ARTHROPODA

ByShrim Gautam •
· H. Stannius and Von Seibold coined the term Arthropoda (Arthros = jointed; podos = legs).


VVI Examples to Remember of Phylum Arthropoda for Entrance Exams of Nepal are:
  • Peripatus (The connecting link between Annelida and Arthropoda)
  • Aranea (Spider)
  • Palamnaeus (Scorpion)
  • Palaemon (Prawn)
  • Cancer (Crab)
  • Astacus
  • Lepas
  • Daphnia (Water flea)
  • Cyclops
  • Cray Fish
  • Scolopendra (Centipede)
  • Julus (Millipede)
  • Lepisma (Silverfish)
  • Cockroach, Grasshopper
  • Dragonfly
  • Housefly, Mosquito
  • Springtails, Bristle tails
  • Butterflies, Moths
  • Limulus (King crab), Living fossil.


Characteristics of Phylum Arthropoda

· Triploblastic, segmented animals with joined appendages, organ-system level of body organization,
· Tube within tube body plan.
· Coelomate, true coelom restricted to gonads in adults.
The body cavity is filled with hemolymph i.e. blood (Haemocoelomate).


A. External features
a) Bilaterally symmetrical
b) Body is metamerically segmented; their number is generally fixed (tegmatization)
c) Body is covered externally by a hard chitinous exoskeleton; in the form of plates (sclerites) secreted by the underlying epidermis.

The chitinous exoskeleton is shedded off periodically; the process is called Ecdysis/Moulting.

d) Body is divisible into the head, thorax, and abdomen. In some, the head and thorax are often fused to form cephalothorax (in Crustacea and Arachnida).
e) Cephalisation: A true head is present that bears a pair of compound eyes, antenna, and feeding apparatus.
f) Muscles present in continuous sheets of both striated and unstriated muscles.


B. Digestive system
· Alimentary canal is complete, straight.
· Mouth bears mouthparts for ingestion of food.
· Mouthparts are provided with lateral jaws.
· Mouth is modified for biting, chewing, sponging, piercing, siphoning, etc.


C. Circulatory system
· Open type, lacunar type i.e. blood flows in lacunae.
· Heart is dorsal; distributing blood by artery to organs and tissues, from where it returns through the body spaces to the heart.
· Blood is colourless and contains WBCs only.


D. Respiration
  • Crustaceans via General body surface or gills
  • Insecta, diplopoda, chilopoda via Trachea
  • Arachnida via Book lungs

E. Excretion
  • Arachnida: Coxal Glands
  • Crustaceans: Green or Antennary Glands
  • Insects: Malpighian Tubules
Aquatic forms via Green glands or antennary or maxillary glands and Ammonotelic
Terrestrial arthropods via Malpighian tubules or coxal gland and Uricotelic


F. Nervous System
· Annelidan type
· Consisting of paired dorsal ganglia over the mouth, a ring or collar around the gullet, and a ventral chain of ganglia.
· Endocrine system present. Consists of neurosecretory cells in the brain, base of compound eyes, and glands in the thorax. Pheromones have also evolved.
· Highly developed muscular system with numerous separate muscles.
· Sense organs are well-developed.

(a) Eyes
· Simple or compound
· Compound eyes give a Mosaic vision as each lens forms its own image.
· Unit of the simple eye is called ocelli and the unit of the compound eye is called ommatidia.
(b) Antennae: For perceiving odour or for the sense of touch.
(c) Taste receptors: Located in the feet of insects.
(d) Sound receptors are present in chirping crickets and cicadas.
(e) Statocysts for balancing.


G. Reproduction
· Unisexual
· Fertilization is internal in terrestrial forms, external in aquatic.
· Oviparous (ovoviviparous in scorpions)
· Development may be direct or indirect.
· In some, the development occurs by Metamorphosis i.e. series of changes from the egg – larva – pupa – adult.


Classification of Phylum Arthropoda

Phylum Arthropoda is divided into the following 6 classes.


I. Onychophora (Velvet worm or walking worm)
· Connecting link between Annelida and Arthropoda.
Annelidan characters
· Body worm-like.
· Integument thin, soft, lacking exoskeleton
· 12–14 pairs of parapodia-like unjointed appendages.
· Excretion by paired nephridia
Arthropodan characters
· Tracheal respiration
· Appendages ending in claws
· Coelom reduced
· Salivary glands present
Others
· Distinct head with a pair of antennae, a pair of simple eyes, a pair of oval papillae, a pair of jaws.
· External segmentation absent.

Examples: Peripatus


II. Crustacea
(Crusta= Hardshell)
· Aquatic (marine/freshwater)
· The body is covered externally by the chitinous cuticle.
· Body divisible into cephalothorax (head fused with the thorax) and abdomen.
· The cephalothorax bears 5 pairs of appendages (two pairs of antennae, one pair of mandibles, and two pairs of maxilla) and a pair of stalked compound eyes.
· Abdomen has pleopods or biramous appendages.
· Eyes form appositional (in bright) and super positional image (in dark).
· Respiration takes place by gills or body surface. 
The respiratory pigment is hemocyanin or metallic copper (light blue in colour).
· Excretion by a pair of antennary glands or green glands. They are ammonotelic.
· Sexes separate with distinct sexual dimorphism.
· Development indirect with zoea larva.

