HOMOPTERA 421 



covers colonies of them. The three following species are widely dis- 

 tributed and are common. 



The woolly apple aphis, Eriosoma lamgera. — This plant-louse, on 

 account of its woolly covering and the fact that it is a serious pest of 

 the apple, is known as the woolly apple aphis, although the apple is 

 its secondary host. This insect not only has a complicated series of 

 generations but the life-cycle is subject to variations ; its usual course 

 is as follows : 



The winter-eggs are deposited in crevices of the bark of elm. 

 From these eggs stem-mothers hatch in the spring and pass to the 

 young leaves, where they produce either the well-known leaf -curl of 

 the elm or, when a group of terminal leaves are affected, what has 

 been termed a rosette, which is a cluster of deformed leaves. Within 

 these pseudogalls the second generation is produced; this consists of 

 wingless agamic females. The offspring of these, the third generation, 

 become winged and migrate from the elm to the apple. Here they 

 produce the fourth generation, the members of which live on the 

 water-shoots or the tender bark of the apple, and are wingless. The 

 fifth generation also consists of wingless agamic females. Some of 

 these develop on the bark of the branches, which apparently ceases 

 to grow at the point of attack but swells into a large ridge about the 

 cluster of plant-lice, leaving them in a sheltered pit ; the aphids also 

 frequently congregate in the axils of the leaves and the forks of the 

 branches. Other members of this generation pass to the roots of the 

 tree, where they produce knotty swellings on the fibrous roots. The 

 sixth generation consists, in part, of winged agamic females which 

 migrate from the apple to the elm, where they produce the seventh 

 generation. This generation, the last in the series, consists of the 

 males and oviparous females, both of which are beakless and wing- 

 less. These pair and each female produces a single egg, which is 

 found in a crevice of the bark with the remains of the body of the 

 female. 



The course of events outlined above may be modified in two ways : 

 first, it is said that the sexual forms are sometimes produced on the 

 apple; and second, some members of the sixth generation do not 

 develop wings and migrate, but are wingless and produce young that 

 hibernate on the apple. This species infests also mountain ash and 

 hawthorn, as secondary hosts. 



The elm-feeding generations of this species that cause the leaf- 

 curls and rosettes have been known as Schizoneura americdna. And 

 there are also found during the summer aphids on tender elm bark 

 which are believed to belong to this species and which have been 

 described under the name Schizoneura rileyi. In the Pacific Coast 

 States there is another species of aphid that produces leaf curl on elm. 

 This is Schizoneura itlmi, a European species, which in Eiu-ope has 

 been found to migrate to Ribes. 



The alder-blight, Proctphilus tesselldtus. — A woolly aphid that is 

 foimd in dense masses on the branches of several species of alder is 

 known as the alder-blight. Colonies of this species are easily foimd 



