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 
has 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 it is 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 
■ De Geer vi. 75. Latr. Hist. Nat. xiv. 371. 
b Jacquin Collectan. iii. /. xxiii./. 7. c De Geer uli supr. 
