THE INSECT WORLD. 



141 



the surface of the ground, feeding- on the juices of roots, and 

 transforming in the spring of the seventeenth year into a winged 

 adult. No less than twenty-two broods have been tabulated in 

 the United States, chiefly through the efforts of the late Dr. C. 

 V. Riley, so that we know approximately how each is distributed, 

 and are able to foretell with certainty when the insects will appear 

 and about what territory they will cover. A further point of 



Fig. 103. 



The periodical cicada, C. septendecim.—a, pupa, ready to change ; b, pupa-skin from 

 which the adult c has emerged ; <f , eggs, taken from the egg-punctures d. 



interest is that in the more southern States the period of develop- 

 ment is somewhat shorter, thirteen years only being required to 

 bring the insect to maturity. Much confusion was caused in 

 arranging the broods until this fact was understood ; but experi- 

 ments have now been made in transferring eggs of the thirteen - 

 year variety to northern regions, and eggs of the seventeen-year 

 variety to southern regions ; the object being to ascertain whether 

 climate would affect the larval period in the first generation. 

 These experimerrts have not yet terminated. This cicada makes 

 up for the long intervals at which it occurs by the enormous 

 numbers in which it appears, and in a "locust" year its loud 



