i 2 The Structure and Special Physiology of Insects 



knobs and horns. The rhinoceros-beetle (Dynastes) (Fig. 19) and the sacred 

 scarabeus are familiar examples of insects with such prominent processes. 



The insect body, as a whole, appears in great variety of form and range 

 of size, as our knowledge of the variety of habit and habitat of insects would 

 lead us to expect. In size they vary from the tiny four-winged chalcids 

 which emerge, after their parasitic immature life, from the eggs of other 

 insects, and measure less than a millimeter in length, to the giant Phasmids 



FIG. 19. Rhinoceros-beetle, Dynastes tityrus, showing chitinous horns. 



(walking-sticks) of the tropics, with their ten or twelve inches of body length, 

 and the great Formosan dragon-flies with an expanse of wing of ten 

 inches. A Carboniferous insect like a dragon-fly, known from fossils found 

 at Commentry, France, had a wing expanse of more than two feet. 

 Insects show a plasticity as to general body shape and appearance that results 

 in extreme modifications corresponding with the extremely various habits 

 of life that obtain in the class. Compare the delicate fragility of the gauzy- 

 winged May-fly with the rigid exoskeleton and horny wings of the water- 

 beetle; the long- winged, slender-bodied flying-machine we call a dragon- 

 fly with the shovel-footed, half-blind, burrowing mole-cricket; the plump, 

 toothsome white ant that defends itself by simple prolificness with the spare, 

 angular, twig-like body of the walking-stick with its effective protective 

 resemblance to the dry branches among which it lives. Compare the leg- 

 less, eyeless, antennaless, wingless, sac-like degraded body of the orange- 

 scale with the marvelous specialization of structure of that compact expo- 

 nent of the strenuous insect life, the honey-bee; contrast the dull colors of the 

 lowly tumble-bug with the flashing radiance of the painted lady-butterfly. 

 But through all this variety of shape and pattern, complexity and degenera- 

 tion, one can see the simple fundamental insect body-plan; the successive 

 segments, their grouping into three body-regions, the presence of segmented 

 appendages on head and thorax and their absence on abdomen (except 

 perhaps in the terminal segments), and the modification of these append- 

 ages into antennae and mouth-parts on the head, legs on the thorax, and 

 ovipositor, sting, or claspers in the abdomen. 



In the character of the structure and functions of the internal organs 



