This method of preparing and mounting insects I can strongly recommend as giving first-rate results; but where the specimens are small they seldom need the soaking in caustic potash which larger ones must have. It is only necessary to leave them awhile in turpentine, especially when they have been first dried with gentle pressure between two glasses, and then mount with balsam in the ordinary way.
Amongst the insect tribes there is abundant employment, especially for the lower powers of the microscope. But if the deeper wonders and beauties of the animal economy are to be sought out and studied, it is desirable that the various parts should be set separately, in order that they may receive a more undivided attention, as well as to render them capable of being dealt with under the higher powers. We will, therefore, briefly consider the treatment which the different portions require.
The eyes of the butterflies, and indeed of almost all insects, afford materials for a study which is complete in itself. When examined with a tolerably high power, instead of finding each eye with an unbroken spherical surface, it is seen that many are composed of thousands of hexagonal divisions, each being the outer surface of a separate portion termed the ocellus. In others these divisions are square; but in all there is a layer of dark pigment surrounding their lower parts. The ocelli may be partly removed from the eye, which will show how their tapering forms are arranged. But here we have to consider how to place them in balsam for preservation. The eye being removed from the insect, and the dark pigment removed by the use of a camel-hair pencil, must be allowed to remain in turpentine at least for some days. The turpentine should then be renewed and the eye well washed in it just before it is to be mounted. It may then be set in balsam in the same way as any other object;—but here a difficulty is met with. The eye being spherical upon the surface required, must necessarily be “folded” or broken in attempting to flatten it. This difficulty may be often overcome by cutting a number of slits round the edges; but some object to this mode of treatment, and where it is practicable it is much more satisfactory to mount one in the natural rounded form and another flat. Instead, however, of mounting the organ whole, four or five slides may be procured from each of the larger ones, as those of the dragon-fly, &c.
The antennæ also are often mounted on separate slides, as being better suited for higher powers and more minute examination than when connected with the insect. These two projecting organs, issuing from the head, are jointed, and moveable at will. They differ very much in form amongst the various species, and are well worth the attention of the microscopist. They are usually mounted with the head attached, and perhaps they are more interesting when thus seen. Some few are very opaque, to prepare which the following method has been advised:—
Bleach the antennæ by soaking in the following solution for a day or two:—
Hydrochloric acid, 10 drops.
Chlorate of potash, ½ drachm.
Water, 1 oz.
This will render them transparent. Wash well, dry, and mount in Canada balsam. Instead of the above, a weak solution of chloride of lime may be used, by which means the nerves will be well shown. Many, however, are rendered transparent enough by simply soaking in turpentine for a longer or shorter time. Where the antennæ, however, are “plumose,” or feather-like, extreme care is required in mounting, though the difficulty is not so great as some seem to think. If they are first dried with gentle pressure, and then subjected to the action of the air-pump in a small quantity of turpentine until the air is thoroughly expelled, they can be easily finished upon the slide, especially when balsam and chloroform are used.
Insects supply us with another series of beautiful objects, viz., the feet.[D] These are sometimes simply dried and mounted without any medium, as before mentioned; but most of them are rendered much more fit for examination by using balsam in their preservation, as it greatly increases their transparency. The smaller kinds may be dried with gentle pressure betwixt blotting-paper, and then immersed for some days in turpentine, without requiring the treatment with liquor potassæ. This immersion will render them beautifully transparent, when they may be mounted in balsam in the usual manner.
[D] See Mr. Hepworth’s interesting articles on the fly’s foot in the second and third volumes of the Microscopic Journal.
It is, however, sometimes found difficult to fix the feet when expanded, in which state the interest of the object is greatly increased. Mr. Ralph recommends the following mode:—“First wash the feet, while the insect is yet alive, with spirits of wine; then holding it by a pair of forceps close to the edge of a clean piece of glass, the insect will lay hold of the upper surface by its foot, then suddenly drop another small piece of glass over it, so as to retain the foot expanded, and cut it off with a pair of scissors, tie up and soak to get rid of air.” Mr. Hepworth says that he never found any difficulty in expanding the foot on a drop of water or well-wetted slide, and laying a thin glass cover over it, tying with thread, drying, and immersing in turpentine.