Sometimes he put into water the delicate viscera of the insects he had suffocated; and then shaking them gently he procured himself an opportunity of examining them, especially the air vessels, which by these means he could separate from all the other parts whole and intire, to the great admiration of all those who beheld them; as these vessels are not to be distinctly seen in any other manner, or indeed seen at all without damaging them, he often made use of water, injected by a syringe, to cleanse thoroughly the internal parts, then blew them up with air and dried them, and thus rendered them durable, and fit for examination at a proper opportunity. Sometimes he has examined with the greatest success, and made the most important discoveries in insects that he had preserved in balsam, and kept for years together in that condition. Again, he has frequently made punctures in other insects with a very fine needle, and after squeezing out all their moisture through the holes made in this manner, he filled them with air, by means of very slender glass tubes, then dried them in the shade, and last of all anointed them with oil of spike, in which a little rosin had been dissolved; by which process they retained their proper forms a long time. He had a singular secret, whereby he could so preserve the nerves of insects, that they used to continue as limber and perspicuous as ever they had been.
He used to make a small puncture or incision in the tail of worms, and after having gently and with great patience squeezed out all their humours, and great part of their viscera, he then injected them with wax, so as to give and continue to them all the appearance of healthy vigorous living creatures. He discovered that the fat of all insects was perfectly dissoluble in oil of turpentine; thus he was enabled to shew the viscera plainly; only after this dissolution he used to cleanse and wash them well and often in clean water. He frequently spent whole days in thus cleansing a single caterpillar of its fat, in order to discover the true construction of this insect’s heart. His singular sagacity in stripping off the skin of caterpillars that were upon the point of spinning their cones, deserves particular notice. This he effected by letting them drop by their threads into scalding water, and suddenly withdrawing them; for, by these means the epidermis peeled off very easily; and when this was done, he put them into distilled vinegar and spirit of wine, mixed together in equal portions, which, by giving a proper firmness to the parts, gave him an opportunity of separating them with very little trouble from the exuviæ, or skins, without any danger to the parts; so that by this contrivance, the nymph could be shewn to be wrapped up in the caterpillar and the butterfly in the nymph. Those who look into the works of Swammerdam, will be abundantly gratified, whether they consider his astonishing labour and unremitted ardour in these pursuits, or his wonderful devotion and piety. On one hand, his genius urged him to examine the miracles of the great Creator in his natural, productions; whilst, on the other, the love of that same all-perfect Being rooted in his mind struggled hard to persuade him that God alone, and not the creatures, were worthy of his researches, love, and attention.
M. Lyonet always drowned first those insects he intended to anatomize, as by these means he was enabled to preserve both the softness and transparency of the parts. If the insect, &c. be very small, for instance one-tenth of an inch, or a little more in length, it should be dissected in water, on a glass which is a little concave; if, after a few days, there be any fear that the insect will putrefy, it should be placed in weak spirit of wine, instead of water. In order to fix the little creature, it must be suffered to dry, and then be fastened by a piece of soft wax; after which it may be again covered with water.
Larger objects require a different process; they should be placed in a small trough of thin wood; the bottom of a common chip box will answer very well, by surrounding the edge of it with soft wax, to keep in the water or spirit of wine. The insect is then to be opened, and if the parts be soft, like those of a caterpillar, they should be turned back and fixed to the trough by small pins; the pins are to be set by a pair of small nippers, the skin being stretched at the same instant by another pair of finer forceps; the insect must then be placed in water, and dissected therein, and after two or three days it should be covered with spirit of wine, which should be renewed occasionally; by these means the subject is preserved in perfection, and its parts may be gradually unfolded, without any other change being perceived than that the soft elastic parts become stiff and opake, and some others lose their colour.
M. Lyonet used the following instruments in his curious dissection of the caterpillar of the cossus. As small a pair of scissars as could be made, the arms long and fine; a small and sharp knife, the end brought to a point; a pair of forceps, the ends of which had been so adjusted, that they would easily lay hold of a spider’s thread or a grain of sand. But the most useful instruments were two fine steel needles, fixed in small wooden handles, about 2-3⁄4 of an inch in length.
An observation of Dr. Hooke’s may be very useful if attended to, for fixing objects intended to be delineated by the microscope. He found no creature more troublesome to draw than the ant or pismire, not being able to get the body quite in a natural posture. If, when alive, its feet were fettered with wax or glue, it would so twist and twine its body, that it was impossible any way to get a good view of it; if it was killed, the body was so small, that the shape was often spoiled before it could be examined. It is the nature of many minute bodies, when their life is destroyed, for the parts to shrivel up immediately; this is very observable in many small plants, as well as in insects; the surface of these small bodies, if porous, being affected by almost every change of the air, and this is particularly the case with the ant. But if the little creature be dropped in well rectified spirit or wine, it is immediately killed; and when taken out, the spirit of wine evaporates, leaving the animal dry and in its natural posture, or at least so constituted, that you may easily place it with a pin in what posture you please.[42]
[42] Hooke’s Micrographia, p. 203.
Having thus given a general account of the methods used by Swammerdam and Lyonet, in their examination and dissection of insects, we shall proceed to shew how to prepare several of their parts for the microscope, beginning with the WINGS. Many of these are so transparent and clear, as to require no previous preparation; but the under wings of those that are covered with elytra, or crustaceous cases, being constantly folded up when at rest, they must be unfolded before they can be examined by the microscope; for this purpose a considerable share of dexterity and some patience is necessary, for the natural spring of the wings is so strong, that they immediately fold themselves again, except they are carefully prevented.
One of the most curious and beautiful wings of this kind, is that of the FORFICULA AURICULARIA, or EARWIG, of which we have given a drawing, [Plate XIV.] Fig. 1, represents it considerably magnified, and Fig. 2, the same object of its natural size. When expanded, it is a tolerably large wing, yet folds up under a case not one-eighth part of its size. It is very difficult to unfold these wings, on account of their curious texture. They are best opened immediately after the insect is killed. Hold the earwig by the thorax, between the finger and thumb; then with a blunt-pointed pin endeavour gently to open the wing by spreading it over the fore-finger, gradually sliding at the same time the thumb over it. When fully expanded, separate it from the insect by a sharp knife, or a pair of scissars. The wing should be pressed for some time between the thumb and finger before it be removed; it may then be placed between two pieces of paper, and again pressed for at least an hour; after which it may be put between the talcs without any danger of folding up again.
The wings of the NOTONECTA, or BOAT-FLY, and other water insects, as well as most species of the grylli, require equal care and delicacy with that of the earwig to display them properly.