Question

What could be the basis of classification in Arthropoda?

Answer 1

Hint: The basis of classification of arthropods were found to be habitat, body appendages, organs needed for respiration, the basis through which the body division occurs, and lastly the mode of excretion. The excretory organs found among the organisms of this phylum include – malpighian tubule, green glands, coxal glands, etc. The respiratory organs present are – gills, book gills, book lungs, trachea, etc.

Complete answer:
Arthropoda is the largest phylum of the animal kingdom. It is derived from the words Arthos meaning joint and podos meaning legs. Hence they can be termed as animals having bilateral symmetry and have external segmentation with a hard chitinous exoskeleton and jointed legs or appendages. Arthropods are classified on the above basis into $5$ different classes :
1. Crustacea – The meaning of crusta is the shell. Organisms of this class have an outer shell or shell-like coating. They are mainly aquatic habitats, some can be terrestrial while some may be parasitic. The cephalothorax is seen in this class and respiration is done by the means of gills or body surface. Excretory organs are modified. Examples: crab, prawn, etc.
2. Myriapoda _ Myrios means ten thousand and podos stands for the foot. So the organisms of this class have ten thousand feet. Their habitat is mostly terrestrial. The body contains numerous segments. Excretion is done by malpighian tubules and respiration through the trachea. Example: millipede, centipede, etc.
3. Insecta – mostly terrestrial in habitat. Body divided into $3$ the distinct region – head, thorax (bearing pair of legs and a pair of wings), and abdomen. Respiration is mainly done by the trachea and excretion is done by malpighian tubules. Example: butterfly, cockroach, mosquito, etc.
4. Arachnida - They are also mostly terrestrial, rarely aquatic. The body is divided into cephalothorax (contains $4$ pairs of legs) and abdomen. Respiration is done by the trachea, book lungs or gills, and excretion by malpighian tubules or coxal gland, or both. Example: spider, scorpion, etc.
5. Onychophora: They are terrestrial. The body is segmented. Respiration is done by the trachea and excretion through nephridia. Examples: Peripatus

Note:
Cephalothorax is a structure formed due to the fusion of the head and thorax. Many arthropods are used by humans as a source of food (crab, prawn). Honey bees produce honey and silkworms produce silk. The insects also bring about pollination. But various arthropods are harmful to crops and also some arthropods (mosquito, housefly) act as a vector or carrier of various diseases.