180 NATURAL HISTORY. [cH. X. 



the aperture towards the other end of the bladder, 

 nothing' tends to hinder the bent sides from meeting 

 soonest at the narrowest part, and so at last closing 

 it up entirely. Here then we have the insect shut 

 up in an oblong sack or bladder. It brings forth 

 young, these prick the gall, and suck the juice on all 

 sides, so that being thus irritated in every direction, 

 it grows in every direction, and consequently a glo- 

 bular form results. And as the punctures are al- 

 ways in a direction farthest from the original aper- 

 ture, that part enlarges least ; and consequently the 

 gall appears to be attached to the leaf by a pedicle 

 or foot-stalk." 



Another class of aphides, which inhabit the poplar, 

 instead of forming galls, contrive to double the leaf 

 so nicely on the nerve which runs through the mid- 

 dle, as to bring the two edges exactly together, and 

 so construct a closed sack. This they effect by 

 pricking the under surface in various places, so as to 

 give rise to small galls, which cause the leaf to curve. 

 Reaumur observes that these punctures must be made 

 according to some definite measure on each side of 

 the centre nervure, or else the edges of the two sides 

 would not exactly meet. Besides these modes of 

 protection, few species of aphides destined to live 



