Aside from their importance as carriers of disease, mosquitoes are notorious as pests of man, and the earlier literature on the group is largely devoted to references to their enormous numbers and their blood-thirstiness in certain regions. They are to be found in all parts of the world, from the equator to the Arctic and Antarctic regions. Linnæus, in the "Flora Lapponica," according to Howard, Dyar and Knab, "dwells at some length upon the great abundance of mosquitoes in Lapland and the torments they inflicted upon man and beast. He states that he believes that nowhere else on earth are they found in such abundance and he compares their numbers to the dust of the earth. Even in the open, you cannot draw your breath without having your mouth and nostrils filled with them; and ointments of tar and cream or of fish grease are scarcely sufficient to protect even the case-hardened cuticle of the Laplander from their bite. Even in their cabins, the natives cannot take a mouthful of food or lie down to sleep unless they are fumigated almost to suffocation." In some parts of the Northwestern and Southwestern United States it is necessary to protect horses working in the fields by the use of sheets or burlaps, against the ferocious attacks of these insects. It is a surprising fact that even in the dry deserts of the western United States they sometimes occur in enormous numbers. Until comparatively recent years, but few species of mosquitoes were known and most of the statements regarding their life-history were based upon the classic work of Reaumur (1738) on the biology of the rain barrel mosquito, _Culex pipiens_. In 1896, Dr. Howard refers to twenty-one species in the United States, now over fifty are known; Giles, in 1900, gives a total of two hundred and forty-two for the world fauna, now over seven hundred species are known. We have found eighteen species at Ithaca, N. Y. All of the known species of mosquitoes are aquatic in the larval stage, but in their life-histories and habits such great differences occur that we now know that it is not possible to select any one species as typical of the group. For our present purpose we shall first discuss the general characteristics and structure of mosquitoes, and shall then give the life-history of a common species, following this by a brief consideration of some of the more striking departures from what have been supposed to be the typical condition. The CULICIDÆ are slender, nematocerous Diptera with narrow wings, antennæ plumose in the males, and usually with the proboscis much longer than the head, slender, firm and adapted for piercing in the female. The most characteristic feature is that the margins of the wings and, in most cases, the wing veins possess a fringe of scale-like hairs. These may also cover in part, or entirely, the head, thorax, abdomen and legs. The females, only, suck blood.