Phylum : Arthropoda

PHYLUM . ARTHROPODA

• It is the largest phylum in the animal kingdom, including 900,000 species. The largest class is insecta with 750,000 species.
• General chacters are :

1. They are triploblastic, coelomate and bilaterally symmetrical animals. The body cavity is full of haemolymph (blood) and it's known as haemocoel. The true coelom is restricted to gonads.

2.  The body is covered by chitinous cuticle, which forms the exoskeleton which is shed at intervals i.e., undergo moulting/ecdysis.

3.  They have a segmented body, each segment bearing a pair of jointed appendages covered by a jointed exoskeleton. Chitinous exoskeleton is secreted by the underlying epidermis.

4. The body is divided into head, thorax and abdomen. In some cases, the head and thorax is fused to form cephalothorax. In insects, the thoracic segments have legs and wings, the abdomen has no legs in insects.

5.  Respiratory organs are gills, book gills, book lungs or tracheal system.

6.  Excretion takes place through green glands or malpighian tubules since nephridia are absent.

 7.   Sensory structures in arthropods are antennae for perceiving odour, eyes, statocysts or balance organs and sound receptors (in chirping crickets and cicadas). Eyes are simple or compound. In honey bees, butterflies, moths and some other insects, the gustatory receptors are present on their feet.

8.  The heart is dorsal and circulatory system is open.

9.  The central nervous system consists of paired pre-oral ganglia connected by commissures to a solid double ventral nerve cord. 

10.  In land arthropods, the fertilization is always internal. 

11.   Arthropods are mostly dioecious, oviparous. In some like the scorpion, the eggs hatch within the female body. They bring forth the young ones alive. They are viviparous. Development is direct or indirect. 

Advancement Over Annelida 

1.    Distinct-head in all species. 
2.    Jointed appendages serving a variety of functions. 
3.    Jointed exoskeleton for protection and muscle attachment. 
4.    Striped muscles arranged in bundles for moving particular body parts. 
5.    Special respiratory organs such as gills, trachea, book lungs in majority of cases. 
6.    Well developed sense organs such as compound eyes, statocysts, auditory organs, taste receptors etc. 
7.    Endocrine glands and pheromone secretion for communication.

CLASSIFICATION 

• The phylum arthropoda is divided into five classes.

Class 1. Crustacea 

(i)    The body is divisible into cephalothorax (head + thorax) and abdomen. 
(ii)    Dorsally, the cephalothorax is covered by a thick exoskeletal carapace. 
(iii)    There are present two pairs ofantennae and a pair ofstalked compound eyes and biramous appendages. 
(iv)    Respiration is carried out either by body surface or by giliis. 
(v)    Excretion takes place usually by antennary glands (green glands). 
(vi)    Sexes are usually separate. Sexual dimorphism is also seen. 
(vii)    Development is usually indirect. 
Examples: Palaemon (Prawn), Macrobrachium (Prawn), Astacus (Crayfish), Palinurus (Lobster), Cancer(Crab), Lucifer(Shrimp), Eupagurus (Hermit Crab), Oniscus (Wood louse, terrestrial), Oaphnia (Water-flea), Cyclops, Balanus (barnacle). Tiny crustaceans such as Oaphnia and Cyclops act as zooplanktons which form important link in the food chain in water. 

Class 2. Chilopoda 

(i)    Body is long, segmented and divisible into head and trunk. 
(ii)    Each trunk segment bears a pair of legs. The first pair of legs are modified into poison claws. 
(iii)    There is a single pair of antennae and ocelli. 
(iv)    Many legs are present. 
(v)    Respiration occurs by tracheae. 
(vi)    Excretion takes place by Malpighian tubules. 
(vii)    Development is direct 

Examples: Scolopendra (Centipede).

Class 3. Diplopoda 

(i)    Body is divisible into head, thorax and abdomen. 
(ii)    There is a single pair of antennae and ocelli.
(iii)    Except first thoracic segment, (if does not have legs) each thoracic segment bears a pair of legs, however each abdominal segment has two pairs of legs. 
(iv)    Respiration occurs by tracheae. 
(v)    Excretion takes place by Malpighian tubules. 
(vi)    Development is generally direct e.g., Julus (Millipede).

Class 4. Insecta (Hexapoda) [Largest number of species] 

(i)    Body is divisible into head, thorax and abdomen. 
(ii)    There is a pair of antennae and a pair of compound eyes. 
(iii)    The thorax consists of three segments with three pairs of legs and usually two pairs of wings. For example, mesothoracic wings which are thick and leathery are called tegmina and metathoracic wings are membranous (in cockroach). Wings may be absent in same insects. 
(iv)    The abdomen may consists of ten segments. 
(v)    Respiration usually takes place by tracheae. (No respiratory pigments). 
(vi)    Heart is tubular and divided into chambers. 
(vii)    Malpighian tubules are the excretory organs. Uric acid is chief excretory waste.   (viii)    Sexes are separate. 
(ix)    Development may be direct or indirect e.g., silver fish, cockroach, bedbug, locust, termites, butter flies, rat flea, beetle, wasp, aphid, glow worm etc. Maggot is the larva of Housefly. 

