Fly, in zoology; a genus of insects belonging to the order of diptera. The mouth is furnished with a fleety proboscis, and two lateral lips; but it has no palpi. This genus is divided into two different sections: 1. Those with simple antennæ. 2. Those which are furnished with a lateral hair or feather. Those have downy bodies, though scarce perceptibly so; and have either a lateral plume or feather on the antennæ, or a simple hair on the side of the antennæ. The pilose have a few hairs scattered upon their bodies, principally upon the thorax; they have either a lateral feather or a lateral hair. Under these divisions are comprehended about 400 different species, as enumerated in Dr Gmelin's edition of the Systema Naturæ. "Variety (as Mr Barbut observes) runs through their forms, their structure, their organization, their metamorphoses, their manner of living, propagating their species, and providing for their posterity. Every species is furnished with implements adapted to its exigencies. What exquintiencing what proportion in the several parts that compose the body of a fly! What precision, what mechanism in the springs and motion! Some are oviparous, others viviparous; which latter have but two young ones at a time, whereas the propagation of the former is by hundreds. Flies are lascivious, troublesome insects, that put up with every kind of food. When storms impend, they have most activity, and sting with greatest force. They multiply most in hot moist climates; and so great was formerly their numbers in Spain, that there were fly-hunters commissioned to give them chase. The vapour of sulphur or arsenic destroys them; and their numbers may be reduced by taking them in phials of honeyed water, or between boards done over with honey." There are 129 species, principally distinguished by the peculiarities in their feelers.

name given to such persons among the Romans as officiously thrust themselves into the company of their superiors and those who despised them, by finding means of getting admittance to entertainments without invitation, and without a welcome: So that musca were the same as parasites, who were frequently by the Greeks termed Musæus. See Parasite.