In this article we will discuss about the classification of phylum arthropoda.
1. Arthropoda is the largest phylum comprising bilaterally symmetrical, triploblastic, coelomate invertebrates.
2. Body segmented but most species exhibit some degree of reduction in metamerism.
3. The segments usually bear paired, lateral and jointed appendages, variously modified as jaws, gills, legs, etc.
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4. The body is covered with an exoskeleton of chitin and protein, which is thick, tough and non-living, covering each segment but remains soft and flexible between segments permitting free movement. Moulting or ecdysis of exoskeleton occurs very often.
5. The muscles are attached to the inside of the exoskeleton and the skeletomuscular system functions as a lever system.
6. The blood vascular system is open; capillaries are absent and the arteries open into irregular spaces, called lacunae or sinuses which collectively form haemocoel. The dorsal heart is a primitively pulsating tube.
7. Respiration is carried out on either through general body surface or through gills or book-gills in aquatic forms and tracheae or book-lungs in the terrestrial forms.
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8. Excretion occurs through Malpighian tubules or by green glands (coelomoducts); nephridia absent.
9. The nervous system is annelidan type; the dorsal brain is connected around the oesophagus with a double ventral ganglionated nerve cord by a pair of circumoesophageal connectives.
10. A distinct cephalization is present.
11. Compound eyes with mosaic vision.
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12. The sexes are usually separate, with sexual dimorphism; gonads and their ducts are usually paired; fertilization internal; development includes little or more metamorphosis; parental care often well-marked.
In present-day classification phylum Arthropoda has been subdivided into seven subphyla—Onychophora, Tardigrada, Pentastomidia, Trilobitomorpha, Chelicerata, Pycnogonidia and Mandibulata.
Subphylum I. Onychophora:
1. The body is caterpillar-like, soft, covered with a thin layer of pigmented cuticle.
2. It is divided into rings, which do not correspond to the segmental arrangement of the appendages.
3. A pair of preoral antennae, a pair of jaws, a pair of oral papillae and many pairs of short, stumpy legs, each with two claws are present.
4. Respiration is carried by tracheae, opening into irregularly scattered pits.
5. The heart is elongated and dorsal and with valvular Ostia.
6. Almost all are viviparous.
Examples: Peripatus, Euperipatus, Ophisthopatus, etc.
Subphylum II. Tardigrada:
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1. Soft-skinned animals of minute size, not exceeding a millimetre in length.
2. Body is vaguely segmented and not divisible into regions.
3. Four pairs of short un-jointed legs, the last pair is terminal and each is provided with two or four claws.
4. Mouth is surrounded by papillae; the buccal cavity contains a pair of horny, sometimes partly calcified teeth.
5. A brain and a ventral nerve cord with ganglia present.
6. Two eye spots at the anterior and are the only sense organs.
7. The gonads in both sexes are saccular and open into the terminal part of the intestine.
8. Live among damp moss; others in fresh or marine water.
Example: Macrobiotus.
Subphylum III. Pentostomida:
1. Tongue or fluke-shaped or cylindrical worms with superficial annulation.
2. On each side of the mouth two pairs of short limbs or only claws present.
3. Alimentary canal straight and simple.
4. Nervous system reduced, consisting of a circumoesophageal ring, a sub-oesophageal ganglion and a simple nerve cord.
5. Heart, respiratory and excretory organs are lacking.
6. Larva with two pairs of jointed legs.
7. All are parasites in vertebrates.
Examples: Railiatiella, cephalobaena, Procephabis, Arum lifers, etc.
Subphylum IV. Trilobitomorpha or Trilob1ta (extinct group):
1. Body divisible into two distinct regions (tagmata)—prosoma and opisthosoma.
2. Two longitudinal furrows divide the oval-shaped body into three distinct longitudinal zones.
3. Antennae paired and un-jointed.
4. Appendages similar and un-specialised.
Examples: Agnostus, Trinuclaus, etc.
