zoology; the name of a genus of insects of the order of aptera, in the Linnaean system. Its body is short, of a roundish figure, and covered with a firm cutaneous skin; the fore-legs are ramose, and serve for leaping and swimming; it has but one eye, which is large, and composed of three smaller ones.

Of this genus, many of which have been reckoned among the microscopic animals, authors enumerate a great number of species. The figure in Plate CCCXV. represents the quadricornis, or four-horned monoculus, a very small species about half a line in length, and of an ash grey colour. From the head arise four antennæ, two forwards and two backwards; all four furnished with a few hairs, which give them the figure of a branch. Between the antennæ, on the fore part of the head, is situated a single eye. From the head to the tail the body goes down, decreasing in shape like a pear; and is composed of seven or eight rings, which grow continually more straitened. The tail is long, divided into two; each division giving rise outwardly to three or four bristly hairs. The animal carries its eggs on the two sides of its tail in the form of two yellowish parcels filled with small grains, and which taken together, nearly equal the insect in bigness. This minute insect is found in standing pools. A number of them being kept in a bottle of water, some will be seen loaded with their eggs, and after a while depositing the two parcels, either jointly or separately.

The name monoculus has been given to this genus, as consisting of individuals which apparently have but one eye: and from the manner in which they proceed forward in the water by leaping, they have also been called water-fleas. The branching antennæ serve them instead of oars, the legs being seldom used for swimming. "The tail, forked in some species, in others simple, serves them for a rudder." Their colour varies from white to green, and to red, more or less deep, doubtless in a ratio to the fragments of the vegetables on which they feed. The red tincture they sometimes give to the water, has made some ignorant men think that the water had turned to blood. Too weak to be carnivorous, they on the contrary fall a prey to other aquatic insects, even to polyps. Their body, compact and hard, is so transparent that in some the eggs with which the abdomen is filled are discernible. The water-parrot and the shell-monoculus, are remarkable. This latter is provided with a bivalvular shell, within which he shuts himself... Monodon himself up, if drawn out of the water. The shell opens underneath, the insect puts forth its antennae, by means of which it swims very expeditiously in various directions, seeking a solid body to adhere to, and then it uses its feet in walking, by stretching them out through the aperture of its shell.

"I preferred a pair of these insects (says our author), last year, in a small glass tumbler, the one male the other female, having a bag filled with eggs affixed on each side the abdomen. In the space of 14 days the increase was astonishing: it would have been impossible to have taken a single drop of water out of the glass without taking with it either the larva or a young monoculus. I again repeated the experiment by selecting another pair; and at the expiration of the last 14 days my surprise was increased beyond measure. The contents of the glass appeared a mass of quick-moving, animated matter; and being diversified by colours of red, green, ash-colour, white, &c. afforded, with the assistance of the magnifier, considerable entertainment."