194 



SYSTEMATIC ZOOLOGY 



The insects as a group are much more highly differentiated 

 than any animals we have considered up to this point ; this is 

 manifest, not only in their structure, but in their modes of life, 

 particularly the care of their young and the living together in 

 colonies with a marked division of labor between different kinds 

 of individuals. They are provided with organs of touch, taste, 

 smell, and hearing, and many have the power of producing 

 sounds ; this is effected in different ways in different insects : 

 in some cases it is the rapid movement of the wings as thev fly, 



which produces a 

 humming sound; in 

 some species there 

 are little leaflike 

 structures near the 

 openings of the 

 trachea?, which by 

 their vibration 

 cause a buzzing as 

 the air is drawn 

 in or forced out ; 

 some scrape their 

 legs against their 

 roughened wings ; 

 others rub the wings 

 together ; and in 

 some the mouth is 

 used to produce 

 sound. Some in- 

 sects are phospho- 

 rescent, emitting a bright light at night ; this light is probably 

 due to chemical changes which take place in the fatty substance 

 within the body. As every observer of insects knows, they vary 

 greatly in their food : some derive their nutriment from decaying 

 vegetable or animal matter; some feed upon living plants, others 

 on living animals ; and some extract the nectar from flowers, 

 but often give as much as thev take by carrying the pollen on 

 their bodies from flower to flower, and thus assist in accom- 

 plishing cross fertilization. 



Insects vary greatly in the length of their lives. The sexually 



FIG. 191. Various forms of insect legs. <j, Mantis, with pred- 

 atory leg; b, 1 . 1 mining leg; .. Acridium, springing 

 leg; d, Gryllotalpa, digging leg; c, Dytiscus, swimming leg. 

 (From Claus and Sedgwick's Text-book.) 



