The flies (Fig. 217) are two-winged insects with mouth parts adapted for lapping (Fig. 218) or sucking. Under the microscope these organs often appear to be composed of needlelike bristles, forming a proboscis protected by a scabbard or sheath. In some flies this weapon is many times as long as the body. The head is well separated from the body, and movable. The eyes are compound and simple, made up of many facets (Fig 219). The wings are gauze-like, often beautiful, and when the fly is in motion, they move in a figure eight, making, it is estimated, 19,800 revolutions a minute. The feet (Fig. 220) enable it to cling to the smoothest surfaces with ease. The little pads are extremely irritating at times when the fly walks over the flesh, tapping here and there with its soft tongue, in which all parts except the labium are rudimentary. The latter has a broad tip for licking or lapping. The flies breathe by spiracles, and are among the most active of all insects, and the bravest, attacking man and beast, and refusing to be driven off, despite the most active and spirited defense. [Illustration: FIG. 220.--Foot of a fly.] [Illustration: FIG. 221.--Flesh fly, eggs and young in various stages of development.] The development of the house fly is a familiar process. The eggs of the flesh fly, as an example (Fig. 221), are small, white objects which hatch into maggots. These change gradually, finally becoming pupæ, then assuming the adult form. The bluebottle fly (Fig. 222) is one of the best known. The house fly is found in greatest numbers near stables, as there, in the piles of refuse, the eggs are deposited, hatching in twenty-four hours. The young appear as fleshy, soft, footless worms or maggots, which are ravenous, and live upon the most fœtid matter for two weeks, when they change into a pupa, a barrel-shaped, cocoonlike form. For two weeks this remains motionless, when out of it breaks the perfect house fly, soon to deposit its eggs and help to produce the tens of millions of flies which swarm wherever human beings are found. [Illustration: FIG. 222.--Bluebottle fly and young.] Among the many species of flies some are bloodsuckers, as the horse flies. The robber flies are the hawks of the race, carrying off other insects, even large dragon flies. The many species of horse flies attack horses and cattle, and the animals are often driven to a frenzy by their approach. An entire herd will recognize the approach of these insects and stampede. Many of the flies deposit their eggs upon the hairs or nostrils of horses. Flies exist in countless varieties, from harmless creatures to some in Africa which are deadly to cattle and horses; from the ordinary fly whose larva lives in cheese to others which thrive in alcohol and wine. In California the larva of one species is found in Lake Mono, where no other animal can live. Hundreds of bushels of them are sometimes washed upon the beaches, constituting a favorite food for the Indians.