DEVELOPMENT OF COPIDOSOMA GELECHLE. 355 



This leads to a discussion of mixed broods, and especially to a 

 consideration of the question as to how such broods have come 

 into existence. The obvious explanation of their origin is the 

 one given above, viz., that they arise from two eggs. Marchal 

 and Silvestri, who have studied the development of polyembry- 

 onic insects, both offer this explanation. They support the con- 

 clusion by citing the fact that two (or more) parasitic eggs are 

 sometimes laid in the egg of the host. According to Marchal, 

 such eggs develop independently, each producing a distinct 

 polygerm and consequently a distinct brood. If the two eggs 

 are of the same sex potentiality, the individuals developing from 

 them will be either all females or all males, according to whether 

 or not the eggs are fertilized or unfertilized. The dual origin of 

 these double broods naturally elude detection in lots that have 

 emerged. But if one of the two eggs is unfertilized and the other 

 fertilized, the result will be a mixed brood, consisting of males and 

 females. This conclusion of Marchal and Silvestri is strongly 

 supported by the facts of polyembryonic development in the 

 armadillos, in which it has been conclusively demonstrated 

 (Fernandez, '09, Patterson, '13) that all of the embryos of a given 

 pregnancy are the product of a single egg. As a result, mixed 

 litters are never found in these mammals. 



That mixed broods may arise from two eggs in Copidosoma 

 is supported by the fact that two polygerms are sometimes 

 found in a single Gnorimoschema larva. However, certain facts 

 concerning the condition of mixed broods in this species, make 

 it doubtful whether the origin of all such broods can be explained 

 in this obvious way. Careful dissections of something over a 

 hundred parasitized Gnorimoschema larvae have revealed only 

 two cases in which a single larva contained more than one 

 polygerm. Since 6.17 per cent, of all broods are mixed, and 

 since a similar number of unmixed broods would have a dual 

 origin, we should expect to find over 12 per cent, of all parasitized 

 larvae containing two polygerms, but instead, less than 2 per cent, 

 are found. 



Another line of evidence which is not in harmony with the 

 view that mixed broods are always the product of two or more 

 eggs, is the great preponderance of females in certain lots. Of 



