Insects are probably the most specialized animals in existence today. As a group they have persisted since the early Palaeozoic Era while other species flourished and passed into extinction, largely because of their ready ability to adapt body structure to conform with changing conditions in the world about them. At present some 300,000 species of insects are known to science; no two exactly alike in either form or habits. In the gradual evolution of such a multitude from a single stock Nature has developed some curious and interesting types. Although most insects are fashioned on about the same pattern of internal body structure, they vary tremendously, as everybody knows, in external form or shape of the outside covering. Now an insect, as compared with man, is built wrong-side-out; that is, with the skeleton on the outside and the muscles, blood vessels, and nerves concealed safely within. The fact that the hard part, or exo-skeleton, is in contact with the outside environment has led to constant changes in this structure so that the insect may better carry on its life activities. The kind of ground over which it must travel, the type of food that it captures, the place in which it builds its nest; all determine what particular kind of body skeleton will better the insect's chance for success in its struggle for existence.
To illustrate just a few of these adaptations we turn to some familiar insects and examine the legs, those most useful of appendages combining the functions of arms, hands, and feet. (see below)
1 - The Biting Louse (Order Mallophaga) that infests mammals has a sharp claw on the tip of each leg so that it may better cling to the hair of its host.
2 - The House Fly (Order Diptera) has a leg typical of many insects; clawed at the end and bearing several soft, cushioned pads that enable it to stick to smooth surfaces.
3 - The Water Strider (Order Hemiptera) has taken to the water. It skates about on the surface film on lightweight limbs that at like snowshoes to prevent the owner from breaking through.
4 - An insect that strikes terror to the hearts of small water organisms is the Predaceous Diving Beetle (Family Dytiscidae). It has developed a heavy fringe in hairs on its feet that serve as paddles to drive it through the water.
5 - The Engraver Beetle (Family Scolytidae) must force its body through the bark of trees so wears a pair of spurs. Jagged projections on the tibia engage the sides of the tunnel.
6 - The Mole Cricket (Order Orthoptera) pushes aside the dirt with massive fore-legs covered with teeth like a steam-shovel. It excavates in search of vegetable food.
7 - The Honey Bee (Order Hymenoptera) is one of the most specialized of all insects. On the hind-leg there are three interesting structures; the pollen basket fringed with hairs, the heavy wax pincers, and the pollen comb at the tip.
8 - The Grasshopper (Order Orthoptera) depends on powerful muscles in his jumping legs to carry him out of danger. The fore-legs are small and used for transfering food.
9 - The fore-legs of the Praying Mantis (Order Orthoptera) has developed into a vicious sword with which unsuspecting insects are cut down from ambush. The Mantis is not averse to using members of his own family if pinched for food.
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