Fig. 57—Mosquito larvæ and pupæ (T. incidens) with their breathing-tubes at the surface of the water.
Fig. 58—Anopheles larvæ (A. maculipennis) resting at the surface of the water.
A few kinds feed habitually some distance below the surface, others on the bottom, while still others feed always at the surface. With one or two exceptions, the larvæ must all come to the surface to breathe ([Figs. 53]–[57]). Most species have on the eighth abdominal segment a rather long breathing-tube the tip of which is thrust just above the surface of the water when they come up for air. In this tube are two large vessels or tracheæ which open just below the tip of the tube and extend forward through the whole length of the body, giving off branches here and there that divide into still smaller branches until every part of the body is reached by some of the small divisions of this tracheal system that carries the oxygen to all the tissues. The length of the breathing-tube is correlated with the feeding-habits of the larvæ. Anopheles larvæ which feed at the surface have very short tubes ([Fig. 58]), others that feed just below the surface have breathing-tubes as long or very much longer than the ninth abdominal segment. The last segment has at its tip four thin flat plates, the tracheal gills. These too are larger or smaller according to the habits of the larvæ. Those species that feed close to the surface and have the tip of the breathing-tube above the surface most of the time have very small tracheal gills, while those that feed mostly on the bottom have them well developed.
When first hatched the larvæ are of course very small. If the weather is warm and the food is abundant they grow very rapidly. In a few days the outer skin becomes rather firm and inelastic so it will not allow further growth. Then a new skin forms underneath and the old skin is cast off. This process of casting off the old skin is called molting, and is repeated four times during the one, two, three or more weeks of larval life.
PUPA
With the fourth molt the active feeding larva changes to the still active but non-feeding pupa ([Fig. 59]). The head and thorax are closely united and a close inspection will reveal the head, antennæ, wings and legs of the adult mosquito folded away beneath the pupal skin. Instead of the breathing-tube on the eighth segment of the abdomen as in the larva, the pupa has two trumpet-shaped tubes on the back of the thorax through which it now gets its air from above the surface. The pupal stage lasts from two to five or six days or more. When the adult is ready to issue the pupal skin splits along the back and the mosquito gradually and slowly issues. It usually takes several minutes for the adult to issue and for its wings to become hard enough so it can fly. In the meantime, it is resting on the old pupal skin or on the surface of the water, where it is entirely at the mercy of any of its enemies that might happen along and is in constant danger of being tumbled over should the water not be perfectly smooth.