Subphylum V. Arachnomorpha or Chelicerata:
1. Body divided into cephalothorax and abdomen.
2. Cephalothorax consists of one preoral and five postoral segments, each bearing a pair of appendages.
3. Appendages of first postoral segment are known as chelicerae.
4. Abdomen consists of 12 to 13 segments and a telson.
5. Second abdominal segment bears genital aperture covered by operculum, which is a modified abdominal appendage.
6. Eyes compound, degenerated in most cases.
7. Median ocelli present.
8. Respiration by book-lungs, book-gills, tracheae and skin.
Class 1. Merostomata:
1. Aquatic arthropods.
2. Abdomen subdivided into a 7-segmented mesosoma and a 5-segmented metasoma.
3. Mesosoma bears 4 to 5 pairs of similar appendages.
4. A caudal style present.
5. Respiratory organs gills.
Subclass i. Xiphosura:
1. The prosoma has an un-segmented carapace, the opisthosoma has a dorsal shield with serrated edges; prosoma and opisthosoma are joined by a transverse hinge.
2. The horseshoe-shaped carapace of the prosoma is marked by a median longitudinal ridge and lateral grooves.
3. The prosoma bears a pair of lateral compound eyes and a pair of anterior, median ocelli.
4. Three-jointed, chelate chelicera followed by five pairs of similar appendages, all of which are chelate except the fifth, which bear terminal spines.
5. Generally marine.
Examples: Limulus (distribution—North America), Tachypleus, carcinoscorpious (South- East Asia).
Subclass ii. Eurypterida (Extinct):
Example: Eurypterus
Class 2. Arachnida:
1. The body is usually divisible into cephalothorax and abdomen.
2. The cephalothorax bears sessile, usually simple eyes, two pairs of cephalic appendages, chelicerae and pedipalpi, and four pairs of legs.
3. Antennae are always absent.
4. Respiratory organs are either tracheae, or book-gills or book-lungs.
5. Heart is tubular and ordinarily not divided into chambers.
6. Sexes are almost always separate.
7. Development usually direct.
Examples: Scorpio, Palamnaeus, Argiope, Mynossus, etc.
Subphylum VI. Pycnogonida:
1. Marine, spider-like; chelicerae short, pedipalpi long and segmented;
2. At the anterior end a distinct proboscis is present.
3. Genital pores thoracic.
4. Third pair of appendages, the ovigers in male carry the eggs.
5. Reduced abdomen bears two to three segments.
6. Partially sedentary marine animals.
7. Young are parasites on soft bodied invertebrates.
Example: Nymphon.
Subphylum VII. Mandibulata:
1. Body divided into head, thorax and abdomen.
2. Three preoral segments are fused and bear antennae, compound eyes and simple eyes.
3. The fourth segment bears a pair of mandibles.
4. Respiratory organs gill, tracheae or skin.
Class 1. Crustacea:
1. The body is elongated and divisible into head, thorax and abdomen. In many cases, the head is fused with the thorax and form the cephalothorax.
2. Head bears two pairs of antennae and at least three pairs of masticatory appendages in addition to others.
3. The abdomen is usually segmented and often bears appendages.
4. Appendages perform varied functions like feeding, cutting, swimming, feeling, etc.
5. The appendages are typically biramous, each having a basal portion and two distal branches.
6. Respiration by gills.
7. Coelom is restricted to the gonads as gonocoel.
8. The chitinous cuticle is usually impregnated with calcium carbonate.
Subclass i. Cephalocarida:
1. Nineteen trunk segments behind a horseshoe-shaped head; only anterior nine trunk segments bear appendages.
2. Both pairs of antennae are short.
3. Eyes absent.
4. Hermaphroditic with genital opening on ninth segment.
Example: Hutchinsoniclla.
Subclass ii. Branchiopoda:
1. Varying number of trunk segments provided with appendages of a more or less uniform character, usually foliaceous and rarely leg-like.
2. Abdomen without appendages; terminally a pair of un-jointed or many-jointed caudal style.
3. A cephalic carapace is often present; it may be either shield-like or bivalved.
4. Paired compound eyes.
5. The antennules and second maxillae reduced.
6. Larva is a nauplius or metanauplius.
Examples: Branchipus, Triops, Lepidurus, Daphria, Polyphemus, etc.
