OF THE LIBELLULA OR DRAGON FLY.
In the libellula we have an instance of those insects which are termed in the pupa state, semicompleta, that is, such as proceed from the egg in the figure which they preserve till the time arrives for assuming their wings; and who walk, act, and eat as well before that period as afterwards.
Of all the flies which adorn or diversify the face of nature, there are few, if any, more beautiful than the libellulæ: they are almost of all colours, green, blue, crimson, scarlet, and white; some unite a variety of the most vivid teints, and exhibit in one animal more different shades than are to be found in the rainbow. It is not to colour alone that their beauty is confined, it is heightened by the brilliancy of their eyes, and the delicate texture and wide expansion of their wings. The larva of the libellula is an inhabitant of the water, the fly itself is generally found hovering on the borders thereof.
These insects are produced from an egg, which is deposited in the water by the parent; the egg sinks to the bottom, and remains there till the young insect finds strength to break the shell. The larva is hexapode, and is not quite so long as the fly; on the trunk are four prominences or little bunches, which become more apparent, in proportion as the larva increases in size and changes its skin. These bunches contain the rudiments of the wings, which adorn the insect when in its perfect state.
The head of the larva is exceedingly singular, the whole fore part of it being covered with a mask, which fits it more exactly than the common mask does the human face, having proper cavities within to suit the different prominences of the face; it is of a triangular form, growing smaller towards the bottom; at this part there is a knuckle which fits a cavity near the neck, on this it turns as on a pivot. The upper part of this mask is divided into two pieces or shutters, which the insect can open or close at pleasure; it can also let down the whole mask whenever it pleases. The edges of the shutters are jagged like a saw. It makes use of the mask to seize and hold its prey. There is a considerable difference in the shape of these masks in different species of the libellula, some having two claws near the top of it, which they can thrust out or draw in, as most convenient; these render it a very formidable instrument to the insects on which they feed.
These animals generally live and feed at the bottom of the water, swimming only occasionally: their manner of swimming, or rather moving in the water, is curious, being by sudden jerks given at intervals; but this motion is not occasioned by their legs, which at this time are kept immoveable and close to the body; it is by forcing out a stream of water from the tail that the body is carried forward; this may be easily perceived, by placing them in a flat vessel, in which there is only just water enough to cover the bottom. Here the action of the water squirted from their tail will be very visible; it will occasion a small current, and give a sensible motion to any light bodies that are lying on the surface thereof. This action can only be effected at intervals, because after each expulsion the insect is obliged to inhale a fresh supply of water. The larva will sometimes turn its tail above the surface of the water, and eject a small stream from it as from a little fountain, and that with considerable force.
The pupa differs but very little from the larva, the bunches containing the wings grow large, and begin to appear like four short thick wings. It is full as lively as the larva, seeking and enjoying its food in the same manner: when it is arrived at its full growth, and is nearly ready to go through its last change, it approaches the edge of the water, or comes entirely out of it, fixing itself firmly to some piece of wood or other substance, by its acute claws. It remains for some time immoveable; the skin then opens down the back, and on the head; through this opening is exhibited the real head and eyes, and at length the legs; it then creeps gradually forward, drawing its wings, and then the body out of the skin. The wings, which are moist and folded, now expand themselves to their real size; the body is also extended till it has gained its proper dimensions, which extension is accomplished by the propelling force of the circulating fluids. When the wings and limbs are dry, it enters on a more noble state of life: in this new scene it enjoys itself to the fullest extent, feasts on the living fragrance issuing from innumerable openings, sports and revels in delight, and, having laid the foundation for its future progeny, sinks into an easy dissolution.
The dragon fly is of a ferocious and warlike disposition, hovering in the air like a bird of prey, in order to feed on and destroy every species of fly; its appetite is gross and voracious, not confining itself to small flies only, but the large flesh-fly, moths, and butterflies, are equally subjected to its tyranny. It frequents marshy grounds, where insects mostly abound.
The female of the CYNIPS or GALL INSECT, which has no wings, passes through no transformation; while the male, which has four wings, passes through the pupa state before it becomes a fly. The only change, though a considerable one, which takes place in the female gall insect, is this, that after a certain time it fixes itself to the branch of a tree, without being able to detach itself; it afterwards increases much in size, and becomes like a true gall; the female, by remaining thus fixed for the greater part of her life, to the place where she was first seen, has very little the appearance of an animal; it is in this period of their life that they grow most and produce their young, while they appear a portion of the branch they adhere to; and what is more singular, the larger they grow, the less they appear like animals, and whilst they are employed in laying thousands of eggs, seem to be nothing but mere galls. The genera of gall insects are very extensive; they are to be found on almost every shrub and tree.
The APHIDES or PLANT LICE, to arrive at their respective state, pass through that of the semicomplete pupa, and their wings do not appear till they have quitted their pupa state; but as in all the families of the pucerons there are many which never become winged, we must not forget to observe, that these undergo no transformation, remaining always the same, without changing their figure, though they increase in size and change their skin. It is remarkable, that amongst insects of the same kind, some individuals should be transformed, while others are not at all changed. These insects will be considered more fully in another part of this chapter.
