Leech, or Hirudo, L. a genus of insects comprising several species; the most remarkable of which are :

1. The medicinalis, or Medicinal Leech, from three to four inches in length; its body is of a dark brown colour, with six yellow spots on the back ; and a similar yellow line on each side; (though, in some seasons, these distinguishing marks are rather imperfect, and almost totally disappear) : its head is smaller than the tail, and adheres very firmly.—This species is viviparous, inhabits clear running waters, produces in July only one young insect at a time, and is valued for its use in drawing blood.

2. The sanguifuga, or Horse-Leech, which inhabits stagnant waters, and is larger than the preceding ; its skin is smooth and glossy ; the body is depressed ; the back is of a dusky colour; .and the belly of a light green, with a yel-low lateral margin.

3. The Geometra, or Geometrical Leech, is only 1 1/2 inch long, and has a smooth, glossy skin, of a dusky brown colour, but in some seasons is greenish, spotted with white. When in motion, its back is elevated, so as to form a kind of ridge, and it then appears to measure the space it passes over, like a compass ; its tail is remarkably broad ; and the insect holds as firmly by it, as by the head. This species is commonly found attached to stones in shallow running waters ; and it likewise fastens itself on trout and other fish, after the spawning season.

The medicinal leech is the only one used for local bleedings. It may be applied with great safety and advantage, especially for obstinate head-aches arising from fulness of blood, as well as in many cases of external inflammation, with a view to extract the thick and superfluous humour with which the vessels are overcharged. If the leech will not readily fix itself, a few drops of milk may be rubbed on the spot where it is to perform the operation ; or a little blood may be drawn, by making a slight puncture, after which it will immediately settle. When employed for relieving the piles, or to extract blood from the gums, it is requisite to secure the insect with a piece of rush, to prevent it either from creeping into the anus or gullet, in whichi cases it would occasion great distress, in the stomach or intestines. To induce the leech to quit its hold when it adheres longer than required, oil of tartar, volatile alkali, pepper, or acids, are occasionally employed ; though a little common salt, strewed on its head, will answer the same purpose. On the contrary, if it be intended to draw a larger quantity of blood, the tail of the leech should be cut off; in consequence of which it continues to draw blood, in order to repair the loss it has sustained. The discharge occasioned by the puncture of a leeeh, is easily stopped with brandy, vinegar, etc. or may be kept open by applying warm fomentations.

As the geometrical leech often occasions great damage among trout and other fish, it has been recommended to throw a little salt water into the pond ; but we doubt whether this expedient, by remedying one evil, would not be productive of still greater injury, by destroying the fish.