The mouth, also, with its organs, is an interesting object in many insects. That of the common fly is often made use of, and is comparatively easy to prepare. By pressing the head, the tongue (as it is commonly termed) will be forced to protrude, when it must be secured by the same means as the foot, and may be subjected to the soaking in turpentine, and mounted as usual. The honey-bee is, however, very different in formation, and is well worth another slide; indeed, even in insects of the same class, the differences are many and interesting.
Another worthy object of study is the _respiration_ of insects, which is effected by tracheæ or hollow tubes, which generally run through the body in one or more large trunks, branching out on every side. These terminate at the surface in openings, which are termed _spiracles_, or breathing organs. The _tracheæ_ often present the appearance of tubes constructed by a twisted thread, somewhat resembling the spiral fibres of some plants. These are very beautiful objects, and are generally mounted in balsam, for which reason they are mentioned here; but as they evidently belong to the “dissecting portion,” they will be fully treated of in another place.
Amongst the parasitic insects a great variety of microscopic subjects will be found. As these are usually small, they may be killed by immersion in spirits of turpentine; and if at all opaque, may be allowed to remain in the liquid until transparent enough, and then mounted in Canada balsam.
The acarida, or _mites_ and _ticks_, are well known; none, perhaps, better than those which are so often found upon cheese. Flour, sugar, figs, and other eatables, are much infested by them; whilst the diseases called the _itch_ in man, and the _mange_ in animals, are produced by creatures belonging to this tribe. These animals are sometimes mounted by simply steeping them in turpentine, and proceeding as with other insects. The “Micrographic Dictionary” gives the following directions as to mounting _parts_ of these:--“The parts of the mouth and the legs, upon which the characters are usually founded, may be best made out by crushing the animals upon a slide with a thin glass cover, and washing away the exuding substance with water; sometimes hot solution of potash is requisite, with the subsequent addition of acetic acid, and further washing. When afterwards dried and immersed in Canada balsam, the various parts become beautifully distinct, and may be permanently preserved.”