EMBRYOLOGY OF COCCIDS 111 



the possibility of the function of the organism as the 'Keimbahm 

 determinant,' as Mecznikow surmised. The fact that these 

 organisms are present throughout the life history of scale insects 

 is in favor of the view just mentioned. 



However, I cannot think that the presence of these symbiotic 

 organisms is altogether without significance. The species known 

 to harbor these symbiotic organisms, so far as present researches 

 go, belong without exception to the suborder Homoptera, which 

 are characterized by a long sucking proboscis, very dehcate 

 membranous wings, and thin dorsal body wall. Coccids may 

 remain more or less stationary, feeding in the same place, for a 

 long period of time. It is clear, therefore that such Homoptera 

 as the scale insects and the aphids are apt to be exposed to changes 

 in the external conditions because they cannot change places as 

 easily as other insects. 



It may be said that the part most susceptible to the environ- 

 mental change is the sex cells. As a rule, the sex cells of the 

 Homoptera, unlike those of the Coleoptera, in which the hard 

 elytra cover the dorsal surface of the body, lie beneath a thin, 

 almost unprotected dorsal body wall. Unless some special 

 means be provided for their protection, the sex cells would be in 

 danger of injury. In free moving Homoptera, such as Jassids, 

 Membracids, Flugorids, Cercopids, the means of avoiding injury 

 to the germ cells are found in quick movements by which these 

 insects are able to take refuge by either dropping to a more 

 favorable place or running behind the trunk or leaves of the 

 host plant. In the Coccids, Aphids, and Psyllids this change of 

 place cannot be effectively made. The presence of a mass of 

 highly refractory organisms that always surrounds the germ cells 

 in these three otherwise helpless insects strongly suggests a sig- 

 nificance — the protection of germ cells against injury and sudden 

 change in environmental conditions, such as rain, snow, extreme 

 heat, cold, etc. 



JOURNAL OF MORPHOI.i 



