Phylum Arthropoda: Characteristics and Classification

Arthropoda, Earth's largest phylum, encompasses a vast array of fascinating creatures. From insects and crustaceans to arachnids, they dominate terrestrial and aquatic ecosystems. Their joint appendages, exoskeletons, and remarkable adaptations have propelled them to evolutionary success, showcasing the immense biodiversity within this remarkable group.

General Characters of Phylum Arthropoda

Aristotle described crabs and some decapods. He grouped them as malacostraca in Insecta.

The name Arthropod means jointed legs. These animals are meteorically segmented, coelomate and triploblastic.

It is the largest phylum of the Animal kingdom.

1. These animals are multicellular triploblastic, bilaterally symmetrical animals. Some of the anterior segments show cephalization forming a distinct cephalic region.
2. Body is segmented. Segments are limited. Each segment bears a pair of appendages. They are jointed.
3. The body is covered by an exoskeleton. It is made of chitin. The exoskeleton project inside. The projections are useful for muscle attachment.
4. The exoskeleton is cast off periodically and a new one is secreted. It is called ecdysis.
5. Tubular dorsal heart is present.
6. True coelom is greatly reduced. It is represented only by the cavities of the reproductive and excretory organs.
7. The blood is colorless.
8. Circulatory system is open type or lacunar type.
9. The nervous system shows a nerve ring and a double ventral nerve cord.
10. Nephridia are absent. Excretion is by malphighian tubules, green glands, and coxal glands.
11. Antennules are present in some arthropods.
12. Striated muscles are present segmentally.
13. Compound eyes are present.
14. Cilia and flagella are entirely absent.
15. Respiration is by gills, by trachea, or by book kings, or by gill books, etc.
16. Sexes are separate, sexual dimorphism is seen. Gonads and their ducts are paired.
17. Development is direct or indirect. The life history may include a number of larval forms.

Classification of Arthropoda

Phylum Arthropoda is divided into seven subphyla.

1. Subphylum: Onycophora

It includes peripatus. It shows both Annelida and Arthropoda characters. It is a terrestrial form. It is a primitive worm-like organism. It shows pairs of eyes, antennae, and jaws. It shows the tracheal mode of respiration. It has a ladder-like nervous system. It is connecting link between annelids and arthropods. E.g.: Peripatus.

2. Subphylum: Tardigrada

These are soft-skinned animals of minute size. They are 1 mm in size. They live in freshwater or seawater. Ex: Macrobiotus.

3. Subphylum: Pentastomida

A group of worm-like parasites. They do not show circulatory, respiratory, or excretory organs. E.g.: Uguatula: (Parasite in nasal sinules of dog)

4. Subphylum: Trilobitomorpha

All are extinct. Trilobites are preserved as fossils.

5. Subphylum: Chelicerata

Their body is divisible into anterior prosoma cephalothorax, and posterior opisthosoma (Abdomen). Prosoma shows 6 pairs of appendages. The first pair is chelicerae. Hence they are chelicerate. Antennae and true jaws are absent. Respiration is by gills, tracheae, or book kings. Sexes are separate. It is classified into two classes.

1. Class: Merostomata

Marine arthropods. They show gill respiration. They do not show antennae. This is divided into two subclasses.

1. Sub Class: Xiphosura: E.g.: Limuhis (King crab). It shows gill books. It has a horseshoe-shaped carapace, and hence it is called a horseshoe crab.
2. Sub Class: Eurypterida: They are extinct.

2. Class: Arachnida

They are mainly terrestrial. Simple eyes are present. Antennae and jaws are absent. The body is divided into Prosoma; Mesosoma and Metasoma. It is divided into the following orders.

1. Order: Scorpionida: Elongated body. Prosoma is short and unsegmented. Chelicerae are small and 3 segmented. E.g.: Palamnaeus, Buthus.
2. Order: Pseudoscorpionida: Small and scorpion-like arachnids. E.g.: Chelifer.
3. Order: Opiliones (or) Phalangida: These are called harvest spiders. The body is short and oval. E.g.: Phalangium.
4. Order: Acarina: It includes mites and ticks. The body is small and oval. Prosoma and opisthosoma are not distinct. E.g.: Argas (Bird tick)
5. Order: Araneae: This is the largest order. It includes spiders. Respiratory organs are book lungs. E.g.: Agelena.
6. Order: Solifugae: These are sun spiders. They live in dry places. Opisthosoma is of 10 or 11 segments. Telson is absent. E.g.: Galeodes.
7. Order: Thelyphonida: Whip scorpions. They measure 18 to 65 mm in size. E.g.: Mastigoproctus.
8. Order: Schizomida: These are small and nocturnal. Prosoma is unsegmented. E.g.: Thelyphonus.
9. Order: Phrynichida (or) Amblypygi: These are scorpion spiders. Prosoma is unsegmented. Opisthosoma is 12 segmented. The terminal flagellum is absent. Pedipalps are heavy. E.g.: Charinus.
10. Order: Ricinulei (or) Podogona: Tick-like and small arachnids. E.g.: Ricinoides.

