484 ANNUAL REPORT OF NEW- YORK 



SUPPLEMENT. 



other districts. But it now becomes altogether more probable 

 that this sudden tliinning 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 vre 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 the 

 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 



