1909] Observations on a Gall Aphid 89 



tative galls. Thus whenever the galls retained the normal posi- 

 tion of the leaves with the dorsal surface upwards, they were easily 

 filled with rain-water; in most cases, however, they were partly 

 or entirely reversed bringing the ventral surface upwards and 

 directing the long aperture of the gall downwards. Such galls 

 were effectual water-sheds and the inhabitants were always dry, 

 while those that were not reversed carried more or less water 

 mixed with the excretions and exuvia of the plant lice. This 

 liquid was evidently detrimental to the insects, many of which 

 were killed by a parasitic fungus. An ample supply of these 

 brownish dead aphids was sent to Professor Roland Thaxter of 

 Harvard University, who replied as follows : " * * * * the fungus 

 which has attacked them is Entomophthora aphidis, a universally 

 distributed form with which two others are often associated. In 

 your material, however, I see but the single species mentioned." 

 It is clear, then, that the reversal of the gall is a decided advan- 

 tage to the species. I was not able to determine how this reversal 

 was brought about. It was evidently one of the curious effects 

 on the leaf caused by the sucking of the insects, and may have 

 been developed by natural selection for their protection from 

 dampness and disease. The upturned whitish under surface of the 

 leaf-galls rendered the infested plants very conspicuous, and gave 

 them the deceptive appearance of being attacked by a downy 

 mildew. I have, in fact, noticed this phenomenon in the Cheno- 

 podiums in Chautauqua County, N. Y., for several years and had 

 always supposed it to be the work of a fungus until I examined 

 them critically last summer. 



A black aphid, the Aphis rumicis L. of authors, commonly 

 infests the lower surface of the leaves and the inflorescence of these 

 plants. It also sometimes occurs mixed in the colonies of the 

 green aphids under discussion. This habit undoubtedly accounts 

 for Buckton's mixed descriptions. The green and the black 

 forms are entirely distinct species easily separable by pronounced 

 structural differences, as pointed out in the descriptions farther on. 



During the first week in August, 1908, the winged viviparee 

 (viviparous females) of Aphis atriplicis were leaving the galls in 

 great numbers, a general migration being apparently at its 

 height. After the middle of August pupae were not common in 

 the galls, which were almost deserted, except for a few wingless 

 viviparas and larvae. Connold, 1902, observed a similar migration 

 in August in England. I failed to find what became of the winged 



