APHIDIDiE. 



319 



but in the specimens I examined this joint was ringed like the 

 others. 



Common and harmful in Kent. 



Pemphigus spirothecle (Koch). 



This species, like the preceding species, is also found on the leaf- 

 stalks of poplar of the black Italian species (fig. 295), but it makes a 

 corkscrew-shaped gall. It is far from 

 common, and very locally distributed. 

 While living in Cheshire, I only 

 found but one tree, 



growing on 



bank alongside a brook, which annu- 

 ally yielded me a good crop of galls. 

 The queen aphis punctures one side 

 of the leaf-stalk to obtain nourish- 

 ment, and the stalk in consequence 

 bulges on the opposite side. The 

 gnawing and bulging causes the stalk 

 to form a corkscrew - shaped gall, 

 where the stem - mother and her 

 progeny find a home and shelter in the folds. As the fully devel- 

 oped insects are about to escape the galls become more elastic, and 



Fig. 293. Pupal stage 0/ Pemphigus bur- 

 sarius. (From camera-lucida sketch.) 



Fig. 294. Winged form of Pemphigus bursarius. (From camera-lucida sketch.) 



open easily. The gall is of a dull-green colour, and may be looked 

 for from June to September. 



The most abundant crop of those galls I have ever seen was at 

 Peterborough, on the Lombardy poplar. 



