GNAT-WORM, in natural history, a small aquatic insect produced from the egg of a gnat, and which is after its several changes again transformed into a gnat.
These worms do not frequent rivers, but ditches, ponds, and other stagnant waters; where they are found in vast abundance from the middle of May till towards the beginning of winter. This is the reason why watery and marshy places are found most to abound with gnats, and why the wet summers are found to produce the greatest numbers of them; because in dry seasons the ponds and ditches where they are to pass their worm-state are dried up, and the worms killed. These are creatures, however, that one need not go far to seek; since one need only expose a vessel of water in a garden, or any open place, in the summer time, and sooner or later it will not fail to produce plenty of them.
Before they arrive at their full growth, though they are then but small, they are easily found; because they are under a necessity of coming often to the top of the water by having occasion for frequent respiration; and to do this, they are obliged to keep the end of a small pipe they are furnished with from the last ring of their body above water. The end of this pipe is hollow and indented, and forms a sort of funnel upon the surface of the water. It is of the length of about three rings of the body, and is somewhat thicker at its insertion than at its extremity.
The worm is of the third class of those which are transformed into two-winged flies: that is, it has no legs, and has a head of a constant and invariable figure; and has no teeth or moveable jaws formed to play against one another. Their body is long, and their
head is somewhat detached from the first ring, to which it is fastened by a sort of neck. This first ring is the longest and largest of all, and seems a sort of corcelet to the worm. The creature has eight rings besides this. These grow smaller as they approach the hinder extremity.
While the worm is young, the body is whitish or greenish; but when it is at its full growth, and draws near the time of its change, it becomes greyish. The great transparency of the body of this worm gives a fine view of what passes within it; and it is at any time easy to see the motion of the intestines by which the food is pushed on towards the anus. The two principal tracheæ are also seen very distinctly in this creature. They are two white tubes placed in a direction parallel to one another, and run from the first ring to the tube of respiration.
This worm several times changes its skin in the course of its life. After three changes of this kind, which usually happen in the space of three weeks, it undergoes a fourth, in which the old skin is as easily thrown off as before; but the animal now appears in a new form, viz. that of a nymph. It is now shorter and rounder than before; and the body is so bent, that the tail is now applied to the under part of the head: this, however, is only its form in a voluntary state of rest, for it can yet move; and, when it pleases, extends its tail, and swims as swiftly as before.
All the parts of the future gnat may be seen in this nymph; the skin of it is extremely thin and transparent, yet sufficiently tough and firm for the use for which it is intended. It is uncertain how long the animal lives in this nymph state; but after the time is accomplished, its change into the gnat is very quick, and attended with great danger to the animal, since multitudes of them are drowned in the act of getting out.