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.