I have already, under another head[912], considered the annual sleep, or winter state of torpidity of insects, during which an intermission for the most part of muscular motion and action takes place. I shall now make a few observations with respect to their diurnal sleep, which may very properly have its place in the present letter. That insects, usually so incessantly busy and moving in every direction, require their intervals of repose, seems to call for no proof. We see some that appear only in the day, and others only in the night, others again only at certain hours; which leads to the conclusion, that when they withdraw from action and observation, it is to devote themselves to rest and sleep. The cockchafer flies only in the evening; but if you chance to meet with it roosting in a tree in the earlier part of the day, you will find it perfectly still and motionless, with its antennæ folded and applied to the breast:—we cannot indeed say that its eyes are shut; for as insects have no eyelids, that sign of sleep can never be found in them. Again, if a Lepidopterist goes into the wood to capture moths in the day-time, he finds them often perched on the lichens that cover the north side of the trunk of a tree, with their wings and antennæ folded, and themselves without motion, and insensible of his approach and their own danger. Thus it was that I captured that rare insect the lobster-moth (Stauropus Fagi) in the New Forest. Some, however, have asserted that the caterpillar of the silkworm, except when they moult, never intermits feeding day or night, and consequently does not sleep: but the accuracy of this statement, both from analogy and observation, admits of great doubt. Malpighi informs us that these caterpillars for an hour and more, twice a day, remain immoveable with their heads bent down as though asleep, and even if disturbed, resume again the same inactive posture[913]; and other larvæ in great numbers certainly seem to have regular intermissions from eating of considerable duration: those called Geometers, for hours together remain motionless projected from a twig, to which they adhere by their posterior prolegs alone; and the processionary caterpillars make only nightly sorties from their nests, passing the day in inaction and repose[914]. Bees have been often seen by Huber, when apparently wearied with exertion, even in the middle of the day, to insert the half of their bodies into an empty cell, and remain there, as if taking a nap, without motion for half an hour or longer[915]; and at night they regularly muster in a state of sleep-like silence. Mr. Brightwell once observed an individual living specimen of Haltica concinna, which appeared to remain motionless on the same spot of a wall for three successive days.
Before concluding these remarks on the Internal Anatomy and Physiology of Insects, I shall explain to you, as you will probably feel inclined occasionally to pursue the subject, the best mode of dissecting them.—By far the most useful dissecting instruments for this purpose are very fine-pointed and sharp scissors, as these will enable you to divide the integument and separate other parts with much less risk of injuring their delicate structure than any knife. These scissors are what Swammerdam chiefly used; and he had some so extremely small and fine, that he was necessitated to employ a lens when he sharpened them. If to these be added a sharp and fine-pointed knife or two, some needles fixed in handles, also fine-pointed—(you will find them more convenient than any other instrument for detaching minute parts and fibres,) a pair of fine and accurately adjusted pliers, and an assortment of camel's-hair brushes,—you will be nearly set up as an Entomological dissector. You will still, however, require a small dissecting table, with a projecting and moveable arm for lenses of various descriptions, so as to admit both the hands to be employed upon the subject under examination; and for this purpose probably no contrivance can be better adapted than that of Lyonet, of which the figure in Adams On the Microscope will convey a better idea than any description[916].
Previously to dissecting any insect, it must be killed by plunging it into boiling water, which is recommended by Lyonet, or spirits of wine or of turpentine; and it is often useful to let larvæ remain a few days in the latter, by which means the vessels become firmer and stronger. The parts of pupæ become much more distinct if they are boiled for a few minutes: and the same mode may be adopted in the examination of spiders.
The most convenient mode of proceeding, which was that also of Lyonet, is to dissect the insect in water, or, to avoid putridity, in diluted spirits,—if small, upon a concave glass, to which it should be fastened by means of a little melted wax; if larger, in the bottom of a common chip box, surrounded with a border of wax to retain the fluid. The integuments of the insect, being carefully divided longitudinally with scissors, should if flexible be turned back, and fixed by small pins stuck in by a fine pair of pliers, while the skin at the same time is stretched by another. After making such observations as present themselves without further dissection, the viscera must be cautiously extracted, washing away the fat which surrounds them with spirits of turpentine, in which it is soluble, applied by camel's-hair pencils. After separation they may conveniently be examined by putting them into water, and gently shaking them so as to cause the parts to unfold. If endowed with the patience of Swammerdam, you may even arrive at injecting these minute parts with wax or coloured fluids, conveyed by delicate glass tubes having one end as fine as a hair, which he also employed to fill the viscera with air; and afterwards drying them in the shade, and anointing them with oil of spike in which a little resin had been dissolved, he succeeded in preserving them. If it is not convenient to finish the dissection of an insect at once, it should be covered with spirits of wine. Swammerdam found a mixture of spirits and distilled vinegar very useful for keeping caterpillars previously to dissecting them, as it consolidated the parts[917].
And now having brought to a close my long wanderings in this ample and intricate field, and having threaded, as well as my slender powers and limited knowledge enabled me, the infinite turnings and convolutions of this Dædalean labyrinth—the Anatomy and Physiology of insects,—will you not own that the volume of wonders I have laid before you proves irrefragably that, though these minims of nature apparently rank so low in the scale of being, yet in their structure, instead of being, as might be expected, more simple, they are infinitely more complex and highly wrought than those animals that are placed the nearest to ourselves? the Creator in the latter doing every thing by a beautiful simplicity; while in the former, the more to magnify his power and skill, because they afford no apparent space for it, by a wonderfully curious and intricate multiplicity: and whether we study the one or the other, we shall in both trace the footsteps of that adorable Love which has shown attention to the comfort and well-being of the lowest insect, as well as of the highest of his creatures.
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