572 APPENDIX. 
upon scraping some of it off with a penknife, 1 found it was a 
white substance of a fatty nature. In this case, then, the fat 
must have been exploded on all sides with considerable vio- 
lence through the pores of the body. Probably this was a 
more intense degree of the plethoric disease. When I exa- 
mined this appearance the fly had fallen off, and I could not 
find it. 
In looking over some letters long since received from 
J. Hobart Briggs, Esq., (a most diligent and accurate observer 
and delineator of natural objects,) after my account of the 
diseases of insects was printed ; in one I found the details of 
a singular instance of Acariasis which had escaped my recol- 
lection, but which ought not to be lost.—In July 1817 he 
found a small spider, not bigger than those called Spinners, in 
his garden, which appears to belong to Walckenaér’s third 
family of Therzdion, to the thorax of which were attached 
four oblong bright scarlet Acarz, each of which was as large 
as the thorax itself. He afterwards met with another spider 
still smaller, attacked by two of these swoln parasites, one of 
which appeared to him nearly to equal the whole spider in size. 
The Acarus was probably either the Leptus Phalangit*, or 
the Astoma parasiticum?. 
IV. 
The observations on the chemical composition of insects° 
were printed before the publication of the first number of the 
Zoological Journal, in which is an able memoir, to which we 
must refer our readers for further information on that sub- 
ject. 
* De Geer vii. 117. é. vii. f.5. Latr. Gen. Crust. et Ins. i. 161. 
> De Geer Jbid. 118. t. vii. f. 7,8. Latr. Ibid. 162, 
© Vou. HI. p. 395 —. 
