The sexes are separate. The testes and ovaries produce sperms, and eggs respectively.


Life history of insects is characterized by metamorphosis (Meta = change + Morphe = form). This is an abrupt change of form or structure during the life cycle. There are three morphologically distinct stages in the life cycle; the egg “finally” develops into larva which is converted into motionless pupa that finally develops into an adult. It some primitive insects the metamorphosis is incomplete. The larva resembles adult and is called nymph or instars’. It lives in the same habitat as adult.


Phylum Arthropoda is a large group consisting of great variety among them. Some of its important classes are as follows.

  1. Class Crustacea:

These arthropods are aquatic and have gills for respiration. On the dorsal side of the cephalothorax the exoskeleton is in the form of carapace. In the exoskeleton deposition of salts in addition to chitin makes it more firms. The appendages are modified for capturing food, walking, swimming, respiration and reproduction. Coelom is reduced and is in the form of hemocoel. Head has two pairs of antennal appendages, one pair of mandibles (jaws) and two pairs of maxillae. Sexes are mostly separate e.g. Daphnia, Cyclops, Crabs, lobsters, prawn, wood louse etc. (Fig. 10.8)


Fig. 10.8 Example of class crustacean

2.   Class Insects:

This is the largest group not only of arthropoda of all the animal kingdom and has great variety. Insects are found everywhere, many show social behavior. The body in insects has three distinct regions head, thorax and abdomen. There is a pair of antennae and compound eyes on the head. The head is usually vertical to the body and jaws are ventrally placed. The thorax has three segments in which are present three pairs of jointed legs and in many one or two pairs of wings. Abdomen has varying number of segments. Brain is formed of fused ganglia and double nerve cord is ventral. Sexes are separate and animals are oviparous. Metamorphosis takes place during development e.g. dragonfly, mosquito, butterflies, moths, wasps, and beetles etc. (Fig. 10.9).


Fig. 10.9 Insects

Most spiders have eight eyes placed in such a way as to give them panoramic view of the predators and prey.

3.   Class Arachnida:

Body has the anterior segments that are fused to form a combined cephalothorax, with a pair of appendages called chelicerae with claws, two pairs as pedipalps and four pairs of legs. There are no antennae and no true jaws. Abdomen may be segmented or un-segmented with or without appendages. Respiration is by gills lungs. Or special structures called book lungs; excretion is by the malpighian tubules. Eyes simple, sexes are separate. They are oviparous (lay eggs). No true metamorphosis e.g. scorpions spiders, mites and ticks.


Fig. 10.10 Arachnids

4.    Class myriapoda:

The body is divided into large number of segments each having a pair of legs. A pair of antennae and a pair of eyes are present on head e.g. centipedes and millipedes