INSECTS 



In the United States there are numerous species of 

 "annual" cicadas, so called because they appear every 

 year, but their life histories are not actually known in 

 most cases. These species are called "locusts," "harvest 

 flies," and "dog-day cicadas" (Fig. 112). They are the 

 insects that sit in the trees during the latter half of sum- 

 mer and make those long shrill sounds that seem to be 

 the natural accompaniment of hot weather. Some give 

 a rising and falling inflection to their song, which re- 

 sembles zwing, zwing, zwing, zwing, (repeated in a long 

 series); others make a vibratory rattling sound; and still 

 others utter just a continuous whistling buzz. 



During the interval between the times of the appear- 

 ance of the adult cicadas, the insects live underground. 

 The periodical cicada comprises two races, one of which 

 lives in its subterranean abodes for most of seventeen 

 years, the other for most of thirteen years. Both races 

 inhabit the eastern part of the United States, but the 

 longer-lived race is northern, and the other southern, 

 though their territories overlap. Most of our familiar in- 

 sects complete their life cycle in a single year, and many 

 of them produce two or more generations every season. 

 For this reason we marvel at the long life of the periodical 

 cicada. Yet there are other common insects that normally 

 require two or three years to reach maturity, and certain 

 beetles have been known to live for twenty years or more 

 in an immature stage, though under conditions adverse 

 for transforming to the adult. 



Throughout the period of their underground life the 

 cicadas have a form quite different from that which they 

 take on when they leave the earth to spend a brief period 

 in the trees. The form of the young periodical cicada 

 at the time it is ready to emerge from the ground is shown 

 in Plate 5. It will be seen that it suggests one of those 

 familiar shells so often found clinging to the trunk of a 

 tree or the side of a post. These shells, in fact, are the 

 empty skins of young cicadas that have discarded their 



[184] 



