GNAT, in zoology. See Musca.
There is no species of insects so particularly troublesome to mankind as the gnats. Others give more pain with their stings, but it is only by accident that we are struck by them; but the gnats thirst for our blood, and follow us about in whole companies for it. There are many marshy places in our country where the legs and arms are all the summer swelled to an enormous size by the bitings of these insects, and in many other countries they are yet more troublesome than with us.
All the naturalists have of late years employed the microscope to examine the parts of this little animal; and Swammerdam, Hooke, Bonanni, Lewenhoek, &c. have given very good accounts and very valuable drawings of the creature.
The trunk of the gnat, or the instrument with which it strikes the flesh and sucks the blood from animal bodies, is of a very curious construction. The piercer, or, more properly, the piercers of this instrument are all entirely hid in the sheath of the instrument which makes what we call the trunk, and is the only part naturally offered to our view. This trunk appears to be cylindric in the greatest part of its length; and is covered with scales not unlike those on the nerves of the creature's wings, and resembling small leaves. Near its end it has a little swelling, where there is an oblong button broader at its insertion than
at its point: the end of this button is furnished with an aperture out of which the creature occasionally thrusts a fine point. Many naturalists have observed this point. Swammerdam considered it as a single pointed body formed to pierce the skin; but Lewenhoek discovered that it was made up of a vast number of pointed bodies. There is no occasion, however, for all the accuracy of Swammerdam, nor all the power of the magnifying-glasses of Lewenhoek, to discover that this is a complex body; a common small magnifying glass and a little attention will at any time discover it.
If a gnat is held by the corcelet between the fingers, and a little squeezed, the trunk will frequently be seen to open lengthwise on each side; sometimes only a little way, at others almost its whole length; and a fine glossy reddish filament shews itself at the opening of this case. This filament is bent and turned inwards, and one very soon distinguishes that it is indeed a congeries of a great number of filaments: these one may easily separate in some measure, from one another by any pointed instrument; and very often the filament of itself separates into several in the bending. It is plain, therefore, that the instrument destined to pierce the skin and suck the blood is of a complex structure; that what we might naturally take for this instrument, is only its case or sheath; and that this case or sheath, instead of a plain cylindric body is really a composition of two semi-cylindric ones, which the animal has the power of separating from one another on proper occasions.
The best way to get a regular sight of the trunk of this creature and of the manner of using it, is to suffer a gnat to settle upon the hand, and not to disturb him in the operation; but, with a magnifying glass in the other hand, to observe all his motions. In this case, we may first observe a small and slender point thrust out of the case, and the animal try several different parts of the skin with this sharp instrument. When this is done, it chooses that part which is most easily pierced, and where there lies a vessel underneath capable of furnishing as much blood as it will have occasion to suck. As soon as it has made its choice, the wound is immediately given: and since the point of the compound piercer cannot be protruded so far out of the case as it is necessary it should be to strike to a proper depth, the use of the slit in this case is seen; for while the button at the end of this remains firmly applied to the orifice of the wound, where the piercer is introduced, and supports that feeble and delicate instrument from bending, the case opens at the slit, and its two sides bend to give room to the piercer to penetrate; and, at length, when the piercer is sunk to its utmost depth, the two extremities of each piece touch, and the two sides are brought close together.
The several species of gnats have great variety in their trunks. One of them deserves to be remarked, as having no need of the button at the end of the case, common to all the rest, to support it while it enters the flesh. This has a case on which it rests itself as on a seventh leg; from which it darts a piercer, which without any support is of sufficient strength to penetrate the flesh, and do its office for the animal. This species of gnat has two very long beards placed above its trunk, and terminated by an end covered
Gnat. vered with white scales. What remains of these beards is covered with brown scales, the body of the gnat also is brown, and the corcelet reddish.
Though it is easy to find that the trunk of a gnat is composed of several pieces, yet it is by no means so easy to say what the precise number of them is. The best microscopes often shew the whole a single body, its several parts being so extremely well joined; and when they have been found to be more than one, it is yet extremely difficult to say how many they are. Leuwenhoek believed them to be four in number; and Swammerdam, who at first believed the whole to be a single filament, afterwards thought he discovered six pieces going to its composition.
After separating the piercer of the gnat wholly from its sheath, if it be cut transversely near its base, or insertion in the head, and the section laid upon the plate of a microscope, and there touched with an extremely fine pointed instrument, it may be divided into four, and sometimes into five separate pieces. Two of these may be often seen to come out of a third as out of a canal or tube. The seeming necessity of a tube in this instrument for sucking the blood, has made many ready to persuade themselves that they have actually seen one: but if we follow the analogy of nature in her other works, we shall find there is no absolute necessity for such an organization in this part; since, in the gad-fly, the several pieces of which the piercer is composed are of themselves able to form a tube for the conveying of the blood.
Out of the immense number of gnats that one sees in summer, few can have any chance, even once in their lives, of sucking the blood of the larger animals; the rest, however, are by no means doomed to perpetual famine: the herbs of the field afford them a sufficient nourishment; for the gnats, like many other insects, are partly carnivorous, and partly otherwise, feeding equally on flesh and vegetables.
The wings of gnats are of a very curious structure, and worthy of an attentive observation. It is well known, that, on touching the wings of butterflies, a coloured powder is left upon the fingers; which, tho' to the naked eye it appears a mere shapeless dust, yet when examined by the microscope is found to consist of beautiful and very regularly figured bodies resembling feathers and scales. The generality of flies have nothing of this kind; but a close examination of the wings of the gnat will shew that they are not wholly destitute of them: they are bestowed much more sparingly indeed upon the gnat than on the butterfly; but they are arranged with great regularity.
The wings of the gnat, like those of most other insects, are of a cartilaginous substance, friable, and transparent like a flake of talc; and the circumference and many parts of the inner surface of the wing are strengthened by slender but firm ribs, which are divaricated into several ramifications. These appear to us to be mere strait fibres; but they are probably hollow, and perform the office of vessels for the carrying of fluids or air necessary to the support of the wing, as well as to strengthen it. In the wings of butterflies there are similar ribs, but they are there all hid by the scales: but it is not so in the gnat; for in its wings, as in those of the other flies, these ribs seem naked. The assistance of the microscope, however, shews that they
are not absolutely so in the wings of gnats, but these nerves or ribs, with their several ramifications, look like as many stalks of a plant covered with small oblong leaves. The several scales that are attached to these ribs make acute angles with them, and are directed towards the end of the wing. The number of these scales is very small in comparison with those of the butterfly-clas; but they make a lighter and more elegant ornament. There are some species which have the intermediate spaces of the wing also adorned with these scales; but they are in these only thinly scattered. The intermediate spaces of the wings, when they have no scales are finely wrought and pointed: the inner edges of the wings are always bordered with a row of scales in form of a fringe; which, in some species, is composed of scales all of the same size, and in others is made up of many various lengths: and the exterior edge of the wing, which is surrounded by a rib much thicker and stronger than the interior, is not fringed with a series of scales, but is beset at proper distances with a kind of prickles.
The ordinary shape of the wings of the gnats is that of an oblong battledoor, one end of which is broader and the other more pointed. The narrower end is that from which goes the stalk by which it adheres to the rib. The other end is sometimes more, sometimes less round, and is sometimes a little hollowed in the middle. Some of these are much longer in proportion to their breadth than others; and some of them have their extremity formed into an open crescent. All have a number of fine lines running longitudinally through the whole scale.