THE STRANGE CYCLE OF THE CICADA 



is spent underneath the ground, in an undeveloped con- 

 dition known as the pupa state. The "shells" are the 

 cast-off pupa -cases, or final "moults," of the insects 

 when they come up after 

 their long sojourn within the 

 earth. 



At several points in the 

 United States seventeen-year 

 cicadas appeared in the 

 spring of 1902, while in other 

 parts there were none. This 

 uncovers one of the curious 

 facts in the insect's nat- 

 ural history. Somewhere 

 throughout the continent 

 there appears, almost every 

 year, a brood wliich is limit- 

 ed to a certain belt of coun- 

 try of greater or less extent. 

 Entomologists, by keeping 

 the track of these broods, 

 have been able to predict 

 their appearance within cer- 

 tain zones. For example, in 

 the western suburbs of Phil- 

 adelphia immense numbers 

 of cicadas appeared in the 

 summer of 1885. Their visit 



was predicted and announced by the author several 

 months before it occurred. The only knowledge need- 

 ed for this was that a brood had appeared in 1868; and 

 the only ability, that of adding seventeen to these 

 figures. In like manner, by adding seventeen again, a 



229 



THE SEVENTEEN - YEAR CICADA 

 AND ITS PUPA-CASE 



