590 EVOLUTION OF ANIMALS Part V 



nitrogenous polysaccharide (C:{i.H-,4N40oi )x that is insoluble in water, dilute 

 acids, and the digestive juices of many animals. Chitin is extremely resistant 

 to decay and has been analyzed from the remains of beetles that lived in the 

 Eocene Period of 25 million years ago. In addition to covering the body, the 

 cuticle lines the fore- and hindgut, the air-tubes and the ducts of surface 

 glands. 



An insect is an air-breathing arthropod with a distinct head, thorax, and 

 abdomen. The in-cut sharpness with which these parts are set off suggested 



Fig. 30.1. Insects live almost everywhere and in unimagined places and ways. 

 They represent perfection of adjustment and success. Silverfish, Thermobia 

 domestica, a wingless insect about half an inch long, a rapid runner and skillful 

 dodger. The various domestic species live in the warmest places in houses, eat glue, 

 starch and paper, and are pests in libraries. Out of doors other species frequent 

 moist fallen leaves. (Courtesy, Ross: Entomology. New York, John Wiley and 

 Sons, 1948.) 



the name insect. The head bears most of the sense organs, the thorax includes 

 those of locomotion, and the abdomen those of reproduction. All adult insects 

 have six legs, thus the name. Class Hexapoda. As adults, they usually have 

 either one or two pairs of wings. The primitive wingless ones are the Thysa- 

 nura, silverfish and firebrats and the Collembola, springtails (Fig. 30.1). 

 Male and female organs are in separate individuals and fertilization is internal. 



Abundance, Reproductive Capacity and Size. There are some six times as 

 many species of insects as of all other animals (Fig. 30.4; Table 30.1). At 

 least 685,900 have been described but there is no complete catalogue, and 

 the estimates shift with many additions and changes due to duplications. Many 

 new species are still being discovered, especially in the tropics. The number 

 in any one locality is relatively small, varying greatly with the climate. Only 

 15,449 are given in "A List of the Insects of New York (state)" published in 

 1928 (Cornell University). 



Individual insects are countless. The two or three hundred tent caterpillars 

 in one web swell to enormous numbers when they are compounded with those 

 in an unsprayed apple orchard. Mayflies emerge from the water by millions, 

 fly for a brief period, then fall to the ground and mounds of them, accumulat- 

 ing under the lamps in lakeside parks, are cleared away by shovelfuls. In 

 some pantries and kitchens, the supply of cockroaches is like a never-failing 



