Gmt , a name commonly given to the family culicidoe, of the proboscidean division of the order diptera or two-winged insects; the cousin of the French, the mosquito of the United States. The gnats belong to the genus culex (Linn.), which is characterized by a soft, elongated body; long legs; large head and eyes; long, many-jointed antenna?, most plumose in the males; uniform and hairy palpi, longest in the males; a sucking proboscis, formed of a membranous sheath enclosing from two to six sharp bristles or lancets, which take the place of jaws, and whose punctures, therefore, are properly called bites; the side pieces of this apparatus serve not only as suction tubes, but as supporters and protectors of the lancets; wings horizontal, delicate, and many-veined; the winglets, two little scales behind the wings, and moving with them, are small; behind these are the knobbed balancers or poisers. The old genus culex was divided by Meigen into three, and was by him restricted to such gnats as have the palpi in the males longer than the proboscis, and very short in the females; the other two were anopheles (Meigen), in which the palpi of the males are as long as the proboscis, and cedes (Hoff-mannsegg), in which they are very short in both sexes; to these were afterward added sabethes, with ,palpi shorter than proboscis; megarhinus, with very long recurved proboscis and short palpi; and psorophora, with a small appendage on each side of the prothorax.

Other genera, ill-characterized for the most part, have been added by modern systematise. The names gnat and mosquito are also given in some places to members of the family tipuladoe; and our own mosquitoes belong to several genera, among which is the genus culex, properly confined to the more northern regions of the continent. Dr. Harris mentions five species of culex and one of anopheles as found in New England; to these many species and several genera must be added. Some species are active by day, others only by night, but both are equally fond of human blood; the former are found principally in marshes and damp woods and rarely in houses, and are of more brilliant colors than the nocturnal species. The males with plumed antennas do not annoy us by their bites, but simply flit from flower to flower, sipping the dew and sweet juices, requiring but little if any food, propagating their species, and soon after perishing. The female gnats are most persistent biters and annoying musicians, at almost all seasons of the year; from the tropics to Lapland and arctic America, man is obliged to adopt some contrivance to protect himself from their attacks, either the thick coat of grease of the northern regions, the sand bed of the tropics, the smoky smudge of the woods, or the mosquito bars and curtains of civilized life.

Gnats have been known to appear in such swarms as to constitute an insect plague, darkening the air like clouds of smoke, arresting the progress of invading armies, and rendering whole districts for the time uninhabitable; attacking not only man but beasts, and, even when not biting, filling every crack and corner with their countless multitudes. When we consider the immense number of these insects, and the comparatively small proportion which can ever taste human blood, we must admit, what experiments with sweetened fluids have confirmed, that vegetable juices form the food of the greater number of females, and perhaps the natural food of all; many males probably do not eat at all. The sacking apparatus is admirably contrived for obtaining fluids, animal or vegetable, and these insects are provided with a sucking stomach independent of the proper digestive cavity. The sucker is well described and figured by Reaumur in his "Memoirs;" the flexible sheath gives support to the lancets while they penetrate the skin; the point of the combined lancets is sharper than the finest needle, so that the size of each of the several weapons must be very small; the wounds made by this instrument would be insignificant, were it not for an irritating secretion from the proboscis, which in some delicate skins produces obstinate itching, and, in rare instances, even irritable ulcers. - The metamorphoses which gnats and mosquitoes undergo are very curious.

The eggs are deposited in almost any natural or artificial receptacle for fresh water, and are arranged in a boat-shaped form; fixing herself by the four anterior legs to some object at the surface of the water, the female crosses her hind legs in the form of the letter X; bringing the latter close to the end of the body, on a level with the water, the first egg is received and retained in place by the crossed legs; as the eggs are extruded they are placed side by side vertically, adhering firmly together by the glutinous substance which covers them; when the stern of the egg raft is properly raised, it is pushed further from the body by the succeeding ova, always retained in place by the legs on the sides; when the raft is about half made and its shape is determined, the legs are uncrossed and placed parallel, and the prow of the boat is narrowed and raised like the stern. The boat is always of the same shape, containing from 250 to 850 eggs, and is abandoned by the mother to the mercy of the winds and waves, which can neither sink, wet, nor break it up; even a temperature below freezing cannot destroy the life within these eggs.

