428 SYNOPTIC COLLECTION. 



way to a third generation (fig. 8, pupa ; fig. 9, adult), 

 but these are winged agamic females. These "winged 

 migrants " fly from the plum tree to the hop plant. The 

 fourth generation or the first on the hop plant is repre- 

 sented in the act of crawling in fig. 10. Several genera- 

 tions may be produced parthenogenetically on the hop 

 vine, fig. 1 1 representing the normal parthenogenetic 

 female of the sixth generation. In the autumn, winged 

 females (fig. 12, pupa; fig. 13, adult) are again devel- 

 oped and these "return migrants" fly back to the plum. 

 They are agamic and soon produce young which, how- 

 ever, develop into the sexual wingless females already 

 described. Later the winged males appear and sexual 

 reproduction follows. 



This remarkable specialization in development is cor- 

 related with specialized structures and with social habits. 

 The thoracic segments, which, generally speaking, are 

 distinct in the Hemiptera, are more or less fused together 

 in these forms ; especially is this the case with the 

 mesothorax and metathorax, the boundaries of which are 

 difficult to make out. 



The dorsal tubes on the abdomen of both the sexual 

 and the parthenogenetic forms which discharge the sweet 

 liquid, "honey dew," is a unique adaptation on the part 

 of the Aphides which brings them into harmonious rela- 

 tions with other insects that are remotely connected with 

 them genetically, such as ants. 



The grape aphis, Phylloxera vastatrix Planchon, has 

 carried specialization so far that there are several phases 

 or types of the species, and it is probable that in one 

 type, Gallaecola or the gall-inhabiting form, the male has 

 ceased to exist. 



According to Riley 1 the first galls that appear on the 

 grape leaves in the spring time are formed mainly by 

 young Phylloxera that hatch on the roots of the grape 



1 6.th Ann. Rep. Nox. and Benef. Insects Mo., 1874, p. 37. 