6. Subphylum: Pycnogonida

These are small, marine, and spider-like arthropods. Their body shows one cephalic somite, 3 or 4 trunk somites, and a vestigial abdomen. Respiratory or excretory organs are absent. The mouth is present on a long proboscis. E.g.: Nymphon, Pycnogonum (Sea spider)

7. Subphylum: Mandibulata

These are terrestrial, freshwater or marine animals. Their body shows two or three parts. Antennae are 1 or 2 pairs. Mandibles are 1 pair. Maxillae are 1 or 2 pairs. Walking legs are 3 or more pairs. Respiration is by gills or tracheae. Excretion by Malphighian tubules or green glands. Sexes are separate. This is divided into 3 classes.

Class 1: Crustacea

These are mostly aquatic and gill breathers. Their exoskeleton is hard and impregnated with lime salts. The head shows 2 pairs of antennae and 3 pairs of jaws. Respiration is by gills or body surfaces. Excretion is by antennal glands.

There are more than 30,000 known species of crustaceans. This is divided into 8 sub-classes.

1. Sub Class: Cephalocarida: Most primitive, marine, and bottom-dwelling arthropods. The body is small, slender, and divided into a horseshoe-shaped head and a trunk of 18 segments. Eyes and carapace are absent. Larva is a metanauplius. E.g.: Hutchinsoniella.
2. Sub Class 2: Branchiopoda: These are small, primitive, and free-living. These are mostly freshwater crustaceans. Antennules are the second maxillae that are either reduced or absent. Mandibles are with or without a vestigial palp. Larva is nauplius. Parthenogenesis is very common. The sub-class is divided into 4 orders.
   1. Order: Anostraca (Fairy shrimps): E.g.: Branchipus, Artemia.
   2. Order: Notgstraca (Tadpole shrimps): E.g.: Trijxs arid Lepidurus.
   3. Order: Conchostraca (Calm shrimps): E.g.: Lynceus, Lepteatheria.
   4. Order: Cladocera (Water fleas): E.g.: Daphrila
3. Sub Class 3: Ostracoda (Seed Shrimps): These are small, free-living freshwater or marine arthropods. The body is laterally compressed and poorly segmented. The body is entirely enclosed within a bivalved carapace with an adductor muscle. Larva is nauplius. Parthenogenesis is quite common. The sub-class is divided into 4 orders.
   1. Order: Myodocopa: E.g.: Cypridina.
   2. Order: Cladocopa: E.g.: Polycope.
   3. Order: Podocopa: E.g.: Cypris, Drawinula.
   4. Order: Platycopa: E.g.: Cytherelia.
4. Sub Class 4: Mystacocarida: These are primitive and marine. Their body is microscopic and elongated. Thorax shows 4 segments, each with a pair of simple and reduced appendages. The abdomen is limbless. E.g.: Derocheilocarus.
5. Sub Class 5: Copepoda: These are free-swimming and freshwater or marine arthropods. Their body is small, elongated, and composed of a head, thorax, and abdomen. The carapace is reduced or absent. Larva is nauplius. E.g.: Cyclops, Calanus.
6. Sub Class 6: Branchiura: These are temporary ectoparasites of fresh water, marine fishes, and some amphibians. Ex: Argulus (carp lice).
7. Sub Class 7: Cirripedia (Barnacles): These are marine, sessile, parasitic, or fixed to rocks, boats, etc. The body is relatively large and imperfectly segmented. The animal is enclosed in a carapace made of two folds of skin called the mantle, which is supported by calcareous plates. Typically 6 pairs of biramous thoracic appendages are present. The abdomen is rudimentary. The sub-class is divided into 5 orders.
   1. Order: Thoracica: E.g.: Lepas, Balanus.
   2. Order: Acrothoracia: E.g.: Trypetesa.
   3. Order: Ascothoracica: E.g.: Synagoga, Dendrogaster
   4. Order: Apoda: E.g.: Proteolepas.
   5. Order: Rhizocephala: E.g.: Sacculina, Peltogaster.
8. Sub Class 8: Malacostraca: These are large crustaceans marine and freshwater arthropods. The body is segmented, with a head of 6 segments, a thorax of 8, and an abdomen of 6. Carapace may be present, vestigial, or absent. Typically 19 pairs of appendages are seen. The sub-class is divided into 2 series.
   1. Series: Leptostraca: These are exclusively marine and primitive malacostracans with 21 body segments.
      • Order: Nebaliacea: E.g.: Nebalia.
   2. Series: Eumalacostraca: These are marine, freshwater, or terrestrial malacostracans with 20 body segments.
      • Super Order 1: Syncarida: It includes two orders.
        1. Order: Ahapsidacea: Freshwater forms. E.g.: Anapsides.
        2. Order: Bathynellacea: They are small. Pleopods are reduced. E.g: Bathynella
      • Super Order 2 : Peracarida: It is divided into 5 orders.
        1. Order: Mysidacea: Many are marine and few are freshwater form. E.g.: Mysis.
        2. Order: Cumacea: They are small and minute. They are seen in deep littoral parts of the sea. E.g.: Diastyles.
        3. Order: Tanaidacea: They are minute marine arthropods. E.g.: Tanais.
        4. Order: Jsopoda: Body is dorsi -ventrally flat. The carapace is absent. E.g.: Oniscus, Ligia.
        5. Order: Amphipoda (Sand hoppers): The body is elongated and laterally compressed. E.g.: Gammarus.
      • Super Order 3: Hopolocarida: It has a single order.
        1. Order: Stomatopoda (Mantis shrimps): These are marine arthropods. E.g.: Squilla, Pseudosquilla.
      • Super Order 4: Eucarida: It includes two orders.
        1. Order: Euphausiacea: They are marine. They form the food of whales. E.g.: Euphausia (Krill).
        2. Order: Decapoda: These are marine freshwater or terrestrial forms. E.g.: Penaeus, Hippa.