The larvae come out in a few days from the lower end of the eggs, which are arranged somewhat like the seeds of the ripe sunflower, and the empty shell boat is soon destroyed by the weather. The larvas of gnats and mosquitoes are the well known "wigglers" seen in warm weather in almost every collection of standing water; they remain, as it were, suspended from the surface of the water, head downward, breathing air by means of a respiratory tube which goes off at an angle from near the end of the body, communicating with the tracheae; the tube and the terminal joint are provided with radiating hairs; the head is round, distinct, with antennas and ciliated organs which keep up a constant current of water toward the mouth, and bring within their reach the minute animalcules on which they feed; the thorax and ten-jointed abdomen are furnished with lateral pencils of hairs. If disturbed, these larva quickly wriggle to the bottom, but soon come again to the surface and suspend themselves by the respiratory tube. Some species are comparatively free from hairs in this condition.

After remaining in the larva state from five to fifteen days, according to the weather, and changing their skins two or three times, they are changed into pupa?, called tumblers from the manner in which they roll over and over in the water by means of the fin-like paddles at the end of the tail; they are very quick in their motions, and swim with the head upward; the respiratory openings are at the end of two tubes situated just behind the head, so that the little tumblers remain near the surface, head upward, to take in air; in this state, which lasts five or ten days, according to circumstances, the insect takes no food; the future gnat can be distinguished through the transparent covering of the pupa. When the perfect insect is ready to come forth, the pupa skin bursts open on the back toward noon on a warm, still, sunny day, and the head of the gnat makes its appearance, followed soon by the thorax; this is a process of great danger to the insect, as the slightest breeze would tip over the emerging form, and consign it to certain death in the water; after it has succeeded in raising its body except the tail, and stands erect like a mast in the pupa shell boat, it extricates the front pair of legs and places them for support on the surface of the water; the heavy and wet wings are now slowly unfolded, that the sun and air may dry them; this effected, the danger is over, and the other legs are drawn forth and extended on the edge of the pupa case, the body is stretched out, the antenna) and proboscis elevated; by this time the wings are dry and fully expanded, and the insect flies off to revel among the flowers or in search of blood, according to the sex.

The source of the buzzing noise has been much discussed by naturalists, and is still the subject of dispute; it has been ascribed to the mouth by Mouffet, to the friction of the base of the wings against the chest by Kirby; the wing-lets, the poisers, the motion of the wings, the rapid passage of air through the thoracic stigmata, and the vibrations of the thorax from the contraction of the muscles of the wings, have been supposed to be the cause by other entomologists; by whatever organ it be produced, Siebold says it is always due to the action of voluntary muscles, and has no connection with the respiratory system. It is probable that the sound is produced by the combined action of the wings and by the thoracic vibrations consequent thereon. It has been estimated by Baron de la Tour that the gnat vibrates its wings 50 times in a second. This very rapid movement probably depends on a peculiar form of muscle which has been detected in the mosquito and other diptera; the fibrillae are not bound together as in ordinary striated muscles, but are separate and parallel, formed by the aggregation in a linear series of little disks with regular interspaces; contraction of these independent fibrillae takes place by the approximation of these disks to each other; some are contracting while others are relaxed, so that a constant and rapid movement of the wings is secured.

It is certainly a remarkable example of the extent of modern microscopic investigation, that the minute muscles of the wings and legs of the mosquito can be dissected and studied. Some of the biting culicidoe do not make a boat of eggs, but string their ova end to end; others deposit them in soft mud or in dry sand; but all require moisture in the larva state. As the eggs are developed into the perfect insects in about three weeks, many broods are hatched in the course of the warm season, fully explaining their occurrence in large numbers; fortunately only a small portion of the pupa3 succeed in extricating themselves from their cases; thousands of them perish by drowning, and are devoured by fish, reptiles, and aquatic insects; the perfect gnats supply food for carnivorous insects, • the great tribe of fly-catching birds, and the bats. - The family of tipuladoe are also called gnats; these are often seen performing their aerial dances during the summer, and in sheltered places even in mild days in winter, preferring the decline of day; these dancing companies are said always to consist exclusively of males; any attempt to intrude upon their sportive circles shows their quickness of vision and of motion, as the whole company is at once removed to a distance.

These gnats sometimes crowd into houses in immense numbers.

1. Female (greatly magnified). 2. Male.

1. Wing of gnat, showing nervures and small cells. 2. Termination of abdomen of male. 3. Termination of abdomen of female. 4, 5, 6. Modes of operation of gnat's sucker. 7. Gnat's eggs. S. Boat of gnat's eggs.