosis that is simple or primitive.
Young stages are called nymphs.

Order 5. Orthoptera

Large or medium-sized insects.

Two sets of wings. The forewings, or tegmina, are straight and smooth. Hindwings membranous , and folded when they are at rest.
Mouthparts that chew and bite hard.
Large prothorax. Hindlegs jumping.
Compound eyes are usually 3 or more Ocelli.
Short, long, simple, or segmented cerci.
Metamorphosis is simple.
Examples: Locusts, Grasshoppers, Crickets, etc.

Order 6. Grylloblattodea

Small thysanuriform bugs.
There are no wings.
Mouthparts with bites.
Filiform, can have segmented antennae.
Eyes small and compound, no ocelli.
Metamorphosis is simple.
Example: Grylloblatta.

Order 7. Blattaria

Large to medium-sized insects.
The wings are present, or not.
Long, multi-segmented as well as filiform.
Mouthparts chewing and biting types.
The prominent as well as segmented.
Simple metamorphosis.
Examples: Periplaneta, Blatta.

Order 8. Phasmida

Large leaf-like or stick-like insects.
Usually, they are long, filiform and with many segments.
Small compound eyes, ocelli 2, or 3 or not present.
Mouthparts with bites.
The wings are present, or not.
The cerci are small and divided.
Examples: Phyllium, Carausius (Stick insect).

Order 9. Mantodea

Medium-sized to large-sized insects.
A tiny, triangular head has big compound eyes that have three or more Ocelli.
Mouthparts with bites.
A very elongated and elongated prothorax.
Forelegs are modified to catch and holding prey.
Small, big or even without wings.
Metamorphosis is simple.
Examples: Mantis (praying mantis).

Order 10. Dermaptera

Small – to medium-sized insects.
An elongate body with a smooth or chitinous cover.
Mouthparts that bite type.
The abdomen is equipped with a forceps-like cerci located at the rear end.
Metamorphosis is simple.
Example: Forficula.

Order 11. Diploglossata

Small insects.
Depressed body covered in short hairs.
Mouthparts with bites.
There are no eyes or wings.
Two cerci that are segmented is present.
Metamorphosis that is simple.
Example: Hemimerus.

Order 12. Plecoptera

Medium – to large-sized.
Soft-bodied, elongated and flattened insects.
Broadhead has two filiforms of 50 to 100 segmentsed antennas.
Moderate compound eyes , with three or more or none Ocelli.
Mouthparts with bites.
Two pairs of similar veined wings. The hind pair with an enormous anal area.
11 segmented abdomen. The final segment is shortened and bears long, filiform multisegmented cerci.
Hemimetabolous or simple metamorphosis.
The aquatic larva is referred to as Naiads.
Some examples: Stoneflies, Salmon flies.

Order 13. Isoptera

The insects are polymorphic and social in colonies within the caste system.
Small to medium-sized, mostly soft-bodied and light-colored insects.
Small to very big head and has moniliform short or long , many segmented antennae.
Mouthparts chewing or biting.
Eyes with a dingy compound that has 2 or none of the or no.
Wings in two sets when there are two pairs, with similar shape, size and the venation.
Simple and short cerci or 2-8 segments.
Simple metamorphosis.
A colony is made up of males and females who are winged and sterile workers who are wingless soldiers.
Examples include: termites, white ants.

Order 14. Zoraptera

Minute wing or wingless insect.
Antennas with nine segments.
Compound eyes and ocelli can be found in winged forms but are not present in forms with no wings.
Mouthparts that bite type.
The cerci are short and divided.
Simple metamorphosis.
Example: Zorotypus.

Order 15. Embioptera

Small, slender insects that have flattened bodies.

The head’s large size bears an antenna with a filiform shape.
Females with small compound eyes typically larger than males. No Ocelli.
Mouthparts that bite chewing.
Thorax almost length of the abdomen.
The first tarsal segment of the forelegs that has been enlarged to include spinnerets, glands, and glands.
Two pairs of wings at present, membranous and similar in size and size and.
Metamorphosis that is simple or with hemimetabolous.
Example: Oligotoma.

Order 16. Corrodentia

Small insect with wings or winged wings.
The head is large and free. It has long or short filiform antennae.
Usually, large compound eyes are not accompanied by the ocelli.
Mouthparts modified for biting.
No cerci.
Simple metamorphosis.
Some examples: Booklice, Bark lice, Dust lice.