Examples:
Palaemon (Prawn)
Cancer (Crab)
Astacus
Lepas
Daphnia (Water flea)
Cyclops


III. Myriapoda
(Myrios= Ten thousand; podos=feet)
· Body is divisible into head and numerous segmented trunk.
· Respiration by trachea.
· Head with 1 pair of antennae.
· Excretion by malpighian tubules.
· It is further divided into 2 main orders: Diplopoda and Chilopoda which include Millipede and Centipede respectively.

(a) Chilopoda
· Body divisible into head and trunk.
· Each segment has a pair of legs.
· The first pair of legs is modified into poison claws.
Example: Scolopendra (Centipede)


(b) Diplopoda
· Body divisible into the head, thorax, and abdomen.
· First thoracic segment without appendages.
· Each other thoracic segment has a pair of legs; however each abdominal segment has 2 pairs of legs.

Example: Julus (Millipede)


Comparision between Millipede and Centipede
S.N Millipdede S.N Centipede
1. Most species have a cylindrical body. 1. Flattened body.
2. Two pairs of legs per segment. 2. One pair of legs per segment.
3. Usually have 70-100 pairs of legs. 3. Usually have 10- 30 pairs of legs.
4. Herbivorous 4. Carnivorous



IV. Insecta (The largest class of animal kingdom)
(Latin: Insecta = incised; i.e. divided into distinct parts)
· Body divided into 3 regions; head, thorax, and abdomen. The body is bilaterally symmetrical and segmented.
· Head consists of six segments; bears a single pair of antennae, 2 compound eyes, and the mouthparts.
· Thorax consists of 3 segments bearing 3 pairs of legs (Hexapoda) and usually one or two pairs of wings.
· Abdomen bears eleven or fewer segments and also the genital apertures which are situated near the anus at the posterior end of the body. The abdomen is unappendaged.
· Excretion is performed by fine Malpighian tubules. Uricotelic.
· Respiration by branched; cuticle-lined Trachea.
The trachea carries oxygen directly to the tissues from paired apertures; the spiracles or stigmata, which lie on the sides of the thorax and abdomen.
· Heart is tubular and is divided into chambers.
· Sexes are separate.
· Change from larva to adult is known as Metamorphosis or Metabola. Based on it, insects are

a) Ametabolous insects
· Insect without metamorphosis e.g. Lepisma (Silverfish)

b) Paurometabalous insects
· Insect with gradual metamorphosis e.g. cockroach, grasshopper.
· Egg Þ Nymph Þ Adult

c) Hemimetabolous insects
· Insect with incomplete metamorphosis e.g. Dragonfly.

d) Holometabolous insects
· Insect with complete metamorphosis e.g. Housefly, mosquito,es etc.
· Egg -> Larva -> Pupa -> Adult


Sub-classes of Insecta

On the basis of the presence or absence of wings, they are divided into two sub-classes.

A. Apterygota
· Wingless insects
· Have appendages on the abdomen, used for walking.
· Metamorphosis absent.

Examples: Silverfish, Springtails, Bristle tails, etc.


B. Pterygota
· Wings present
· Metamorphosis present: either complete or incomplete.
· Exopterygote insects e.g. Grasshopper, cockroach, locusts, crickets, etc. have simple metamorphosis with nymphal stage.
· Endopterygote insects have a complex metamorphosis and have young stages termed larva. Examples: Houseflies, mosquitoes, butterflies, moths, etc.


Types of Mouthparts in Insects

i) Biting and Chewing type e.g. cockroaches, grasshoppers.
ii) Chewing and Lapping type e.g. honey bees, wasps.
iii) Piercing and Sucking type e.g. bugs, mosquitoes.
iv) Sponging type e.g. houseflies.
v) Siphoning type e.g. butterflies, moths, etc.

· In houseflies and mosquitoes, the hind wings are modified as short stumpy outgrowths; the halteres, or the balancing organs.


V. Merostomata
· Marine arthropods
· Carapace (dorsal shield of the cephalothorax) is horse-shoe shaped and convex.
· Abdominal appendages are modified into book gills (respiratory).
· Abdomen is wide and unsegmented.
· Tip of the abdomen has a long spine-like telson.

Examples: Limulus (King crab) also called a Living fossil.


VI. Arachnida
· Terrestrial arthropods
· Body divisible into two regions Prosoma (cephalothorax) and opisthosoma (or abdomen). Antennae absent.
· Prosoma has a pair of simple eyes and six pairs of appendages (two chelicerae, two pedipalpi, and four pairs of walking legs).
· Abdomen lacks appendages.
· Respiration takes place by book lungs or trachea.
· Sexes separate.
· Excretion by malpighian tubules or coxal gland.

Examples: Aranea (Spider)
Palamnaeus (Scorpion)



Tags:
Previous Post Next Post

Main Tags