41 i ' CE S T 
the months of larva: in general; and round the orifice of 
the mouth are placed fome proje&ing mammillae, which 
are imperforate, and perhaps ferve the purpofe of feelers. 
The larva, having arrived at its full growth, eft'efts its 
efcape from the abfcefs, by p re fling again(l the external 
opening, which bccafions its-enlargement by the points 
prefled upon being gradually abforbed. When the open¬ 
ing has thus obtained the fize of a imall pea, the larva 
writhes itfelf through, and falls from the back of the ani¬ 
mal to the ground, and, 1'eeking a convenient retreat, 
becomes a cliryjalis, as reprefen ted .at fig. 4. Thefe larvae 
never change or throw off their ikin, the fame ferving 
them through their whole growth ; and it at length alfo 
ferves to form the (hell of the chryfalis. The fack which 
enclofes the larva beneath the fkin, is formed of a tough 
thick membrane, rough on the infide; and the pus l'e- 
creted by it, is moftly of a yellow colour. After the exit 
of the caterpillar, the wound in the Ikin is moftly clofed 
up and healed within a few days. 
The chryfalides continue in that ftate from about the 
latter end of June until about the middle of Auguft, when 
the fly appears. The perfect infeft, on leaving the chry¬ 
falis, forces open a very remarkable marginated triangu¬ 
lar lid, or operculum, (fee fig. 5.) which may be traced in 
the fkin of the larva, and is Jituated on one fide of the 
fimall end. 
The GEfinis bovis, in its perfeft ftate, (fig. 6.) is the 
largeft of the European fpecies of this genus, and is very 
beautiful. Although its effects on the cattle have been 
fo often remarked, yet the fly itfelf is rarely feen or taken, 
as the attempt would be attended with confiderable dan¬ 
ger. The pain it inflifts in depofiting its egg is much 
more fevere than in any of the other fpecies. When one 
of the cattle is attacked by this fly, it is eafily known by 
the extreme terror and agitation of the whole herd ; the 
unfortunate objeft of the attack runs bellowing from 
among them to fome diftant part of the heath, or the neareft 
water; while the tail, from the feverity of the pain, is held 
with a tremulous motion ffraight from the body, in the 
direftion of the fpine, and the head and neck are alfo 
firetched out to the utmoft. The reft, from fear, gene¬ 
rally follow to the water, or difperfe to different parts of 
the field. When the oxen are yoked to the plough, the 
attack of this fly is attended with real danger, as they 
become perfeftly uncontroulable, and will often run with 
the plough direftly forwards, through the hedges, or 
whatever obftrufts their way. There is provided, on this 
account, to many ploughs, a contrivance immediately to 
fet them at liberty on fuch an occafion. 
The heifers, fteers, and younger cattle, are the moft 
frequently attacked by this fly, and have in general a 
greater number of botts than others : the ftrongefl and 
hrvdthieft beafts feem conftantly to be preferred by it; and 
this is a criterion of goodnefs in much efteetn with the 
dealers in cattle. The choice of a found healthy fubjeft 
for the depofition of the eggs, is probably caufed by the 
folicitude of the parent for the fafety of its offspring. 
And the tanners alfo ob/erve, that their beft and ftrongefl 
hides have the greateft number of bott-holes in them ; 
for, although the fkin heals up on the exit of the larva, 
it is not with the fame matter as the original fkin ; which 
has been remarked by late phyfiologifts, and which this 
curious faft fufficiently confirms. 
The female fly is very quick in performing the opera¬ 
tion of depofiting its egg ; lhe does not appear to remain 
on the back, of the animal more than a few feconds ; and 
the cow never attempts to lafh this infeft off with her tail, 
which fhe performs fo dextroufly when attacked by other 
flies. The whole of this genus of infefts appear to have a 
ftrong diflike to moifture, fince the animals find afecure 
refuge when they get into a pond or brook, where the Ta- 
bani, Conopes, and other flies, follow without hefitation, 
but the CEftri rarely or never; and, during cold, rainy, 
or windy, weather, they are not to be feen. 
