Phylum Arthropoda

Phylum Arthropoda (Greek arthron = joint, podos = leg) includes animals such as cockroach, butterfly, scorpion, millipede and prawn. It is the largest phylum in the animal kingdom, both in number of species and in individuals. The name refers to the phylum's defining feature: jointed appendages.

Arthropods are found in virtually every habitat and may be solitary or colonial; most are free-living, though barnacles are sedentary, and a few, such as the female mosquito and bed bug, are blood-feeding parasites (sanguivorous).

The body is bilaterally symmetrical, triploblastic, truly coelomate (eucoelomate), and metamerically segmented, following the tube within tube plan and reaching the organ-system level of organisation. It is covered by a tough, non-living, chitinous exoskeleton, which must be periodically shed and replaced as the animal grows — a process called moulting or ecdysis. The body is typically divided into head, thorax and abdomen.

The digestive system is complete. The circulatory system is open, with blood (haemolymph) flowing through body spaces called the haemocoel rather than through a closed network of vessels. Respiration is carried out by gills, tracheae, book lungs or book gills depending on the group. Excretion occurs through green glands, Malpighian tubules or coxal glands, depending on the group.

The nervous system consists of a nerve ring and a double, ventral, ganglionated nerve cord. Sense organs are well developed, including antennae and simple or compound eyes. Sexes are usually separate, often with sexual dimorphism, fertilisation is generally internal, and development may be direct or indirect through metamorphosis. Some arthropods, such as honey bees, reproduce by parthenogenesis (development of offspring from an unfertilised egg), and some, like honey bees, ants and termites, show polymorphism (different body forms within a species for different roles). …