Fig. 67.—The mosquito, Culex sp.; showing eggs (on surface of water), larvæ (long and slender, in water), pupa (large headed, at surface), and adult (in air). (From living specimens.)

The mosquito (Culex sp.) (fig. [67]) lays its eggs in a sooty-black little boat-shaped mass which floats lightly on the surface of the water. In a few days the larvæ, or "wrigglers," issue and swim about vigorously by bending the body. The head end of the body is much broader than the other, the thoracic segments being markedly larger than the abdominal ones. The head bears a pair of vibrating tufts of hairs, which set up currents of air that bring microscopic organic particles in the water into the wriggler's mouth. At the posterior tip of the body are two projections, one the breathing-tube (the wriggler coming often to the surface to breathe), and the other the real tip of the abdomen. The wriggler, although heavier than water, can hang suspended from the surface film by the tip of its breathing-tube. It changes in a few days into the pupa, which, instead of being quiescent as with most flies, can swim about. It has a large bulbous head end and the posterior end of the body bears a pair of swimming-flaps. It takes no food. When ready to change to the adult mosquito the pupa (which, unlike the wriggler, is lighter than water) floats at the surface of the water, back uppermost. The chitinous cuticle splits along the back and the delicate mosquito comes out, rests on the floating pupal skin until its wings are dry, and then flies away. Only the female mosquitoes suck blood. If they cannot find animals, mosquitoes live on the juices of plants. They are world-wide in their distribution, being serious pests even in Arctic regions, where they are often intolerably numerous and greedy. Recent investigations have shown that the germs which cause malaria in man live also in the bodies of mosquitoes, and are introduced into the blood of human beings by the biting (piercing) of the mosquitoes. It is probable also that the germs of yellow fever are distributed by mosquitoes in the same way. By pouring a little kerosene on the surface of a puddle no mosquitoes will be able to escape from the water.

Fig. 68.—The house-flea, Pulex irritans; a, larva; b, pupa; c, adult. (The fleas are probably more nearly related to the Diptera than to any other order of insects.) (After Beneden.)

Lepidoptera: the moths and butterflies.—Technical Note.—Obtain specimens of a few moths, and compare with the butterfly already studied; note especially the character of antennæ. Obtain miscellaneous specimens of larvæ, pupæ, and cocoons of any moths or butterflies. Note the variety in colors, markings, and skin coverings of the larvæ; note the shape and markings of the pupæ. Rear from eggs, larvæ, or pupæ in breeding-cages any moths and butterflies obtainable (for directions for rearing moths and butterflies see Chapter [XXXIV]), keeping note of the times of molting and of the duration of the various immature stages. If the eggs of silkworms can be obtained the whole life cycle of the silkworm moth can be observed in the schoolroom. The larvæ (worms) feed on mulberry or osage orange leaves, feeding voraciously, growing rapidly and making no attempts to escape. The molting of the larvæ can be observed, the spinning of the silken cocoon, and the final emergence of the moth. The moths after emergence will not fly away, but if put on a bit of cloth will mate, and lay their eggs on it. From these eggs, which should be kept well aired and dry, larvæ will hatch in nine or ten months (if the race is an "annual").

The Lepidoptera (figs. [69]-[74]) include all those insects familiarly known to us as moths and butterflies; they are characterized by their scale-covered wings (fig. [69]) and long nectar-sucking proboscis composed of the two interlocking maxillæ. They undergo a complete metamorphosis (fig. [70]) and their larvæ are the familiar caterpillars of garden and field. These larvæ have biting mouth-parts and feed on vegetation, some of them being very injurious, for example the army-worms, cut-worms, codlin moth worms, etc. The adult moths and butterflies take only liquid food, or no food at all, and are wholly harmless to vegetation. The structure and life-history of a butterfly has already been studied, and in the more general conditions of structure and life-history there is much similarity in the many insects of this order. The eggs are usually laid on the food-plant of the larva; the larva feeds on the leaves of this plant, grows, molts several times, and pupates either in the ground or in a silken cocoon or simply attached to a branch or leaf. There are about six thousand species of moths and butterflies known in North America, and they are our most beautiful insects.

Fig. 69.—A small, partly denuded part, much magnified, of a wing of a "blue" butterfly, Lycæna sp., showing the wing, scales and the pits in the wing-membrane, in which the tiny stems of the scales are inserted. (Photo-micrograph by Geo. O. Mitchell.)