2. CEftrus eqtii, the liorfe gad-fly. This fpecies has 
R U S. 
been extremely well deferibed by Mr. Bracy Clark, in the 
3d vol. of the Tranfaftions of the Linnaean Society. It 
is fmaller than the bovis; and of a yellowifh-brown co¬ 
lour, with a dufky band acrofs the tliorax, and the tip of 
the abdomen of Similar colour ; the wings are whitifh, 
with a pale-duiky bar acrofs the middle of each, and two 
dufky fpots at the tip. 
The manner in which the young larvae, or botts, are in¬ 
troduced into the ftomach and bowels of the animal they 
infeft, is Angularly curious, and cannot be better deli¬ 
vered than in the words of the ingenious obferver. 
“ When the female has been impregnated, and the eggs 
are fufficiently matured, fhe feeks among the horfes a fure¬ 
ject for her purpofe, and, approaching it on the wing, (he 
holds her body nearly upright in the air, and her tail, 
which is lengthened for the purpofe, curved inwards and 
upwards ; in'this way fhe approaches the part where fhe 
defigns to depofit her egg ; and, fufpending herfelf for a 
few feconds before it, fuddenly darts upon it, and leaves 
her egg adhering to the hair; fhe hardiy appears to fettle, 
but merely touches the hair with the egg held out on the 
projected point of the abdomen. The egg adheres by 
means of a glutinous liquor fe ere ted with it. She then 
leaves the horfe at a final] diftance, and prepares a feconci 
egg; and, poifing herfelf before the part, depofits it in 
the fame 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 fometimes placed on one 
horfe. The horfes, when they become ufed to this fly, 
and find that it does them no injury by fucking their 
blood, hardly regard it, and do not appear at all aware of 
its infidious objeft. The fkin of the horfe is always 
thrown into a tremulous motion on the touch of this in¬ 
feft, which merely arifes from the very great irritability 
of the fkin and cutaneous mufcles at this feafon of the 
year, Auguft and September, cccafioned by the continual 
teafing of the flies, till at length thefe mufcles aft invo¬ 
luntarily on the flighted touch of any body whatever. 
The infide of the knee is the part on which thefe flies are 
moft fond of depofiting their eggs, and next to this on 
the fide and back part of the ffioulder, and lefs frequently 
on the extreme ends of the mane; but it is a fact worthy 
of attention, that the fly does not place them'promifcu- 
oufly aboutthe body, but conftantly on thole parts which 
are moft liable to be licked with the tongue ; and the ova 
therefore are always fcrupuloufly placed within its reach. 
The eggs thus depofited, I at firft fuppofed were loofened 
from the hairs by the moifture of the tongue, aided by its 
roughnefs, and were conveyed to the ftomach, where they 
were hatched ;” [and fo we had Hated it in the article 
Entomology;] “ but on more minute fearch I do not 
find this to be the cafe, or at leaft only by accident; for, 
when they have remained on the hairs four or five days, 
they become ripe, after which time the flighted applica¬ 
tion of warmth and moifture is fufficient to bring forth in 
an inftant the latent larva. At this time, if the tongue 
of the horfe touches the egg, its operculum is thrown 
open, and a finall aftive worm is produced, which readily 
adheres to the moift furface of the tongue, and is from 
thence conveyed with the food to the ftomach. If the 
egg itfelf be taken up by accident, it may pafs on to the 
intellinal canal before it hatches; but, in this cafe, it§ ex- 
iftence to the full growth is more precarious, and certainly 
not fo agreeable, as it is expofed to the bitternefs of the 
bile. I have often, with a pair of feiflars, clipped off fome 
hairs with eggs on them from the horfe, and, on placing 
them in the hand, moiftened with laliva, they have hatched 
in a few feconds. At other times, when not perfeftly 
ripe, the larva would not appear, though held in the hand 
under the fame circumftances for feveral hours; a fiiffi- 
cient proof that the eggs themfelves are not conveyed to 
the ftomach. 
“ It is fortunate for the animals infefted by thefe infefts, 
(adds Mr. Clarke,) that their numbers are limited by the 
hazards they are expofed to. I fhould fufpeft, near a hun- 
a dred 
