The class Insecta are small cold blooded animals with a head, thorax and a segmented abdomen. Adults have one pair of antennae on their heads, and six legs, and 0, 2 or 4 wings on their thorax. Immatures may have 0. 6 or more legs and are wingless. Caterpillars have 6 true legs and temporary larval legs that are called prolegs. Some fellow members of the phylum Arthropoda that are not insects are Arachnids, Crustaceans, millipedes [Diplopoda] and centipedes [Chilopoda].
Arachnids have the head and thorax combined into a cephalo [head] thorax[chest] and an abdomen, and they have 8 legs on their cephaolothorax. Spiders, daddy-long-legs, scorpions, mites and ticks are arachnids. Spiders which have smooth and unsegmented abdomens are distinguished from their close relatives the daddy-long-legs who have segmented abdomens.
Crustaceans also have a cephalothorax and abdomen and have 10 legs. Common aquatic crustaceans are lobsters, crabs and horseshoe crabs and crayfish while the sowbugs [pillbugs or roly-polys] live on land.
Millipedes have a head and a long round or squarish segmented abdomen and are land animals. They have two pairs of legs per segment and are sometimes called thousand-leggers. They are slow moving plant eaters and curl up into a spiral when disturbed.
Centipedes have a head and long segmented flattened abdomen and have 1 pair of legs per segment. They are carnivores and have fangs on their head. They run as they hunt for small animals and insects that they feed on.
Many people are very afraid of insects. Of the 1,000,000 described insect species only 2% are harmful to man and this includes agricultural, animal and plant pests. The only insects and arthropods that are out to get man are: mosquitoes, horse and black flies, and fleas and lice; and among the arachnids, ticks and chiggers [a type of mite] because the females need a blood meal to lay their eggs. Mosquitoes are infamous for their disease carrying around the world. Malaria has been one of the major diseases of man for centuries and is still a major problem around the world. The housefly, which can carry 30 million bacteria on its body, is also a major disease vector. It is linked to many gastrointestinal illnesses in man where there is poor sanitation.
Bees and silkworms are very economically important and beneficial to man. The Honey bee is the principal pollinator of our flowers, fruits, nuts and vegetables. Wild bees, some wasps, flies and beetles are also pollinators, but none have the specialized structures to comb pollen from their hairs and store pollen in sacs on their legs as do the honey and bumble bees. Moreover, honey bees produce honey which for centuries was our only natural sweetener.
The silk industry is totally dependent on the domesticated Bombyx mori moth and caterpillar. Silkworms produce yards of silk from their silk glands when they spin their silken cocoons. Workers unravel the silken strand from the cocoon and the strands are woven into silken fabric from ties and other articles of clothing are made.
This is a short introduction to insects and their kin. I hope that this introduction will stimulate you into exploring these fascinating animals too.