Phylum Arthropoda

Phylum Arthopoda: This is the largest phylum in the animal kingdom. In other words, there are more animals in this phylum than in any other. In fact, arthropods comprise 75% of the animal types identified by zoologists so far.

Prawns (crustaceans), spiders, scorpions and insects of all kinds comprise this phylum of creatures with jointed (arthros means jointed) legs. Their body is segmented and is covered by an exoskeleton of protein and chitin. This exoskeleton is cast off (moulting) periodically to allow the body to grow. The segments of the body are grouped either into two regions (cephalothorax and abdomen) as in spiders or into three regions (head, thorax and abdomen) as in insects.

Arthropods have an open circulatory system. The heart has pores. It pumps blood into a cavity called haemocoel that surrounds the body organs. The body organs are bathed in this blood called haemolymph. Respiration takes place through different organs in different arthropods. Crustaceans (lobsters, prawns, shrimps, barnacles, etc.) have gills and most insects have a system of air tubes called the tracheal system. The openings of these air tubes are called spiracles.

Spiders and scorpions have fine leaf like structures called book lungs in the abdomen. The openings of book lungs can be closed to prevent water loss.

Excretion occurs through green gland in crustacea and through Malpighian tubules in other arthropods. Many arthropods, like flies, lobsters and crabs, have compound eyes.

Spiders have special organs in the abdomen called spinnerets that help them spin webs. They also have poison glands like, scorpions.

Centipedes have simple eyes, many legs (one pair per segment) and a pair of poison claws.

Millipedes too have simple eyes and many legs (two pairs per segment) but they do not have poison claws.

Some arthropods like ticks and mites are parasitic, and carry diseases. King crabs and sea spiders are marine. Crustaceans have a hard covering called carapace. They are aquatic.

Most insects, on the other hand, are terrestrial. Some like silver fish are wingless, while many have wings. Many insects (mosquitoes, flies, butterflies, etc.) change form during the course of growth. The eggs hatch into larvae, the larva passes through a pupal stage and then comes the winged adult.

This process is called "metamorphosis. Louse is an ectoparasite. Beetles are among the most abundant of insects. Many insects like bees, ants and termites are social—they live in large colonies. Reproduction among arthropods is sexual and the sexes are separate.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

The organs which spiders use for spinning webs are called _______________________
A. Spiracles	B. Spicules	C. Spinneretes	D. Carapaces
Right Option : C
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Question : 2

Mouth parts of cockroach are of type _______________-
A. Biting and chewing		B. Piercing and sucking
C. Siphoning		D. Spoging
Right Option : A
View Explanation