The Grasshopper 347 



POLYEMBRYONY 



In 1904 P. Marchal described an interesting observation. He found 

 that in two small parasitic Hymenoptera ( ), a 



Chalcid ( ) — Encyrtus ( ) — 



which lay eggs in the developing eggs of the small moth Hyponomeuta 

 ( ) and a Proctotrypid ( ) — 



Polygnotus — which infests a gall-midge — Cecidomyid ( ) 



— larva, the nucleus of the egg of the insects divided, and each such 

 particle of nucleus became a complete new embryo. A mass, or chain, 

 of embryos is thus produced which lie in a common cyst and develop 

 as their larval host develops. In this way over a hundred embryos may 

 result from a single egg. Marchal pointed out the analogy of this phe- 

 nomenon to the artificial polyembryony that has been induced in 

 Echinoderms ( ) and other eggs by separating the 



blastomeres, and suggested that the abundant food-supply afforded by 

 the host-larva may be favorable for this multiplication of embryos, which 

 may be, in the first instance, incited by the abnormal osmotic pressure 

 on the egg. 



When many embryos develop from a single egg in the way just 

 described, it is called polyembryony. 



H. H. Newman has shown that in the ant-eater, armadillo, in which 

 three to nine embryos commonly form in different species, all develop 

 from a single egg. The fertilized egg does not split into separate parts 

 but evaginates in different portions to form separate embryos. 



ALTERNATION OF GENERATIONS 



A true alternation of generations has been found in Hymenopterous 

 gall flies (Fig. 226), in which a complete asexual generation (complete 

 from egg to adult) succeeds a complete sexual generation (egg to adult), 

 each generation being parasitic on a different host plant. The adults 

 in each case bear no resemblance to each other ; in fact, they have not 

 only been described as different species, but actually as different genera. 



EMBRYOLOGY 



The flapping of wings or the "singing" of the male grasshopper 

 attracts the "unfertilized" females. The sperm are then injected into 

 the female receptacle, from whence they work their way into the various 

 eggs. 



The zygote thus formed, begins to segment mitotically, forming the 

 embryo on top of the yolk close to the egg-shell. There are two 

 protective membranes, the innermost being known as an amnion 

 ( ) or chorion ( ), and the 



outer as the serosa ( ). As soon as the embryo has 



used up the yolk as food, it is ready to hatch. 



