CH. X.J THE PLANT-LOUSE 179 



It has been already stated, that the formation of 

 the true gall-nut is owing to the deposition of an 

 egg in some part of a plant. But in the formation of 

 the protuberances inhabited by the aphides there 

 IS this remarkable difference; tliat the parent, in- 

 stead of burying her offspring, buries herself, and 

 then, as the walls of lier wonderful mansion rise up 

 around and enclose her, she begins to people her 

 abode. When Reaumur examined the smallest of 

 these at its first formation, he found it tenanted by 

 one old aphis only : — when he examined a larger 

 gall-nut he found, in addition to the old aphis, one 

 or two young ones — and in a protuberance of a 

 larger size still, he discovered a more abundant po- 

 pulation. The mode by which the insect is at last 

 thoroughly enclosed is thus described by him : — " Let 

 us imagine that the mother-aphis, still young, pricks 

 the leaf; the punctured spot swells all around the 

 insect, and consequently it becomes enclosed within 

 a little cavity. If it continue to prick it at the low- 

 est part of this cavity, this place will go on swelling 

 in length, so as to become oblong or cylindrical. 

 Let us conceive that the insect always continues to 

 puncture it forwards ; as soon as the gall has risen 

 to a certain height above the superior surface of the 

 leaf, the insect is no longer in its original position ; 

 Tiz. on the plane of the inferior surface of the leaf. 

 Here then it is that there is a small opening into the 

 incipient bladder ; this aperture is only an indenta- 

 tion in the leaf. As soon as the insect removes from 



