484 
ANNUAL REPORT OF NEW-YORK 
SUPPLEMENT. ' 
other districts. But it now becomes altogether more probable 
that this sudden thinning in the ranks of these creatures is caused, 
not by their migration, but by their increase being suddenly 
arrested by this insect. Recently the field mice have been very 
abundant all over our country, and complaints were everywhere 
made of apple and other trees being girdled and destroyed by 
them in the winter of 1855-56. The same causes which pro¬ 
duced such unusual numbers of these vermin appear to have 
favored the increase of other small animals also. In my own 
vicinity at least, the squirrels, having been quite plenty in the pre¬ 
ceding years, became unusally numerous last year, and from the 
readiness with which individuals containing parasites were then 
obtained, it is evident that the males were generally infested with 
these insects. The present year, sportsmen inform me there is a 
remarkable paucity in the numbers of these animals, not a quar¬ 
ter as many being now present in the forests as were found there 
a year ago. This diminution it can scarcely be doubted, has been 
occasioned by the insect of which v.e are treating. And when¬ 
ever the squirrels are becoming multiplied these parasites will 
rapidly increase their numbers also. We know what a multitude 
of eggs a single bot-fly glues to the hairs of a horse’s fore legs. 
If this squirrel-fly is similarly prolific what a host of these unfor¬ 
tunate animals will a single female mutilate, since she places only 
one or two eggs in each ! By some mysterious instinct she undoubt¬ 
edly knows -whether a squirrel is already inoculated, and thus 
avoids consigning a single one of her progeny where it will be 
forestalled and unable to obtain the amount of nourishment which 
it requires. Hence, when the numbers of these insects become 
but moderately increased, as each female will be intently on tho 
alert to dispose of her stock of eggs, it will scarcely be possible 
for a male squirrel anywhere to escape them. 
Emasculated individuals are met with belonging to each of the 
species of squirrel common in our country. It is a fly bred from 
the striped squirrel which I have described above. Whether this 
same fly attacks our other squirrels also, or whether each kind ol 
squirrel has a distinct species of bot-fly peculiar to it, future 
observers must determine. As there are two species of these 
insects residing under the skin of our American rabbits it is quite 
