Sources : Gnat

Pliny the Elder [1st century CE] (Natural History, Book 9, 74; 10, 81; 10, 90; 11, 40-41): [Book 9, 74] ... gnats are [generated] by moisture turning sour... [Book 10, 81] Gnats are [bat's] favorite fodder. [10, 90] Gnats seek for sour things and are not attracted by sweet things. [11, 40] Dogs have a special pest of their own, a maggot that lances particularly their ears, which they cannot protect by their bite. [11, 41] Again, other kinds, namely gnats, are bred by a substance growing sour, and in fact white ones are found even in snow... - [Rackham translation]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 8:13; 12, 8.14): [Book 12, 8.13] The gnat [culex] is named from sting [aculeus] because it sucks blood, for it has a tube in its mouth, like a needle, with which it pierces the flesh so that it may drink the blood. [Book 12, 8.14] Sciniphes are very tiny flies, very troublesome with their stinging. The proud populace of Egypt was struck down by these flies in the third plague. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Worms 9.14-15): [Thomas describes the gnat under the names cynyphes and culex.] [Worms 9.14] Cynyphes, as he says [i.e. Liber rerum], are small flying worms. We read that Moses was scourged by them in Egypt [Exodus 8:17]. They follow the breath of cattle, and especially that of man. They sting, as Andelmus says, with some of the stings sharp enough to cause pain when sleeping. Whence in places where their boldness abounds, delicate people are wont to drape their beds with nets so that the gnats cannot pass through it to do harm. Therefore guard your mind by constant reading and prayer, so that the devil cannot enter to harm you. [Worms 9.15] A gnat [culex] is a worm, as Isidore says, and the name is derived from its sting, because it sucks blood. For it has in its mouth a pipe in the manner of a spur, with which it pierces the flesh of animals, in order to drink the blood. As the Experimentator says, the gnat loves the light so much that it sometimes burns itself on the light. Isidore: gnats seek acid and avoid sweets (as Pliny says). And woe to these detractors, who always treat the evil works of men by scattering, and care for no good, so that they may recite, for example, of such in the song of Deuteronomy: Their wine is the venom of serpents and the deadly poison of the asp [Deuteronomy 32:33]. If you want to get rid of gnats on the road or while sleeping, eat garlic and they will not bother you. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book12.12): A Gnat is a little flie, and is called Culex, and hath that name of Aculeo, a sting: for he sucketh bloud, & hath in his mouth, a pipe like a prick, & therewith he pearceth the flesh for to suck the bloud, and is accounted among Volatiles, as the Bee is, though he have a body of a worme, with many feete: For hee hath wings and flyeth therewith, and is gendred of rotted or corrupt vapours of carrens, and corrupt place of marreyes. By continual flapping of wings, he maketh noyse in the aire, as though he hurred: and sitteth gladly upon carraines, botches, scabbes, and sores: and is full noyfull to scabbed Horses, and sore backed, and grieveth sléeping men with noyse and with biting, and waketh them of their rest, and flyeth about most by night, and pearceth and biteth members upon the which he sitteth, and draweth toward lyght, and gladly he seeth lyght, and so unwarely he falleth into a Candle or into the fire, and for coveting to sée lyght, he burneth himselfe ofte. And is best féeding to swallowes, for Gnats be taken for best beloved meate to swalowes that flye in the aire, & hunt flies. And they are called Sciniphes, that is to understand small flies, but most noyfull in stings: for Scines in Gréeke, is Musca in Latine, a flye in English: and therof commeth Scinomia, a houndes flye, & with such flies the land of Aegypt was smitten, as Isidore sayth. And Scinomia is the worst kinde of flyes, with greater bodye and broader wombes, than other flyes, and lesse flight: but they be full tender, and cleave fast in the members of beasts, on the which they smite. In wooll, haire, and bristles of beasts, and namely in hounds, such a flye hideth hir selfe, and sucketh and drinketh bloud: insomuch that she gnaweth the flesh, and maketh it swell with botches and pimples, as it fareth in eares of old hounds, the which eares such flyes gnawe and make them swell and full of sores. And so it is no wonder though such Flyes stinke right foule, that are fedde with such corrupt meates and humours. - [Batman]