OESTRUS. 
Mouth with a simple aperthfe, and not ex- 
serted ; feelers two, of two articulations 
orbicular at the tip, and seated each side in 
a depression of the mouth ; antennae of three 
articulations, the last subglobular, and fiir- 
nished with a bristle on the fore-part, placed 
in two hollows on the front. The face of 
this singular genus is broad, depressed, vesi- 
cular, and glaucous, and has some sort of 
resemblance to the ape kind. They are 
extremely troublesome to horses, sheep, and 
cattle, depositing their eggs in different 
parts of the body, and producing very pain- 
fiil tumours, and sometimes death. The 
larva are without feet, short, thick, and 
annulate, and often furnished with small 
hooks. There are tvvelve species, named 
from the animals which they infest : thus we 
have O. bovis, O. equi, O. ovis, O. hominis, 
&c. The principal European species is the 
O. bovis, or ox gad fly, which is, the size of 
a common bee, and is of a pale yellowish 
colour, with the thorax marked with four 
longitudinal dusky streaks, and the abdo- 
men by a black bar across the middle ; the 
lip is covered with tawny orange-coloured 
hairs ; the wings are pale-brown, and un- 
spotted. The female of this species, when 
ready to deposit her eggs, fastens on the 
back of a heifer, or cow, and piercing the 
skin with the tube situated at the lip of the 
abdomen, deposits an egg in the puncture, 
and then proceeds to another spot at some 
distance fiom the former, repeating the 
same operation, at intervals, on many parts 
of the animal’s back. The pain which this 
operation occasions is extreme ; and hence 
cattle, as if foreseeing their cruel enemy, 
are observed to be seized with the most 
violent horror when apprehensive of the 
approaches of the female oestrus, flying in- 
stantly to the nearest pond or pool of water ; 
it having been observed that this insect 
rarely attacks cattle when standing in water. 
The eggs are laid in August or September, 
and the larvae remain till the following sum- 
mer before they undergo the change to the 
pupa state. At this period they force them- 
selves out of their respective cells, and fall- 
ing to the ground, creep beneath the first 
convenient shelter, and lying in an inert 
state become contracted into an oval form, 
but without casting the larva skin, which 
dries and hardens round them. When the in- 
cluded insect is ready for exclusion, it forces 
open tlie top of the pupa coat, and emerges 
in its perfect form, having remained within 
the chrysalis somewhat more than a month. 
We shall give an account of the O. equi, 
VOL. V. 
from the Transactions of the Linnman So- 
ciety, drawn up with great accuracy by 
Mr. Clarke. “ When the female has been 
impregnated, and the eggs are sufficiently 
mature, she seeks among the horses a sub- 
ject for her purpose ; and approaching it on 
the wing, she holds her body nearly upright 
in the air, and her tail, which is lengthened 
for the purpose, curved inwards and up- 
wards : in this way she approaches the part 
where she designs to deposit her egg ; and, 
suspending herself for a few seconds before 
it, suddenly darts upon it, and leaves her 
egg adhering to the hair : she hardly appears 
to settle, but merely touches the hair with 
the egg held out on the projected point of 
the abdomen. The egg is made to adhere 
by means of a glutinous liquor secreted with 
it. She then leaves the horse at a small 
distance, and prepares a second egg, and, 
poising herself before the part, deposits it 
in the same way. The liquor dries, and 
the egg becomes firmly glued to the hair : 
this is repeated by various flies, till four or 
five hundred eggs are sometimes placed on 
one horse. The horses, when they become 
used to this fly, and find that it does them 
no injury, as the Tabani and Conopes, by 
sucking their blood, hardly regard it, and 
do not appear at all aware of its insidious 
object. The skin of the horse is always 
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 continual teasing of 
the flies, till at length these muscles act in- 
voluntarily on the slightest touch of any 
body whatever. 
“ The inside of the knee is the part on 
which these flies are most fond of deposit- 
ing their eggs, and next to this, on the side 
and back part of the shoulder, and, less 
frequently, on the extreme ends of the 
mane. But it is a fact worthy of attention, 
that the fly does not place them promiscu- 
ously about the body, but constantly on 
those parts which aremost liable to be licked 
with the tongue ; and the ova, therefore, are 
always scrupulously placed within its reach. 
“ The eggs thus deposited I at first sup- 
posed were loosened from the hairs by the 
moisture of the tongue, aided by its rough- 
ness, and were conveyed to the stomach, 
where they were hatched : but on more 
minute search I do not find this to be the 
case, or at least only by accident ; for, 
when they have remained on the hairs four 
or five days they become ripe, after which 