Subclass iii. Ostracoda:
1. Indistinctly segmented body bearing usually up to two pairs of non-foliaceous trunk appendages.
2. Posterior part of trunk is terminated by a pair of caudal styles.
3. Body is entirely enclosed in a bivalved carapace.
4. Both pairs of antennae well-developed and used for swimming.
5. The larvae typically nauplius, enclosed within a bivalve carapace.
Examples: Cypris, Notodromus, etc.
Subclass iv. Mystacocorida:
1. Body divided into a head, five-segmented thorax, and six-segmented abdomen; the last four throcic and all abdominal segments are free.
2. No abdominal appendages except on the last, which carries a pair of caudal styles.
3. Sexes separate; earliest larval stage is Nauplius.
Example: Derocheilocaris.
Subclass v. Copepoda:
1. Elongated, segmented body divided into head, thorax and abdomen.
2. Thorax six-segmented, but usually the first segment and sometimes second one too fuses with the head to form cephalothorax.
3. Each thoracic segment bears a pair of appendages; of these the first is always, and the last is often, uniramous and others are biramous.
4. Except for caudal rami, the abdominal segments do not possess appendages.
5. No carapace; no compound eye; a median nauplius eye is usually present.
6. The genital aperture is on special genital segment preceding the abdomen.
7. Larvae Nauplius.
Examples: Cyclops, Calanus, Lernaea, Ergasilus, etc.
Subclass vi. Branchiura:
1. Body divided into head, thorax and abdomen.
2. A bilobed cephalic fold arises from the head and fuses with the sides of the first thoracic segment to form a carapace-like structure.
3. Thorax has four pairs of biramous appendages; abdomen un-segmented, without appendages, bilobed and ends with a minute pair of caudal claws.
4. Mouth suctorial; eyes compound.
5. Both pairs of antennae reduced; the first one clawed. Often the first maxillae are modified to form a pair of suckers.
6. The genital aperture opens at the basis of fourth thoracic legs.
7. Sometimes the eggs hatch to a nauplius larva, but commonly the young have all adult appendages on hatching.
Example: Argulus.
Subclass vii. Cirripedia:
1. Almost exclusively marine, sessile, either parasitic or free-living. In parasitic forms the structures may become so modified that almost all traces of arthropodan features disappear.
2. Body imperfectly segmented; carapace forms a pair of mantle folds, which usually enclose the animal entirely and is supported by a system of calcareous plates forming a hard shell.
3. Six similar pairs of biramous appendages are present in most cases, but the posterior region of the body is limbless and rudimentary
4. The antennules of the larva give rise to the organs of attachment and then become vestigeal; the antennae usually disappear altogether.
5. Eyes absent.
6. Mostly hermaphrodite; the young hatch as nauplius larva.
Examples: lepas (goose barnacle), Balanus (acorn barnacle), Synagoga, Proteolepas, Sacculina, etc.
Subclass viii. Malacostraca:
1. Body divisible clearly into head, thorax and abdomen and a telson constituting a segmental region.
2. Distinct segmentation present; with a fixed number of 20 segments typically, rarely 21; the head constitutes six, thorax eight and abdomen six or seven segments.
3. Each segment bears appendages except the first.
4. Posterior margin of the head commonly fuses with one or more thoracic segments to form a cephalothorax; no typical caudal style.
5. Paired sessite or stalked compound eyes mostly present.
6. The antennules are often biramous.
7. The male gonopore always on the eighth, and female gonopore on sixth thoracic segment.
8. The nauplius stage often occurs before hatching.
Examples: Nebalia, Palaeocaris, Anaspides, Bathynella, Squilla, Thermosbaena, Spelaeo- griphus, Gnathophansia, Mysis, Leptocuma, Gammarus, Caprella, Asellus, Cryptoniscus, Euphansia, Thysanopoda, Macrobrachium, Penaeus, Palinurus, Scyllarus, Homarus, Astacus, Eupagurus, Cancer, Lithodes, Portunus, Uca, etc.
Class 2. Chilopoda:
1. Body usually dorsoventrally flattened.
2. Number of legs vary from 15 to over 100.
3. First pair of thoracic appendages are maxillipeds and function as poison claws.
4. Each leg bears 7 segments.
5. Segment just anterior to telson is the genital segment.
6. A pair of gonopods are present.
7. Respiratory organs are tracheae.
8. Habitat humid areas; nocturnal and carnivore.
Examples: Scolopendra, Geophilus, Scutigera, Lithobius, etc.
