18 
comes firmly glued to the hair : this is repeated by these flies till 
four or five hundred eggs are sometimes placed on one horse. 
The skin of the horse is usually thrown into a tremulous motion 
on the touch of this insect, which merely arises from the very 
great irritability of the skin and cutaneous muscles at this season 
of the year, occasioned by the heat and continual teasing of the 
flies, till at length these muscles appear to act involuntarily on the 
slightest touch of any body whatever. See PI. I. fig. 1. 
The inside of the knee is the part on which these flies are most 
fond of depositing their eggs, and next to this on the side and 
back part of the shoulder, and less frequently on the extreme 
ends of the hairs of the mane. But it is a fact worthy of atten¬ 
tion, that the fly does not place them promiscuously about the 
body, but constantly on those parts which are most liable to be 
licked with the tongue ; and the ova, therefore, are always scru¬ 
pulously placed within its reach. Whether this be an act of rea¬ 
son or of instinct, it is certainly a very remarkable one. I should 
suspect, with Dr. Darwin,* it cannot be the latter, as that ought 
to direct the performance of any act in one way only. Which¬ 
ever of these it may be, it is, without doubt, one of the strongest 
examples of pure instinct, or of the most circuitous reasoning any 
insect is capable of. 
The horses, when they become used to this fly, and find it does 
them no injury, as the Tabani and Canopes, by sucking their 
blood, hardly regard it, and do not appear at all aware of its insi¬ 
dious object. During warm sunny weather this process is seen 
performed on commons, and in the fields. 
The eggs thus deposited, (see fig. 2, and when magnified fig. 3,) 
I at first supposed wei'e loosened from the hairs by the moisture 
of the tongue, aided by its roughness, and were conveyed to the 
stomach, where they were hatched : but on more minute search 
* Zoonomia. Vid. Chapter on Instinct. 
