368 PREPARATION AND MOUNTING 
shown in specimens mounted dry. As before mentioned, 
these tracheze terminate on the outside in openings termed 
spiracles, which are round, oblong, and of various shapes. 
Over these are generally a quantity of minute hairs, forming a 
guard against the entrance of dust. The forms of these are 
seldom alike in two different kinds of insects, so that there 
is here a wide field for the student. ‘Phe dissection, more- 
over, is very easy, as they may be cut from the body with 
a sharp knife or scissors, and mounted in balsam or fluid. 
Many of the larvze afford good specimens, as do also some 
of the common Coleopterous insects. Perhaps, no more 
satisfactory object can be met with to give the student good 
examples of spiracles than the water-beetle Dytiscus, before 
mentioned, as affording such perfectly beautiful suckers. 
They will be found to vary in appearance according to the 
part of the body from which they are taken; but all are 
equally interesting. 
Mr. Lewis G. Mills, LU.B., gives the following account of 
his extracting the poison glands from a spider :—Having 
killed a large spider with chloroform, I left it in water for 
seven or eight days. This treatment usually softens the 
outer skin of insects and causes the viscera to swell, so as 
to burst through the outer integument, and it is in this 
state, perhaps, that the poison glands are most easily dis- 
covered and traced to their points of attachment. I then 
drew the mandibles from the body, and, having placed 
them with a little water on a slide and covered them with a 
piece’ of thin glass, I found that, upon the application of | 
pressure, the two glands shot out and protruded from the 
cases of the mandibles. I tore open one of the mandibles 
with needles, so as to disturb the gland as little as possible. 
The gland then appeared as a closed sac, attached by a 
hollow cord, about the length of the gland itself, to the base 
of the fang, where also was a large bundle of muscular 
fibre. 
Fiso.—The most interesting part of fish to the micro- 
scopic anatomist is undoubtedly the breathing apparatus. 
