Insects 165 



fir. It is usually placed in the genus Chermes, but Borner pro- 

 posed the genus Gillettea in 1909. There are other related forms 

 on different conifers. Brevicoryne brassicae Linnaeus is the gray- 

 ish-appearing aphis so common on cabbage. It has been intro- 

 duced from Europe. 



The Woolly Aphis of the apple (Eriosoma lanigera Hausmann.) 

 abounds on apple trees in many places, and is conspicuous be- 

 cause covered with masses of wool-like wax. A related species 

 (E. americana Riley) migrates between the elm and the apple; 

 but still another (E. ulmi Linnaeus) migrates between elm and 

 currant. The migrations of certain aphids are very remarkable. 

 After living and multiplying for a time on a certain plant, at the 

 proper season of the year they suddenly take flight, and go off 

 to some quite different host-plant. Thence they, or their de- 

 scendants rather, eventually return to the original host. Pro- 

 fessor Gillette points out that these plant-lice are attacked by 

 many enemies, Syrphid flies, Lacewing flies, Ladybird beetles, 

 small Hymenopterous parasites, etc. These rapidly increase, and 

 might exterminate the aphids, did not the latter suddenly depart, 

 leaving their tormentors behind. While the aphids thus have 

 many enemies, they also have friends. These are the ants, to 

 which they afford nourishment. The ants approach them, 

 and from the tail end they exude drops of liquid, which are 

 greedily lapped up. I have seen aphids on a branch, attended by 

 ants. Small wasps came to take the aphids as provision for their 

 nests. The ants would rush at them, as a bulldog rushes at a 

 burglar, and usually succeeded in driving them away. Once in a 

 while, however, the wasp got an aphid. There are some aphids 

 which live under stones, in nests of ants. In 1908, Rohwer de- 

 scribed two species found in nests of ants under stones in Boulder. 

 One of these (Forda olivacea) has since been found to be wide 

 spread and injurious to the roots of grasses and cereals. 



Macrosiphum rosae Linnaeus is the common green or pinkish 

 plant louse found on cultivated roses, with which it has been 

 introduced into this country. M. albifrons Essig is a large species 

 found on lupines; it was taken on the University of Colorado 

 campus. M. rudbcckiac Fitch is the red aphid found on golden- 

 glow in gardens, and the wild Rudbcckia in the mountains. 



The Water-lily Aphis (Rhopalosiphum nymphaeae Linnaeus) 



