The flies and mosquitoes are among the greatest pests and dangers to man. Both are conveyers of disease, and the former, as an agent of destruction, deposits its eggs in meat of all kinds, making it impossible to keep meat in some countries. On the other hand, it should be remembered that the flies are valuable scavengers, hastening the destruction of dead matter which might contaminate the air.

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.

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 foetid 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.

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.

The warfare declared against mosquitoes in America, suggested by Dr. Howard, has attracted widespread attention to these insects, which have rendered many localities absolutely uninhabitable. A Florida physician informed me that in a certain locality horses had been killed by these insatiate bloodsuckers, which are now known to be the carriers of the germs of yellow fever. Over almost every pond or pool in summer they may be seen in countless numbers, filling the air with their disagreeable music.

It is the female mosquito which occasions all the trouble and renders mankind miserable in some of the otherwise most delightful resorts. On the Florida Keys I always had a mosquito bar overhead, not merely over the bed but suspended from the ceiling in midday. Even then these pests would force their way through the meshes.

The development of the mosquito is interesting (Fig. 226). The eggs are deposited as a boat-shaped mass on the surface of the water, where they drift about for several days. The larvÆ appear as wigglers floating in the water, tail upward, and breathing through a tube at the tip of the abdomen which is projected above the water for the purpose. After a while the head grows larger, and several changes ensue. Then the pupa finally appears. This rises to the surface, and out bursts a full-fledged mosquito which, like a man in a canoe, balances itself while its wings dry. A few hours before it was entirely dependent upon the water and swimming in it, but now it appears to be fearful of overturning the frail craft and falling in where it would surely drown. If all goes well, it soon tries its wings and goes buzzing away. The devastation caused by the armed and bewhiskered mosquito is not generally known. Doubtless thousands have lost their lives from this unsuspected cause.

The common gnat (Fig. 227) has habits similar to those of the mosquito. They are often seen floating in the air in great swarms or bands, rising and forming as though in some mystic dance.