132 



AN ECONOMIC ENTOMOLOGY. 



and we find differences in the form of the honey-tubes which are 

 of generic or even sub-family value. Thus, we have some species 

 of Pemphigus entirely without them, or with very small, mere 

 tubercle-like structures. Tliey live on a variety of trees, but 

 perhaps more abundantly on poplar, and make galls, which are 

 sometimes spherical and nearly half an inch in diameter. If we 

 cut one of them after midsummer, we find it full of plant-lice, 



Fig. 96. 



Cock's-comb gall on elm, Colopha nlmicola. — a, elm-leaf showinc; galls; b, winter 

 egg, covered by the skin of the true female; c, larva just hatched ; d, pupa; e, winged 

 adult. 



the progeny of the single specimen which caused the growth of 

 the gall early in the season. There are many other gall-producing 

 lice, perhaps the best-known being the Colopha ulmicola, found on 

 elm, and forming the "cock's-comb" gall. The popular name 

 fairly describes the appearance of the abnormal growth, which 

 is an inch or more in length and about one-quarter of that in 

 height. None of these gall-making species are abundant enough 

 to be seriously troublesome ; but quite the contrary is true of the 



