424 



AN INTRODUCTION TO ENTOMOLOGY 



The cockscomb elm-gall tetraneura, Tetranetira gramlnis. — The 

 life-cycle of this species is quite similar to that of the preceding one. 

 The primary host is elm. The stem-mothers cause the growth of 

 cockscomb-like galls; and the migrants produced in these galls pass 

 to grasses. These migrants differ from those of the preceding species 

 in that vein M of the fore wings is not forked. This species was first 



described from individu- 

 als found on the second- 

 ary hosts and was named 

 Tetraneura gramlnis . Lat- 

 er, forms found on elms 

 were named Tetraneura 

 colophoides. 



For a detailed ac- 

 count of the gall-aphids 

 of the elm, see Patch 

 Cio). 



The poplar-leaf gall- 

 aphid, Thecdbius populi- 

 caulis. — This aphid is 

 common on several spe- 

 cies 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. In the early part of the season each gall is occu- 

 pied by a single wingless female, probably the agamic stem-mother, 

 which by midsimimer becomes the mother of nimierous progeny. 

 These are winged and probably migrate to some other host-plant; 

 but the life-cycle of this species has not been determined. 



The vagabond gall-aphid, Mordwilkoja vagabunda. — This species 

 infests the tips of the twigs of several species of poplar; here it causes 

 the growth of large corrugated galls, which resemble somewhat the 

 flower of the double cockscomb of our gardens. The galls are at first 

 bright green, but later turn black, become woody, and remain on the 

 trees during the winter (Fig. 494). Very little is known regarding the 

 life-cycle of this species. 



Fig. 494. — The vagabond poplar-gall. 

 Walsh and Riley.) 



(From 



Subfamily HORMAPHIDIN^ 



The members of this subfamily are usually gall-makers, resembling 

 in this respect certain members of the Eriosomatinffi, and also re- 

 sembling them in that the antennal sensoria are annular. But in 

 this subfamily the sexual forms are not so specialized by reduction 

 as in the preceding one. In the Hormaphidina?, although the males 

 and the oviparous females are small and wingless, they possess 