Order 17. Mallophaga

Small ectoparasitic insects with no wings.
Elongated or broad Dorsally flattened, tough and well-chitinised body.
Hypognathic head with no the setiform and clavate antennae.
Eyes reduced to compound eyes, no Ocelli.
Mouthparts with bites.
Short legs with sturdy claws that are designed for securing feathers and hair.
No cerci.
Example of a bird lice.

Order 18. Anoplura

Small-sized to minute-sized, permanent an ectoparasitic species on mammals.
A flattened and long body with no wings.
Small with short , 3-5 segmented setiform antennae on its head.
Reduced eyes or no compound eyes or Ocelli.
Mouthparts that pierce and sucking.
Short legs with one claw specially designed for gripping hairs.
No cerci.
Example: Pediculus (Human louse).

Order 19. Ephemerida

Small to medium-sized, soft-bodied insect with a slender body.
Short setiform antennae.
The mouthparts of the mouth are a bit rusty.
Compound eyes that are wee-developed, and 3 ocelli are present.
One or two pairs of delicate numerous veined wings.
Very long multisegmented, filiform, and multisegmented cerci with the same medial caudal filament.
Metamorphosis that is simple or with hemimetabolous.
Naiad (nymphs) are aquatic creatures with stomach tracheal Gills.
Example: Ephemera (mayfly).

Order 20. Odonata

Medium-sized to large-sized fast-flying predaceous insects.
The head of the mobile bears huge compound eyes as well as Ocelli.
Mouthparts that bite type.
Two pairs of similar thin net-veined wings.
Naiads(nymphs) include aquatic species creatures with or without external gills.
Hemimetabolous or simple metamorphosis.
Examples: Dragonflies, Damselflies.

Order 21. Thysanoptera

Minute, slender, terrestrial insects.
A little dorsally compressed, or nearly cylindrical body.
Short, 6 or 9 segments antennae.
Compound eyes, with 3 ocells present.
Mouthparts that are modified to allow piercing, sucking, and chafing.
Two pairs of wings are both narrow and long with only a only a few veins when they are they are present.
10-11 segmented abdomen , with an ovipositor.
Simple metamorphosis.
Example: Thrips.

Order 22. Hemiptera

Small to large oval or elongated and dorsoventrally flattened terrestrial aquatic insects.
2 to 10 or more rarely 25 , segmented, antennas.
Large eyes, with or without Ocelli.
Mouthparts that are piercing and sucking.
Two pairs of wings when present, fore pair typically thickened at the base and membranous on the apical side in Heteroptera and completely membranous for Homoptera.
No cerci.
Anal respiratory filaments are found in certain aquatic forms as well as male coccids contain anal filaments.
Simple metamorphosis.
The majority of them are phytophagous. Some are predaceous.
Examples: Belostoma, Aphids, Cicadas, scale insects.

Division 2. Endopterygota (Holometabolan)

Metamorphosis complex i.e. completes with pupal and larval stages.
The young stages are also known as larvae.
The wings develop inside pupal cases.

Order 23. Megaloptera

Large- or medium-sized insects.
The prognathous head has numerous slender antennae segmented.
Mouthparts with bites.
Two pairs of wings identical in shape, size and size, and.
No cerci.
Complex metamorphosis involving aquatic larvae.
Examples: Sialis, Corydalus.

Order 24. Neuroptera

Small to medium-sized insect species, mostly terrestrial.
Head hypognathous, has huge and well-separated eye sockets.
Large wings, membranous, many- veined.
Long antennae, usually are filiform.
Mouthparts chewing types.
No cerci.
Larvae can be carnivores. Abdominal gills are present in aquatic larvae.
Complex metamorphosis.
Examples: Crysopa (Lacewing), Myrmeleon (Antlion).

Order 25. Raphidiodea

Elongated, small to moderate-sized delicate terrestrial insects.
Setiform, multiple segmented antennae.
Compound eyes appear prominent with three or more Ocelli.
Mouthparts with bites.
Two pairs of wings that are similar.
Female sporting a and slender Ovipositor.
Complex metamorphosis.
Example: Snake flies serpent Flies.

