HOMOPTERA 



117 



The beech-tree blight, Proctphilus imbricator, infests both the twigs 

 and leaves of beech. Like the preceding species it occurs in clusters of 

 individuals, each of which is clothed with a conspicuous downy excretion. 

 These clusters often attract attention by the curious habit that the in- 

 sects have of waving their bodies up and down, the plume-like masses of 

 excretion rendering them very conspicuous. When an infested limb is 

 jarred the aphids emit a shower of honey-dew. Owing to the abundance 

 of this secretion, the branches and leaves of an infested tree become 

 blackened by growths of fungi, as with the preceding species. 



Other aphids produce galls of various shapes and sizes on the leaves 

 of plants. 



The cockscomb elm-gall, Colopha ithnicola. — There are two species of 

 aphids that make similar galls on the leaves of elm. These galls are 

 commonly known as cockscomb elm-galls on account of their shape. 

 Those made by the two species of aphids are so similar that a description 

 of one will apply to the other. In each case the gall is an excrescence 

 resembling a "cock's comb" in form, which rises abruptly from the upper 

 surface of the leaf. It is compressed, and has its sides wrinkled perpen- 

 dicularly and its summit irregularly gashed and toothed. It opens on 

 the under side of the leaf by a long slit-like orifice. 



The poplar-leaf gall aphid, Thecabius populicaulis . — This aphid is 

 common on several species of poplar. It makes a swelling the size of a 

 small marble on the leaf at the junction of the petiole with the blade. 

 This gall is of a reddish tint, and has on one side a slit-like opening. 



Fig. 195. — The witch-hazel cone-gall: a, natural size; b, section of 

 gall, enlarged. (From Pergande.) 



The witch-hazel cone-gall aphid, Hormaphis hamamelidis , causes cone- 

 like galls on the leaves of witch-hazel (Fig. 195). 



