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EVOLUTION OF ANIMALS 



Part V 



for food and space and changes in their microclimates. In general, they do not 

 make long flights. The forays of migratory grasshoppers are exceptions; so 

 are the seasonal migrations of butterflies (Fig. 30.6). 



Insects are carried long distances by air currents. Newly hatched gypsy- 

 moth larvae are buoyed up in the air by small air pockets on the hairs with 



Fig. 30.4. Diagram representing the 

 relative abundance of insects (Hexapoda) 

 and other animals. (Courtesy, Frost: En- 

 tomology. New York, McGraw-Hill Book 

 Co., 1942.) 



which their bodies are covered. They have been captured 300 feet or more 

 up in the air and on strong winds they may travel many miles a day. Insects 

 travel far and wide on human beings and their vehicles — by water, by land, 

 and by air. 



Molting and Metamorphosis. Molting. The young insect grows larger but 

 its cuticle does not. Relief comes to it only with a new and larger cuticle and 

 escape from the old one, that is, by molting. 



As before stated, the integument of insects consists of epidermal cefls and 

 the cuticle that they secrete. The cuticle includes two regions of different 

 chemical content; the outer cuticle, mainly cuticulin, fats and waxes, is re- 

 sistant to injury and has an outermost waxy layer; the inner cuticle is com- 

 posed chiefly of chitin. 



There are several steps in the preparation for molting. (1) The epidermal 

 cells secrete a new outer cuticle which then lies between them and the old 

 cuticle. (2) Specialized epidermal cells secrete molting fluid which passes out- 

 ward through ducts in the new outer cuticle and spreads over its surface. In 

 doing so it separates the new and the old outer cuticles. (3) Molting fluid 

 gradually digests the old inner cuticle. (4) In the meantime, the epidermal 

 cells are forming a new inner cuticle. The molting fluid does not digest this. 



The digested substance of the old inner cuticle is absorbed back into the 

 body. This has been shown by the absorption through the body wall of dye 

 injected between the old and new cuticles. At this time the new cuticle is 

 permeable to water. An insect sheds its old cuticle soon after the new one is 

 completed. Some insects do this too quickly to be clearly observed; others 



