This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
You will probably admit that your own visage would present an appearance not very engaging while concealed by such a mask: but it would strike still more awe into the spectators were they to see you first open the two upper jaw-like plates, which would project from each temple like the blinders of a horse; and next, having, by means of the joint at your chin, let down the whole apparatus and uncovered your face, employ them in seizing any food that presented itself and conveying it to your mouth. Yet this procedure is that adopted by the larvae provided with this strange organ. While it is at rest, it applies close to and covers the face. When the insects would make use of it, they unfold it like an arm, catch the prey at which they aim by means of the mandibuliform plates (fig. 147, a), and then partly refold it, so as to hold the prey to the mouth in a convenient position for the operation of the two pairs of jaws with which they are provided".
* Mem. de la Soc. d'Hist. Nat. p. 48.
(915). The metamorphoses of the Gnat (Culeoc) are not less interesting. The female deposits her eggs upon the surface of the water, in which her offspring are destined to pass the earlier periods of their existence, gluing the ova together at the moment of their extrusion, so as to unite them into a boat-like mass (fig. 177, a) of such beautiful construction that the little bark swims secure from injury even during the roughest weather. The individual eggs are of a conical form (fig. 177, b, a, b, c), and are closed at their inferior extremity by a kind of lid (d), provided to give egress to the mature embryo. The larva (c), represented upon a magnified scale at e, bears not the slightest resemblance to the perfect insect, and is provided with a singular modification of the respiratory apparatus adapted to its habits. The head is large, and carries two ciliated organs (g g), which by their movements bring food towards the mouth: the thorax is even lamer than the head, and is furnished with fin-like bunches of minute hairs, as likewise are the segments of the abdomen. To the extremity of the tail is appended a group of moveable leaflets or fins, so disposed that by their action they sustain the larva at the top of the water, where it generally remains suspended with its head downwards. Such a position would obviously render respiration impossible was there not a corresponding arrangement of the breathing organs to allow of free communication with the air. For this purpose, the respiratory tracheae are found to be connected with a tube appended to the antepenultimate segment of the abdomen, the perforated extremity of which, being raised above the water, procures from the atmosphere the oxygen required for respiration.
After several moults, the larva, having attained its full growth, enters the pupa state, and in this condition still remains an inhabitant of the water, and occupies a position near the surface. A remarkable change, however, is visible in all parts of its structure. The head and thorax (fig. 177, d) are consolidated into one large mass, under which the lineaments of the mature insect may be detected; while the tail still continues to be the agent employed in natation. The condition of the respiratory organs, moreover, is completely altered: the tube fixed upon the antepenultimate segment of the larva has totally disappeared, and instead of it we find two tubes appended to the back of the thorax; these, although they perform the same office as the anal pipe of the larva, are thus displaced in order to correspond with the altered position in which the animal now swims, - the back of the thorax, and not the tail, being nearest to the surface, as represented in the drawing (d.) The necessity for this change of posture, and consequent removal of the apparatus for taking in air from one part of the body to another, will be at once obvious when we consider the circumstances under which the perfect insect, having completed its development, emerges from its pupa investments and enters upon an aerial existence.
The problem to be solved is, how shall the mature gnat escape from the water without being wetted? and when we consider that neither the larva nor the pupa possesses instruments of locomotion capable of enabling it to leave its native element by crawling on shore, the difficulties attending the change appear almost insurmountable. It is evident that, while swimming in the position in which the larva floats (fig. 177, c), the last change could not by possibility be accomplished, as the bursting of the integument would at once admit the water to the submerged gnat, and drown it at the moment of its birth; but by the new arrangement the metamorphosis is easily effected, and that in a manner so beautiful, that it is hard to say which is most admirable, the simplicity of the contrivance, or the perfection with which the object is accomplished. No sooner has the encased imago become fitted for its escape than the pupa, rendered more buoyant, raises its back above the surface; the protruded portion of the pupa-case soon dries, and gradually begins to split in a longitudinal direction, so as to form by its expansion a boat wherein the gnat swims upon the top of its native pond; and sustained in this frail bark, formed by its late skin, it gradually extricates its legs and wings from their coverings, and is kept perfectly dry until the expansion of its instruments of flight enables it to soar into the air and quit for ever the raft so singularly provided for its use.
* Introd. to Entom. vol. iii. p. 126.
Fig. 177. Metamorphoses of the Gnat (Culex pipiens): - A, Boat of eggs. B, a, b, c, some of the eggs magnified; d, another, showing the lid open for the escape of the larva. C, Larva. D, Pupa. E, Larva magnified, showing - e, the respiratory tube; f, the anal fins; gg, the antennae. F, the perfect insect, magnified: a a, antennae; 6, rostrum.