The Diptera include the common House Fly (fig. 16), Bluebottle Fly, Gnat, and many others; their wings are like the Neuroptera, but they have but one pair, the other pair being undeveloped, form little knobs called "balancers." The wings of Flies are generally covered with short stout hairs, as seen in fig. 29; the common Fly feeds upon almost anything, but some of the Diptera feed exclusively on the blood of other animals; the Gnat (Culex pipiens), of which the Mosquito is a variety, deposits its egg in the water on any floating substance, the larva which comes from the egg is aquatic, living with its head downwards, the end of the abdomen only being above the water, in this there is a spiracle or breathing orifice, which enables it to take in the air; the Gnat, when it comes from this larva, stands on its shell for a moment, and then flies off, for it is not suited to live in the water; this history of the early stages of its life, accounts for Gnats being always found in such numbers near pools of water. The Tsetse, which Dr. Livingstone describes as being so fatal to cattle, is of this order; he says, "It is well known that the bite of this poisonous insect is certain death to ox, horse, or dog.... A most remarkable feature in the bite of the Tsetse is its perfect harmlessness to man and wild animals, and even calves, so long as they continue to suck the cows." 9. APTERA (Wingless). To this order belongs the Flea (Pulex irritans), which is furnished with lancet-shaped weapons, through which it sucks the blood of other animals, for it is entirely parasitic, and also the "Chigeo" (Pulex penetrans), which is a troublesome and even dangerous pest in the West Indies; it penetrates the skin of the feet, and deposits its young beneath it. [Illustration: FIG. 30.--FLEA (Pulex irritans) MAGNIFIED.] [Illustration: FIG. 31.--ARACHNIDA.1, Spider (Epeira Diadema); 2, Scorpion (Scorpio).] [Illustration: FIG. 32--LONG-LEGGED HOUSE-SPIDER (Aranæ domestica).]