Gnats are parasites in the same manner as leeches, since, like them, they suck the blood, and live at the expense of others. There is, however, this difference, that the females only are greedy of blood; if this fail them, they live, like the males, on the juices of flowers. Another difference is that they are completely harmless till they have wings, and though they live long under their first form, in damp earth or in water, the duration of their life as perfect insects is of short duration. We need not trouble ourselves about the active larvæ which swarm in stagnant water, nor the chrysalids which float immovable in their natural sepulchre. We give on the next page a representation of a larva of the gnat. The females alone pierce the skin by means of an auger with teeth at the end; they suck the blood, and before they fly away, distil a liquid venom into the wound. This bite seems to have an anæsthetic effect, which does not cause it to be felt till some time after. The little spot around the wound appears as if affected by 118 chloroform. [Illustration: Fig. 8.--Gnat (_culex pipiens_) larva and nymph. (Blanchard).] These parasites repay by an unkind action the assistance which they have demanded from us. Besides the gnats, which belong to the family of _Culicidæ_, there 119 there are also the _Ceratopogon_, and especially the _Simulium molestum_, known in North America under the name of _Black-flies_: "the tormenting black-flies of this country," as the Americans say. Certain Nemocera, known by the name of _Rhagio_, put to flight both man and animals. They are very small; they get into the nostrils, and cause animals to become blind by introducing themselves into their eyes. In addition to these hurtful insects, we find others fatal to the life of animals, and which are a real plague in certain countries. The numerous travellers who have explored the interior of Africa, have almost all spoken to us of a fly which attacks beasts of burden, and kills them in a few hours; this is the Tsetse (_Glossina morsitans_). More than one expedition has failed on account of this dipterous fly. It was this which obliged Green to abandon his plan of reaching Libebe, by causing him to lose one after another all his beasts of burden and of draught. The horse, the ox, and the dog are more especially attacked by this terrible fly between the 22nd and 28th degree of longitude, and the 18th and 24th of south latitude. Happily it does not produce any effect upon man.