Class 3. Symphyla:
1. The number of body segments usually fourteen.
2. 12 pairs of legs are present.
3. A pair of dorsal cerci present on the thirteenth segment.
4. Second maxillae fused to form the labium.
5. Spiracles restricted to the head and the tracheae extend in the first three body segments.
6. Both herbivores and omnivores.
Examples: Scolopendrella, Scutigerella, etc.
Class 4. Pauropoda:
1. Saprophytes, body length is about 1 mm or little more in some cases.
2. 5 segments constitute the head and ten segments are present in the trunk.
3. Antennae branched.
4. Gnatochilarium is formed by the union of first maxillae.
5. Second to tenth segments bear legs.
Example: Pauropus.
Class 5. Diplopoda:
1. The body bears a distinct head but the remaining portion is not distinguishable into different regions.
2. The head bears a pair of many-jointed, short club-shaped antennae, a pair of eyes and 2 or 3 pairs of jaws.
3. The trunk consists of a number of similar segments, each bearing one or two pairs of legs.
4. Respiratory organs are tracheae.
5. Salivary glands are always present.
6. Excretion by Malpighian tubules.
7. Development usually accompanied by a metamorphosis.
Examples: lulus, Polyxenus, Chordeuma, etc.
Class 6. Insecta or Hexapoda:
1. The body is divided into head, thorax and abdomen.
2. The head bears a pair of antennae, a pair of compound eyes and three pairs of mouth appendages.
3. The thorax bears a pair of legs on each of its three segments and generally a pair of wings on each of the posterior two.
4. The abdomen is devoid of appendages in the adult and has a variable number of segments.
5. A liver is absent but salivary glands are always present.
6. The heart is tubular, divided into eight chambers and situated in the abdomen.
7. Respiration by means of air tubes or tracheae.
8. Excretion by means of Malpighian tubules.
9. Sexes separate, development direct or indirect.
Subclass i. Diptera:
1. Abdomen consists of eleven segments both in adult and juvenile.
2. Primarily wingless.
3. Segmented antennae.
4. Caeca present.
5. Malpighian tubules absent or vestigeal.
Examples: Campodea, heterojapyx, etc.
Subclass ii. Protura:
1. Abdominal segments added secondarily through postembryonic growth, making a final complement of eleven segments and a posterior telson.
2. Antennae absent.
3. Primarily wingless.
4. Malpighian tubules present but rudimentary.
Minute, white insects widely distributed; habitat moist litter, bark, etc.
Examples: Pretura, Acerentomen, Eosentomon, etc.
Subclass iii. Collembola:
1. Abdomen with six segments.
2. Usually with three sets of appendages—a ventral tube, a retaining hook on segment 3, which functions with a powerful springing organ on segment 4.
3. Primarily wingless.
4. Tracheael system absent in most species.
5. No Malpighian tubules.
6. Metamorphosis absent.
Known as springtail; live in soil, leaf litter, decaying vegetable materials, etc.
Examples: Podura, Orchesella, Isotoma, Neanura, etc.
Subclass iv. Apterygota:
1. Primarily wingless.
2. Mandibles having only a single point of articulation.
3. Pregenital abdominal appendages usually present.
4. Metamorphosis from juvenile to adult slight or absent.
Example: Lepisma, Pterobius, etc.
Subclass v. Pterygota:
1. Adults winged or secondarily wingless.
2. Pre-genital abdominal appendages lacking in adult.
3. Mandibles articulated at two points; or modified; mouth parts highly variable.
4. Metamorphosis from juvenile slight, or gradual to extreme and complete.
Division 1. Exopterygota:
1. Wings when present form external pads in the late nymphal instars.
2. Extreme metamorphosis between juvenile (nymph) and adult generally lacking.
Examples: Ephemer Periplaneta, Aeschtta (Dragon fly), etc.
Division 2. Endopterygota:
1. Wings, when present, form internal pads.
2. Metamorphosis between juvenile (nymph) and adult more or less extreme, passing through a pupal stage.
Examples: Moths and butterflies, Duormtus, Hypern, Noctua, Trichophaga, Tincola, etc..