Class 2: Myrinpoda

These are terrestrial and air-breathing arthropods. Their body is made of many similar segments. Antennae are 1 pair. Jaws are 3 pairs. Legs are more than 11 pairs. It is divided into 4 groups. Which are not closely related. Nowadays these 4 groups of myriapods are regarded as separate classes.

Group I or Class Diplopoda (millipedes)

These are commonly called millipedes. The body is long and cylindrical with a distinct head and trunk, which has 11 to 100 or more segments according to the species. The name millipede means a thousand leggers. Two pairs of legs are present in each segment.

1. Sub Class 1: Pselaphognatha: E.g.: Polyxenus.
2. Sub Class 2: Chilognatha:
   • Order Oniscomorpha: E.g.: Glomeris.
   • Order Limacomorpha: E.g.: Glomeridesmus.
   • Order Colobognatha: E.g.: Platydesmus.
   • Order Nematomorpha: E.g.: Polymicrodon.
   • Order Polydesmoidea: E.g.: Polydesmus.
   • Order Juliformia: E.g.: Julus.

Group II or Class Chilopoda (Centipedes)

The Chilopoda are commonly called centipedes. The body is slender elongated, and composed of 15 to 180 segments in different species. Chilopoda means "margin foot". Each trunk segment bears a pair of small 7-jointed legs. The first pair of trunk appendages or maxillipeds form the large prehensile poison claws. It is divided into 4 orders.

1. Order Scutigeromorpha: E.g.: Scutigera.
2. Order Lithobimorpha: E.g.: Lithobius.
3. Order Geophilomorpha: E.g.: Geophilus.
4. Order Scolopendromorpha: E.g.: Scolopendra.

Group III or Class Pauropoda

It is a small class, comprising 60 to 70 species. They inhabit moist soil. The body is small. They have a distinct conical head. Trunk segments are eleven a telson is present. E.g.: Pauropus, Brachypauropus.

Group IV or Class Symphyla

Symphyla is a small class of about 120 known species. The body is small white, elongated, centipede-like, composed of 15 to 22 similar segments, 12 of which usually bear legs. E.g.: Scutigerella, Hansaniella.

Class 3: Insecta

These are terrestrial and aquatic arthropods. Respiration is by tracheae. The body cavity is hemocoel. The heart is dorsal and the tubule body is divided into the head, thorax, and abdomen. Antennae are 1 pair, jaws are 3 pairs, walking legs are 3 pairs, and wings are 1 or 2 pairs.

1. Order 1: Isoptera (equal wings): E.g.: Termites or White ants
2. Order 2: Plecoptera (fold wings): E.g.: Stcneflies.
3. Order 3: Embioptera (lively wings): E.g.: Web-spinners (Gligotoma)
4. Order 4: Psocoptera: E.g.: Booklice.
5. Order 5: Anoplura: E.g.: Pediculus (body louse).
6. Order 6: Mallophaga: E.g.: Bird-lice.
7. Order 7: Thysanoptera: E.g.: Thrips (Heliothrips).
8. Order 8: Ephemeroptera: E.g.: Mayflies. (Ephemera).
9. Order 9: Odonata: E.g.: Dragonflies and Damselflies.
10. Order 10: Hemiptera (half wings): E.g.: Belostoma (water bug), Bedbug (Cimex).

Division II: Endopterygota (Holometabola)

Wings develop internally. Metamorphosis is complete including the pupal stage.

1. Order 1: Neuroptera (nerve wings): E.g.: Alder flies, Ant lions.
2. Order 2: Mecoptera: E.g.: Scorpion flies (Panorpa).
3. Order 3: Trichoptera (hair wings): E.g.: Caddis flies.
4. Order 4: Hymenoptera (membrane wings): E.g.: Ants, Bees, Wasps.
5. Order 5: Diptera (two wings): E.g.: Goats, Mosquitoes, House flies.
6. Order 6: Lepidoptera (scale wings): E.g.: Butterflies and Moths.
7. Order 7: Strepsiptera (turning wings): E.g.: Stylopids.
8. Order 8: Coleoptera: (sheath wings): Ex: Beetles.
9. Order 9: Siphonoptera: E.g.: Reas, Pulex.