132 COMPARATIVE ANATOMY. 



present in sufficient or insufficient quantities, as in Apis 

 (bee). 



In the Aphides (plant-lice) an example of alternate gene- 

 ration is exhibited. The egg, which is laid by the parent in 

 the fall, develops the following spring, at which time it un- 

 dergoes segmentation, the resultant larva resembling the 

 parent in being active and provided with limbs. It markedly 

 differs, however, from apparently similar forms of other in- 

 sects in its power of giving birth, through a phase of the 

 process of gemmation, to a progeny like itself. Ova-like 

 buds are formed in the neighborhood of the ovaries, which 

 subsequently assume the outline of the parent insect. Such 

 individuals eight in number are free, active, and each in 

 its turn brings forth eight others like itself. This process 

 is successively repeated within each active gemma as long as 

 the surrounding conditions, which are generally maintained 

 to the eighth or ninth generation, are favorable. In the last 

 brood, brought forth toward the close of the summer, the 

 sexes are for the first time distinct, and the males provided 

 with wings. From each of these forms ova are produced 

 which, after lying inactive during the winter, undergo de- 

 velopment in the following spring as before. It has been 

 estimated that in five generations of offspring a single Aphis 

 may be the progenitor of 5,904,1)00,000 descendants. 



In Cecidomyia (Hessian-fly) a genus of dipterous insects 

 the mature female deposits ova, each having a smooth shell 

 and furnished with a micropyle, beneath the bark of dead 

 ash-trees. They here soon develop into larvae, having no 

 peculiar external characters, but possessing in the posterior 

 portion of their visceral cavity (in immediate relation to, and 

 perhaps having their origin within a fatty tissue), two ovary- 

 like bodies, one on either side. Within these are developed 

 spherical masses which, from their resemblance to eggs, are 

 known as pseudova. Each pseudovum evolves another larval 

 form, so that in time there exists within the tissues of the 

 primary larva as many secondary larvse as there were origin- 

 ally pseudova developed. The parent larva, after changing 

 its exterior into a pupa-like case, dies. The brood remaining 



