210 DISEASES OF INSECTS. 



some new species. The under-side of the abdomen is 

 wholly covered by it, divided in the middle into two lon- 

 gitudinal masses, the anal segment being bare. De Geer 

 lias noticed this or a similar disease, which, when flies are 

 attacked by it, causes the abdomen to swell so as even 

 to burst, and the segments become dislocated. Upon 

 opening the abdomen its found filled with a white unc- 

 tuous substance, which often accumulates (as above de- 

 scribed) on its external surface a . Dr. Host says that 

 in this disease when the animal is dead, the wings, which 

 were before incumbent, become extended, and its almost 

 invisible pubescence grows into long hairs b . De Geer 

 seems to think that these flies are thus affected in conse- 

 quence of having eaten some poisonous food c ; but I 

 rather suspect, as I have observed it become prevalent 

 chiefly in wet seasons, that it arises from a superabun- 

 dance of the nutritive fluid, or of the fat, so that it seems 

 to be a kind of plethora. I once observed a fly fixed to 

 a pane of glass, round which was a semicircle of what 

 appeared to be merely vapour, whose radius was nearly 

 three-fourths of an inch. Taking it for an aqueous 

 fluid that had transpired from the dead animal, I paid 

 no further attention to it at that time : but observing 

 from day to day that the moisture did not evaporate, 

 after two or three months had elapsed, I had the curio- 

 sity to examine it more closely, and, upon scraping some 

 of it off with a penknife, I found it was a white sub- 

 stance of a fatty nature. In this case, then, the fat must 

 have exploded on all sides with considerable violence 

 from half the body or the abdomen. Probably this was 



B De Geer vi. 75. Latr. Hist. Nat. xiv. 371. 



b Jacquin Collectan. iii. t. xxiii./*. 7. c De Geer ubi supr. 