Order 26. Mecoptera

Slender, small to medium-sized predaceous insects.
Usually elongated and with a vertical head.
Long, filiform and numerous antennas with segments.
Similar, long, membranous wings with narrow wings.
Large and well-separated eye compound, Ocelli 3,, or none.
Mouthparts that bite that have a longer beak.
Short, simple, or 2 segmented cerci.
For males, the top of the abdomen is curved and stings like.
Complex metamorphosis.
Examples: Panorpa (Scorpion Flies).

Order 27. Trichoptera

Small to medium-sized, diurnal and nighttime moth-like insects.
Long, filiform and numerous segmented antennae.
A well-developed compound eye with three or none of the or none.
2 wings that are long hairy, folded roof-like over abdomens.
Mouth part bite the mouth.
One or more segmented cerci.
Complex or the holometabolic metamorphosis.
Larvae aquatic.
Examples: Philopotamus (caddis fly).

Order 28. Lepidoptera

Large to medium-sized flying insects from the terrestrial.
The wings and body are covered with flat, overlapping hairs, and scales.
Antennae vary in their frequency, and can be clavate, serrate either hooked or knobbed, or plumose.
Mouthparts sucking are coiled beneath the head.
Large compound eyes, with 2 or not having Ocelli.
Two pairs of wings, the front pair is usually the largest.
Complex metamorphosis.
Larvae is a caterpillar that has three pairs of thoracic leg and 2 to 4 pairs abdominal prolegs, as well as a chewing mouth.
Examples: Butterflies, Moths.

Order 29. Coleoptera

Large and small insects, with the slender integument.
Antennae vary in size and shape typically 11 segments.
Forewings are leathery (elytra). The hindwings are membranous and fold.
Eyes conspicuous but ocelli absent.
Biting the mouth and chewing the same way.
Usually, abdominals have 10 segments.
No cerci.
Complex metamorphosis.
Examples: Beetles, Weevils.

Order 30. Strepsiptera

Minute to endoparasitic and free-living dimorphic insects.
Short flabellate antennae.
Eyes conspicuous but ocelli absent.
Mouthparts that are atrophied bite-type.
Small white fore-wings, big hind-wings that are fan-shaped.
Metamorphosis complex and hypermetamorphosis.
Example: Stylops.

Order 31. Hymenoptera

Small to huge-sized insects, parasites, or social insects.
Male antennas with 12 segments and 13 in female.
Ocelli is usually present.
Two pairs of wings that are membranous, identical. Each side connected during flight.
Mouthparts that are designed for biting and sucking, lapping, and chewing. The female ovipositor usually produces an abrasive stain.
Abdomen is always covered with a the pedicel that is located at the base.
Complex metamorphosis.
Examples: Wasps, Bee, Ants.

Order 32. Diptera

Medium-sized to minute-sized diurnal, nocturnaland aquatic and terrestrial insects.
Slim body with delicate edges.
Antennae variable, either simple or with the addition of.
Large eyes and separate eyes, with three Ocelli.
Wings 1 pair. Fore-wings designed for flight, and hind-wings used as knob-like halters.
Mouthparts piercing-sucking or sponging.
Complex metamorphosis.
Larvae that are limbless and vermiform, also commonly referred to as maggots.
Examples: Houseflies, Mosquitoes, Midges.

Order 33. Siphonoptera

Small to minute-sized insects that are ectoparasitic to mammals and birds.
A laterally compressed body. In the second instance, wingless.
Small head, with or without eyes and Ocelli.
Capitate and short antennae.
Lengthy and leaping legs.
Mouthparts that are piercing and sucking.
No cerci.
Complex metamorphosis.
Examples: Pulex and Xenopsylla (Fleas).

References

https://courses.lumenlearning.com/wm-biology2/chapter/phylum-arthropoda/
https://bugguide.net/node/view/3
https://www.geeksforgeeks.org/phylum-arthropoda/
https://blogs.ubc.ca/mrpletsch/2019/03/07/phylum-arthropoda/
https://www.biologydiscussion.com/invertebrate-zoology/arthropoda/arthropoda-characters-and-classifications-invertebrate-zoology/27691
https://www.biologydiscussion.com/invertebrate-zoology/phylum-arthropoda/phylum-arthropods-characters-and-classification-animal-kingdom/69874
https://www.onlinebiologynotes.com/phylum-arthropoda-general-characteristics-classification/
https://www.notesonzoology.com/phylum-arthropoda/phylum-arthropoda-features-and-classification-with-diagram/1546
https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-arthropoda