Chap. 30 



ARTHROPODS INSECTS, SPIDERS, AND ALLIES 



595 







Fig. 30.5. A dragonfly is ancient history on wings. The form of this one was 

 preserved in stone over 250 million years ago, long before there were birds to fly. 

 Of three great steps in the evolution of insects, wings held straight out from the 

 body came first; wings folded to the body when at rest was second; complete change 

 of form in a single lifetime came third. (Courtesy, Frank M. Carpenter, Harvard 

 University.) 



take several minutes. The insect contracts the muscles of its legs and abdomen 

 forcing blood into the thorax which swells accordingly. Young mayflies swal- 

 low air. The old cuticle cracks along the line on the head and thorax where 

 the inner cuticle has never formed and the other one is weak. A molting insect 

 bucks its thorax upward, wriggles its body free of the old cuticle, and contracts 

 itftpasmodically. This drives blood into the wings and legs which stiffen out 

 as molting is completed. Forcing the blood here and there during molting 

 stretches the cuticle to its utmost, leaving the softer parts in folds that are 

 smoothed out only after further growth. The new cuticle hardens and darkens 

 in a short time, but this is not due simply to exposure to air. If a part of the 

 new cuticle is exposed by the removal of a piece of the old one 24 hours 

 before molting, the new cuticle will neither harden nor darken. 



Metamorphosis. The young insect that crawls out of the eggshell is usually 

 quite unlike the adult it will become. Between hatching and maturity insects 

 increase in size mainly by steps at molting time. Most of them undergo a 

 metamorphosis or change of form. The less the young and adult resemble one 

 another, the greater are the structural changes inside and outside of the body. 



There are three main types of metamorphosis (Fig. 30.7). (1) With slight 

 change of form and no wings ever developed, e.g., the household silverfish 