Reaumur[77] has shewn that the SPIDER FLY, hyppobosca equina, Lin. lays so large an egg, that the fly which proceeds from it is as big as the mother, though the egg does not increase the least in size from the time it is first laid. The insect proceeds also from the egg in the imago or fly state; it is probably transformed in the egg, for Reaumur has found it in the pupa state therein, and having boiled some of their eggs which had been laid for some days, he found the insect in the form of an oval ball, similar to that in which the pupa of flies with two wings are generally found. De Geer is of opinion that the egg itself is a true larva, which, when it is born, has only to disengage its limbs, &c. from the shell which covers it; and he thinks this the more probable, because there is no embryo seen in this egg, but it is entirely filled with the insect; he has also perceived a contracting and dilating motion in the egg, while it was in the belly of the mother, and immediately after it was laid; circumstances which do not agree with a simple egg.
[77] Reaumur, tom. 6, mem. 14.
As M. Bonnet[78] has attempted to give a theory of these various changes, the following extract from it will, I hope, prove agreeable to the reader; it will at least tend to render his ideas of this wonderful subject clearer, and will probably open to his mind many new sources of contemplation.
[78] Bonnet Considerations sur les Corps organises. Contemplation of Nature, &c.
An insect that must cast off its exuvia, or moult five times before it attains the pupa state, may be considered as composed of five organized bodies, inclosed within each other, and nourished by common viscera, placed in the center: what the bud of the tree is to the invisible buds it contains, such is the exterior part of the caterpillar to the interior bodies it conceals in its bosom. Four of these bodies have the same essential structure, namely, that which is peculiar to the insect in its larva or caterpillar state: the fifth body is that of the pupa. The respective state of these bodies is in proportion to their distance from the center of the animal; those that are farthest off have most consistence, or unfold themselves soonest. When the exterior body has attained its full growth, that interior one which is next in order is considerably unfolded; it is then lodged in too narrow a compass, therefore it stretches on all sides the sheath which covers it; the vessels which nourish the external covering, are broken by this violent distension, and ceasing to act, the skin wrinkles and dries up; at length it opens, and the insect is cloathed with a new skin, and new organs. The insect generally fasts for a day or two preceding each change; this is probably occasioned by the violent state in which it then is, or it may be necessary to prevent obstructions, &c. let this be as it may, the insect is always very weak after it has changed its skin, the parts being as yet affected by the exertions they have gone through. The scaly parts, as the head and legs, are almost entirely membranaceous, and imbrued with a fluid that insinuates itself between the two skins, and thus facilitates their separation; this moisture evaporates by degrees, all the parts acquire a consistence, and the insect is then in a condition to act.
The first use that some caterpillars which live on leaves make of their new form, is to devour greedily their exuvia: sometimes they do not wait till their jaws have acquired their full strength; some have been seen to gnaw the shell from which they proceeded, and even the eggs of such caterpillars as have not been hatched.
When we have once formed the idea that all the exterior parts are inlaid, or included one within the other, the production of new organs does not appear so embarrassing, being nothing more than a simple developement; but it is more difficult to form any conception of the changes that happen in the viscera before and after the transformation, the various modifications they undergo eluding our researches. We have already observed, that a little before the change the caterpillar rejects the membrane that lines the intestinal bag: this bowel has hitherto digested only gross food, whereas it must hereafter digest that which is very delicate: a fluid that circulates in the caterpillar from the hind part towards the head, circulates a contrary way after transformation. Now if this inversion is as real as observation seems to indicate, how amazing the change the interior parts of the animal must have undergone? When the caterpillar moults, small clusters of the tracheal vessels are cast off with the exuvia, and new ones are substituted in their room; but how is this effected, and how are the lungs replaced by other lungs? The more we endeavour to investigate this subject, the more we find it is enveloped in darkness.
Whilst the powers of life are employed conformable to the laws of Divine Providence, to change the viscera, and give them a new form, they are also unfolding divers other organs, which were useless to the insect while in the larva state, but which are necessary to that which succeeds. That these interior operations of life may be carried on with greater energy, the animal is thrown into a kind of sleep; during this period, the corpus crassum is distributed into all the parts, in order to bring them to perfection, while the evaporation of the superfluous humours makes way for the elements of the fibres to approach each other, and unite more closely. The little wounds in the inside, which have been occasioned by the rupture of the vessels, are gradually consolidated; those parts which had been violently exercised, recover their tone, and the circulating fluids insensibly find their new channel. Lastly, many vessels are effaced, and turned into a liquid sediment, which is rejected by the perfect insect.
When these various changes are considered, we are surprized at the singularity of the means the Author of nature has made choice of, in order to bring the different species of animals to perfection; and are apt to ask, why the caterpillar was not born a moth? why it passes through the larva and pupa state? why all insects that are transformed do not undergo the same change? These, and a variety of questions that may be started concerning the constituent substances of those existences which appear before us, derive their solution from the general system which is unknown to us. If all were to arrive at perfection at once, the chain would be broken, the creature unhappy, and man most of all. Let us also consider what riches we should have been deprived of, if the silk-worm had been born in its perfect state.
Amongst insects, some are produced in the state in which they will remain during their whole lives; others come forth inclosed in an egg, and are hatched from this into a form that admits of no variation; many come into the world under a form which differs but little from that which they have when arrived at an age of maturity; some again assume various forms, more or less remote from that which constitutes their perfect state; lastly, some go through part of these transformations in the body of the parent, and are born of an equal size with them. By these various changes, a single individual unites within itself two or three different species, and becomes successively the inhabitant of two or three worlds: and how great is the diversity of its operation in these various abodes!
Since it has been shewn that the larva or caterpillar is really the moth, crawling, eating, and spinning, under the form of the worm, and that the pupa is only the moth swathed up, it is clear that they are not three beings, but that the same individual feels, tastes, sees, and acts by different organs, at different periods of its life, having sensations and wants at one time, which it has not at another; these always bearing a relation to the organs which excite them.