372 
ON THE ACARUS, 
and also from the infection lingering many weeks about utensils 
employed by persons having the itch, that it can live for a far 
longer time. 
This insect is seldom found on the outer skin. When it is 
placed on the cuticle, it always seeks the nearest little furrow, 
through which it bores with its trunk until it can get its whole 
body through the upper skin, under which it makes its thread¬ 
like passages in different directions, and of various lengths. 
Raspail and Albin Gras call these passages “ cuniculi,” because 
they resemble those constructed by rabbits. The walls of these 
passages are sometimes so thickened by fibrous matter, that they 
form raised lines. A reddish mark, like a flea-bite without the 
red circle round it, or an oblong point, or a small pimple of a 
black or white colour on the cuticle, may be considered as the 
commencement of one of these passages. This spot usually 
shews itself on the head or at the side of a newly-risen vesicle 
(itch pustule), and in the centre of a small white fleck or spot; 
it is, however, found in different places according to the develop¬ 
ment of the vesicles. 
Raspail and Albin Gras assert that the insects are never found 
in the vesicles, but always at the end of these passages. On the 
contrary, Bonomo, Cestoni, Wichmann, and flecker, sen., who 
have paid much attention to this subject, say that they have 
generally found the insects at the end, and in the securest parts 
of the passages, but that they have also seen them in the blad¬ 
ders. (See Wichmann^s Actiologie der Kr'dtze, second edition, 
1792.) As the passages and the bladders are situated so near 
to one another, a mistake may easily be made. It is seldom 
that two passages are found proceeding from one bladder, although 
many bladders are often seen near to one passage. Those mites 
which Albin Gras drew out of the passages were but one-third 
of their proper size. 
It is yet to be discovered wherein the difference between the 
sexes consists, for no writer has hitherto described it, nor when 
and how the copulation takes place—where the eggs are de¬ 
posited—how long the time of incubation continues—whether, 
as some writers afiirm, the young mites have only six feet, and 
whether they really change their skin several times. 
The acari of the horse were first depicted by Gohier and St. 
Didier, then by Bose, and lastly by Raspail. St. Didier and 
Raspail have also given a short description of them. Bose, 
Huzard sen., Latreille, Dumenil, and Geoffery St. Hilaire have 
likewise published some observations on them, but I have never 
seen them. 
