DISEASES OF INSECTS. 229 



and brocoli (Pontia Brassic<z\ which perish by hundreds 

 from its attacks. As this falls frequently under our no- 

 tice, it will not be uninteresting to give a fuller account 

 of it. Reaumur has traced and related its whole history. 

 One of these little flies that he observed, was so intent 

 upon the business in which she was engaged, that she 

 suffered him to watch her motions under a lens, without 

 being discomposed. She pursued nearly the same plan 

 of proceeding with that of the Ichneumon of the wheat- 

 gnat just described ; except that she repeated her opera- 

 tions frequently on the same caterpillar in different parts, 

 alternately plunging in and extracting her ovipositor. 

 She seemed to prefer the spot where the segments of the 

 body are united, particularly where the eighth meets the 

 ninth, and the ninth the tenth. When the fly had com- 

 pleted its work and quitted the caterpillar, Reaumur 

 gave it food, and it did not seem less lively and vigorous 

 than others of its kind ; in less than a fortnight it as- 

 sumed the pupa ; and in four days the whole of its inte- 

 rior being devoured, it died : but its parasites, perhaps 

 not finding a sufficient supply of nutriment in it, never 

 came to perfection a . Sometimes, however, these little 

 grubs arrive at maturity before the caterpillar has be- 

 come a chrysalis, when they pierce the skin and begin to 

 emerge. First appears a little white tubercle, which gra- 

 dually elevates itself in a direction perpendicular to the 

 body ; while this is doing, a second appears in another 

 place ; and so on, till fifteen or sixteen are seen on each 

 side, giving the caterpillar a very grotesque appearance. 

 By the alternate contraction and relaxation of their bo- 

 dies the grubs effect their complete liberation, which takes 

 a Reaum. ii. 41 7-. 



