HOMOPTERA CICADIDAE 



569 



of the earth, but we have one species, a great rarity, in the ex- 

 treme south of England ; altogether there are about 800 species 

 known. These Insects are seen above ground — so far as the 

 life-histories are at present known — onlj in the perfect condi- 

 tion, the creatures in their earlier stages being subterranean and 

 living on roots. As soon as the individual comes out of the 

 ground it splits open the nymph-skin, and the perfect Cicada 

 emerges. One species — the North American Cicada septendecim 

 —is a most notorious Insect owing to its life-cycle of seventeen 



C 



Fig. 280. — Cicada septendecim. North America. (After Riley.) A, Larva ; B, 

 nymph ; C, nymph skin after emergence of the imago, D ; E, section of twig witli 

 series of eggs ; F, two eggs magnified. 



years. It is considered that the individual, after nearly seven- 

 teen years of underground existence, comes to the surface and 

 lives for a brief period the life of a noisy Insect. This is the 

 only Insect at present known having so considerable a longevity. 

 This fact, and several other peculiarities, have attracted much 

 attention, so that there is an extensive literature connected with 

 the seventeen-year Cicada. It has a wide distribution over the 

 United States, but does not confine its appearance to every 

 seventeenth year, being found somewhere or other — frequently 

 in numerous localities — almost every year. The evidence as to 

 its periodicity has been obtained by taking the locality and other 

 points into consideration as w^ell as the year of appearance. 



