168 PREPARATION AND MOUNTING 



shown in specimens mounted dry. As before mentioned, 

 these tracheae 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 duat. The forms of these are 

 seldom alike in two different kinds of insects, so that there 

 is here a wide field for the student. The 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 larvaa 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 Or. Mills, LL.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 

 ases 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. 



FISH. The most interesting part of fish to the micro- 

 s' copic anatomist is undoubtedly the breathing apparatus. 



