32 



GENERAL ZOOLOGY 



creatures for their few weeks of adult life. In some cases, 

 when the nymphs reach the surface, they build peculiar 

 cones of clay (Fig. 15), several inches in height, over the 

 mouths of their burrows, entirely closing the top of the cone. 

 In the upper part of these they wait the period of their final 

 molt. The formation of these structures has been explained 



Fig. 16. Harvest fly. (Natural size) 

 A, adult emerging from nymphal skin ; B, the cast skin ; C, adult 1 



by some as due to the prevalence of long-continued wet 

 weather at the time of the emergence ; by others, as occa- 

 sioned by heating of the ground in certain localities by the 

 sun, thus bringing the nymphs to the surface before their 

 time. The cicadas have two pairs of transparent mem- 

 branous wings. When the wings are at rest they lie along 

 the sides and back of the body like a sloping roof. 



Aphids. Everyone who has tried to keep plants indoors 

 must have noticed the small, green, oval insects known as 



1 Reprinted by permission from Entomology, by J. W. Folsom, published by 

 P. Blakiston's Son & Co. 



