« VorigeDoorgaan »
ular system, which supplies nutrition, growth, and life, to all of them together.
VI. Almost all insects are oviparous. Nature keeps her butterflies, moths, and caterpillars, locked up during the winter in their egg state; and we have to admire the various devices to which, if we may so speak, the same nature hath resorted, for the security of the egg. Many insects enclose their eggs in a silken web; others cover them with a coat of hair torn from their own bodies; some glue them together; and others, like the moth of the silkworm, glue them to the leaves upon which they are deposited, that they may not be shaken off by the wind, or washed away by rain: some again make incisions into leaves, and hide an egg in each incision; whilst some envelope their eggs with a soft substance, which forms the first aliment of the young animal: and some again make a hole in the earth, and having stored it with a quantity of proper food, deposit their eggs in it. In all which we are to observe, that the expedient depends, not so much upon the address of the animal, as upon the physical resources of his constitution.
The art also with which the young insect is coiled up in the egg, presents, where it can be examined, a subject of great curiosity. The insect, furnished with all the members which it ought to have, is rolled up into a form which seems to contract it into the least possible space; by which contraction, notwithstanding the smallness of the egg, it has room enough in its apartment, and to spare. This folding of the limbs appears to me to indicate a special direction, for, if it were merely the effect of compression, the collocation of the parts would be more various than it is. In the same species, I believe, it is always the same.
These observations belong to the whole insect tribe, or to a great part of them. Other observations are limited to fewer species; but not, perhaps, less important or satis. factory.
1. The organization in the abdomen of the silkworm, or spider, whereby these insects form their thread, is as incontestably mechanical as a wire-drawer's mill. In the body of the silkworm are two bags, remarkable for their form, position, and use. (Pl. XXXIII. fig. 1.) They wind round the intestine; when drawn out, they are ten inches in length, though the animal itself be only two. Within these bags is collected a glue; and communicating with the bags, are two paps or outlets, perforated, like a giater, by a number of snall holes. The glue or gum, being pass
ed through these minute apertures, form hairs of almost imperceptible fineness; and these hairs, when joined, compose the silk which we wind off from the cone, in which the silkworm has wrapped itself up: in the spider, the web is formed from this thread. In both cases, the extremity of the thread, by means of its adhesive quality, is first attached by the animal to some external hold; and the end being now fastened to a point, the insect, by turning round its body, or by receding from that point, draws out the thread through the holes above described, by an operation, as hath been observed, exactly similar to the drawing of wire. The thread, like the wire, is formed by the hole hrough which it passes. In one respect there is a difference. The wire is the metal unaltered, except in figure. In the animal process, the nature of the substance is somewhat changed as well as the form;. for, as it exists within the insect, it is a soft clammy gum or glue. The thread acquires, it is probable, its firmness and tenacity from the action of the air upon its surface, in the moment of exposure; and a thread so fine is almost all surface. This property, however, of the paste is part of the contrivance (Pl. XXXIII. fig. 2.]
The mechanism itself consists of the bags or reservoirs into which the glue is collected, and of the external holes communicating with these bags: and the action of the machine is seen in the forming of a thread, as wire is formed, by forcing the material already prepared through holes of proper dimensions. The secretion is an act too subtile for our discernment, except as we preceive it by the produce. But one thing answers to another; the secretory glands to the quality and consistence required in the secreted substance; the bag to its reception: the outlets and orifices are constructed, not merely for relieving the reservoirs of their burden, but for manufacturing the contents into a form and texture, of great external use, or rather indeed of future necessity, to the life and functions of the insect.
II. · Bees, under one character or other, have furnished every naturalist with a set of observations. I shall in this place confine myself to one; and that is, the relatior which obtains between the wax and the honey. No per. son who has inspected a bee-hive, can forbear remarking how commodiously the honey is bestowed in the comb, and amongst other advantages, how effectually the fermenta tion of the honey is prevented by distributing it into smalı cells. TYze fact is, that when the honey is separated from
the comb, and put into jars, it runs into fermentation, with a much less degree of heat than what takes place in a hive. This may be reckoned a nicety; but, independently of any nicety in the matter, I would ask, what could the bee do with the honey if it had not the wax? how, at least, could it store it up for winter? The wax, therefore, answers a purpose with respect to the honey; and the honey constitutes that purpose with respect to the wax.
This ia the relation between them. But the two substances, though together of the greatest use, and without each other of little, come from a different origin. The bee finds the honey, but makes the wax. The honey is lodged in the nectaria of flowers, and probably undergoes little alteration; is merely collected: whereas the wax is a ductile, tenacious paste, made out of a dry powder, not simply by kneading it with a liquid, but by a digestive process body of the bee. What account can be rendered of facts so circumstanced, but that the animal, being intended to feed upon honey, was, by a peculiar external configuration, enabled to procure it? that, moreover, wanting the honey when it could not be procured at all, it was farther endued with the no less necessary faculty of constructing repositories for its preservation? which faculty, it is evident, must depend primarily, upon the capacity of providing suitable materials. Two distinct functions go to make up the ability. First, the power in the bee, with respect to wax, of loading the farina of flowers upon its thighs. Microscopic observers speak of the spoon-shaped appendages with which the thighs of bees are beset for this very purpose; but, inasmuch as the art and will of the bee may be supposed to be concerned in this operation, there is, secondly, that which doth not rest in art or will--a digestive faculty which converts the loose powder into a stiff substånce. This is a just account of the honey and the honey-comb, and this account, through every part, carries a creative intelligence along with it.
