ON A GAD-FLY GRUB. 699 
not exceed an inch in length ; while the larva of the ox oestrus, at 
full growth, attains to from three to four inches. 
This difference, in itself considerable, is not the only one we 
have to remark. Upon the subject of our inquiry we do not ob- 
serve the six longitudinal lines which, upon the hypoderme of the 
ox, form so many series of tuberculous eminences ranged at certain 
intervals from the first to the last segment. Neither are to be 
seen the five or six eminences surrounding the mouth of the latter ; 
though, to make up for it, we find, at the posterior and inferior side 
of the second segment, a kind of transverse prominence, shooting 
beyond the surrounding skin, and furnished with very small tu- 
bercles, which has no existence in the grub found in the skin of 
the ox. 
These differences, united to those of size, suffice, in our mind, to 
authorise the conclusion, that the larva in question, though it be- 
long to the genus hypoderma, constitutes a distinct species, up to 
the present time undescribed, to which we shall give the name of 
hypoderma equi. 
No doubt we shall be asked what are the habits of this insect 
in its different states 1 Nobody, as yet, that we know of, has 
investigated them. The larva alone being recognised, it is to that 
the few observations apply of which science is in possession. We 
know that this larva is found principally upon horses that have 
been living at pasture in the months of July and August ; and 
that through its presence are caused large indurated knots or 
buttons upon the skin, which are found in the greatest numbers 
along the spine, from the withers to the croup inclusive. These 
buttons, whose volume varies from a lentil to a small nut, have, 
according to M. Loiset, a shape inclining to conical ; and, by care- 
fully separating the hairs clothing them, may be seen on their 
summit a narrow aperture, resembling such as a large needle 
would make. Compressing the tumour causes to issue from this 
opening a minute quantity of purulent serosity, which, after being 
removed, is followed, in the last months of the growth of the 
tumour, by a vesicular point, which is neither more nor less than 
the posterior extremity of a larva. The efforts made to expel this 
are ordinarily ineffectual up to the period at which its growth fits 
it for metamorphosis ; when, by squeezing the pustule forcibly be- 
tween the nails, a vermicular body springs livelily out, as though 
impelled by elasticity, and rolls upon the ground. This body is no 
other than the larva. 
There can be no doubt but that these larvae feed upon the pus 
which forms in the tumours that serve them as habitations up to 
the moment of their transformation ; neither is there any that the 
