Introduction to Arthropods:
The Phylum Arthropoda is largest group of animal Kingdom exceeding in number of species of all other phyla combined. It has over 700,000 species. Animals belonging to the Phylum happen all over the globe at height of over 6,500 meters on mountains to depths of over 6,000 meters in the sea. Various species are adapted for life in air, on land, in soil and in fresh or salt waters. Few are parasites on plants and on or in bodies of other animals. Few are outgoing (live in groups) and different types of colonial insects developed social organizations with division of members in different castes. Several members are of extremely great economics significance, like large shrimps, crabs, and lobsters that are eaten by men.
Features of Arthropoda:
i) Distinctive characteristics of Phylum Arthropoda
ii) The body is generally segmented and bilaterally symmetrical.
iii) Segments are fused to create head, thorax and abdomen.
iv) Most of segments bear the pair of jointed appendages each.
v) Hardened, non-living exoskeleton having chitin in secreted by epidermis; it is periodically moulted as long as animal continues to grow in size.
vi) There is generally highly evolved muscular system, with many separate muscles, made up of finely striated and rapidly contracting fibers.
vii) Digestive tract is complete; mouth parts are given with lateral jaws, mouth parts adapted for chewing or for piercing and sucking liquid food.
viii) Cilia and flagella are not present.
ix) Circulatory system is of open kind; heart is dorsal, distributing blood by artery to organs and tissues, from where it returns through body spaces (haemocoels) to heart.
x) Nervous system is of annelidan kind, comprising of paired dorsal ganglia over mouth, the ring or collar round gullet and ventral chain of ganglia.
Categorization of Arthropods:
Phylum arthropods are separated into classes, which are given below:
i) Crustacean like Prawns, Crabs,Lobsters etc.
ii) Insecta like cockroach, grasshopper, housefly etc.
iii) Arachnida e.g. Spiders, Scorpions, mites, ticks etc.
iv) Myriapoda e.g. Centipedes, Millipede
v) External features of Insects -Cockroach
Almost 70% of all known species of animals are insects. Though they are mostly land animals, they are extensive and adapted to all kinds of environment. They are also only invertebrates which can fly. Many insects feed on plant materials, whereas some feed on animal tissues and wastes. Insects comprise beetles, ants, aphids and grasshoppers. The insect include the well-defined head, thorax and abdomen. The insect performs gaseous exchange via network of open air tubes or tracheae inside body. Such tubes contain openings known as spiracles to exterior.
Habitat and Habit of Cockroach:
It is plentiful in restaurants, storehouses, bakeries, kitchens, boilers and other places where there is warmness, humidity and organic food. Cockroaches dwell in cracks and crevices in walls. Cockroaches are nocturnal insects. In day time they stay inactive in hiding places. But they turn into active during night, run here and there looking for food. They are omnivorous and dispose of vegetable or animal substance. They may eat even non-living materials like paper, leather, cloth etc. Besides, cockroaches are cursorial i.e. they are fast runners and hardly ever fly.
External characteristics of Arachnid-Spider:
The Arachnid has body which is divided into 2 parts, a cephalothorax and abdomen. Cephalothorax has simple eyes, 2 pairs of appendages for feeding and 4 pairs of walking legs. The arachnid generally breathes by tracheae or book-lungs. Spider is the most familiar arachnid glands to spin webs for trapping its prey. 1 pair of its feeding appendages has fangs joined to poison gland. Another arachnid, scorpion, has sharp poison sting on last abdominal segment.
Adaptive characteristics of Arthropods:
i) Arthropods are one of the most successful animals. Success of arthropods is due to some number of reasons. There reasons:
ii) Have exoskeleton that protects them from predators, and in terrestrial species, prevents water loss from body surface. Jointed limbs, , complex movements, capable of swift;
iii) Specialization of segments and appendages for particular tasks like sensory (antenna or feelers) and feeding (proboscis, mandibles, etc.)
iv) Specialized respiratory system for various habitats, like aquatic ones; and well-evolved sensory organs, e.g. eyes (simple and compound) and antennae.
v) Arthropods shed the exoskeletons at intervals to permit their bodies to increase in size. This is called as moulting or ecdysis.
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