The sting also of the bee has this relation to the honey, that it is necessary for the protection of a treasure which invites so many robbers.
III. Our business is with mechanism. In the panorpet tribe of insects, there is a forceps in the tail of the male insect, with which he catches and holds the female. [Pl. XXXIII. fig. 3.) "Are a pair of pincers more mechanical than this provision in its structure? or is any structure more clear and certain in its design?
IV. St. Pierre tells us,* that in a fly with six feet, (! do not remember that he describes the species,) the pair next the head and the pair next the tail, have brushes at their extremities, with which the fly dresses, as there may be occasion, the anteror or the posterior part of its body; but that the middle pair have no such brushes, the situation of these legs not admitting of the brushes, if they were there, being converted to the same use.
This is a very exact mechanical distinction.
V. If the reader, looking to our distributions of science; wish to contemplate the chemistry, as well as the mechanism of nature, the insect creation will afford him an example. I refer to the light in the tail of a glow-worm. Two points seem to be agreed upon by naturalists concerning it: first, that it is phosphoric; secondly, that its use is to attract the male insect. The only thing to be inquired after, is the singularity, if any such there be, in the natural history of this animal, which should render a provision of this kind more necessary for it than for other insects. That singularity seems to be the difference which subsists between the male and the female; which difference is greater than what we find in any other species of animal whatever. The glow-worm is a female caterpillar; the male of which is a fly; lively, comparatively small, dissimilar to the female in appearance, probably also as distinguished from her in habits, pursuits, and manners, as he is unlike in form and external constitution. [Pl. XXXIII. fig. 4, 5.) Here then is the adversity of the case. The caterpillar cannot meet her companion in the air. The winged rover disdains the ground. They might never therefore be brought together, did not this radiant torch direct the volatile mate to his sedentary female.
In this example, we also see the resources of art anticipated. One grand operation of chemistry is the making of phosphorus: and it was thought an ingenious devise, to make phosphoric matches supply the place of lighted tapers. Now this very thing is done in the body of the glow-worm. The phosphorus is not only made, but kindled; and caused to emit a steady and genial beam, for the purpose which is here stated, and which I believe to be the true one.
VI. Nor is the last the only instance that entomology affords, in which our discoveries, or rather our projects, turn ou to be imitations of nature. Some years ago, a
* Vol. i. p. 342
plan was s..ggested, * producing propulsion by reaction in this way: By the force of a steam-engine, a stream of water was to be shot out of the stern of a boat; the impulse of which stream upon the water in the river, was to push the boat itself forward; it is, in truth, the principle by which sky-rockets ascend in the air.
Of the use or practicability of the plan, I am not speaking; nor is it my concern to praise its ingenuity: but it is certainly a contrivance. Now, if naturalists are to be believed, it is exactly the device which nature has made use of, for the motion of some species of aquatic insects. The larva of the dragon-fly, according to Adams, swims by ejecting water from its tail; is driven forward by the reaction of water in the pool upon the current issuing in a direction backward from its body. [Pl. XXXIII. fig. 6.]
VII. Again: Europe has lately been surprised by the elevation of bodies in the air by means of a balloon. The discovery consisted in finding out a manageable substance, which was, bulk for bulk, lighter than air; and the application of the discovery was, to make a body composed of this substance bear up, along with its own weight, some heavier body which was attached to it. This expedient, so new to us, proves to be no other than what the author of nature has employed in the gossamer spider. We frequently see this spider's thread floating in the air, and extended from hedge to hedge, across a road or brook of four or five yards width. The animal which forms the thread has no wings wherewith to fly from one extremity to the other of this line; nor muscles to enable it to spring òr dart to so great a distance: yet its Creator hath laid for it a path in the atmosphere; and after this manner. Though the animal itself be heavier than air, the thread which it spins from its bowels is specifically lighter. This is its balloon. The spider, left to itself, would drop to the ground; but being tied to its thread, both are supported. We have here a very peculiar provision: and to a contemplative eye it is a gratifying spectacle, to see this insect wafted on her thread, sustained by a levity not her own, and traversing regions, which, if we examined only the body of the animal, might seem to have been forbidden to its nature.
I must now crave the reader's permission to introduce into this place, for want of a better, an observation or two upon the tribe of animals, whether belonging to land or water, which are covered by shells.
I. The shells of snails are a wonderful, a mechanical,