• Insects communicate with each other by ectohormones called Pheromones. Pheromones are chemicals secreted outside the body and perceived (as smell by other individuals of same species). They help in communication amongst the organisms of the same species.

Class 5. Arachnida 

(i)    The body is usually divisible into cephalothorax and abdomen. 
(ii)    The cephalothorax bears simple eyes and six pairs of appendages (one pair of chelicerae, one pair of pedipalpi and four pairs of legs). 
(iii)    Antennae are absent. 
(iv)    Respiratory organs are book lungs or tracheae or both. 
(v)    Excretion takes place by Malpighian tubules or coxal glands or both. 
(vi)    Development is generally direct. 
Examples: Tick, Mite, Aranaeus (garden spider), Palamneus (Scorpion), Buthus (Scorpion). 
(vii)    Spiders spin the web by means of proteinaceous secretions of abdominal glands known as "spinnerets". 

• There are two more classes in the phylum Arthropoda. These classes are Onychophora and Merostomata. Peripatus is an important example of the class onychophora which has characters of phylum Annelida and Arthropoda.
• Hence it is called "connecting link". Umulus, (the king crab or Horseshoe crab) is an example of class Merostomata which respires with book gills.
• The king crab is called "living fossil". A living fossil is a living animal of ancient origin with many primitive characters.

Concept Builder

1.    Peripatus is considered as connecting link between annelida and arthropoda as it has unjointed legs and breathes by trachea. 

2.    Larvae of different Arthropods 
        (a) Bombyx(Silkworm) Caterpillar/Silkworm. 
        (b) Beetles, honey bee Grub 
        (c) Musca (Housefly) Maggot 
        (d) Culex, Anopheles Wriggler 
        (e) Pennaeus(marine prawn) Mysis, nauplius, protozoea 
        (f) Cancer(Crab) Megalopa, metanauplius, zoea 

3.    Terga are dorsal plates whereas sterna are ventral plates of exoskeleton. 

4.    Arthrodial membranes join the different sclerites. 

5.    Halters are drumstick shaped, second pair of reduced wings of housefly and mosquito which helps in balancing. 

6.    Eggs of Culex are cigar shaped; they are laid vertically on the surface of water in clusters; airfloats are absent; whereas in Anopheles eggs are boat shaped, laid singly and horizontally; they have airfloat. 

7.    Larva of Culex is bottom feeder whereas larva of Anopheles is surface feeder. 

8.    Adult of Culex lies parallel to the surface and both ends of body deflexed whereas in Anopheles, the body is inclined at an angle of 45° to the surface. So, they can be distinguished with the help of sitting posture. 

9.    In Spider, Spinnerets are present anterior to the terminal anus. They produce silken thread. 

10.    Von Frisch described the process of communication of food source in honey bees. 

11.    Insecticides sprayed over the mosquitoes desensitize their nervous system as well as chemoreceptors and mechanoreceptors ofthe antennae. 

12.    Johnston's organs are present on antennae of mosquitoes. 

13.    The life cycle is generally accompanied with metamorphosis. Larvae and adults may show different feeding habits and occupy different habitats. 
14.    Insects like bees, wasps, beetles, moths and butterflies are good pollinators for important crops. 
15.    Ants, termites and locusts are eaten by Chinese and Indians; Eggs of aquatic bugs are used as food by Mexicans; Ox-warbles are eaten by Red Indians. 
16.    Dragon flies feed on the larvae of mosquitoes 
17.    Glow worm (Firefly) shows bioluminescence. 

18.    Adult Culex and Anopheles can be distinguished with the help of Sitting posture.

19.   The insects may be divided into five groups on the basis of their mode of development.

1.    Ametabola insects – metamorphosis absent. The young ones resemble adult.

ex. Lepisma (Silver fish)

2.    Paurometabola insects – gradual metamorphosis

• ex. Cockroach, Grasshopper, Locusts etc.

3.    Hemimetabola insects – incomplete metamorphosis. Habitat of young ones is different from adults.

ex. Dragon flies (Naiads aquatic but adults aerial)

4.    Holometabola insects – complete metamorphosis

• ex. House flies, Butterflies, Mosquitoes etc

5.   Hypermetabola insects – various forms of larvae

            ex. Blister beetle.