Page:Insects - Their Ways and Means of Living.djvu/391

 in a small quantity of water, provided the latter contains sufficient food material.

The tip of the respiratory tube is furnished with five small lobes arranged like the points of a star about the central breathing hole. When the larva is below the surface, the points close over the aperture and prevent the ingress of water into the tracheae; but as soon as the tip of the tube comes above the surface, its points spread apart. Not only is the breathing aperture thus exposed, but the larva is enabled to remain indefinitely suspended from the surface film (Figs. 174 D, 181 B). In this position, with its head hanging downward, it feeds from a current of water swept toward its mouth by the vibration of the mouth brushes. Particles suspended in the water are caught on the brushes and then taken into the mouth. Any kind of organic matter among these particles constitutes the food of the larva. Larvae of Culex mosquitoes, however, feed also at the bottom of the water, where food material may be more abundant.

The body of the mosquito larva has apparently about the same density as water; when inactive below the surface, some larvae slowly sink, and others rise. But the mosquito larva is an energetic swimmer and can project itself in any direction through the water by snapping the rear half of its body from side to side, which characteristic performance has given it the popular name of "wiggler." The larva can also propel itself through the water with considerable speed without any motion of the body. This movement is produced by the action of the mouth brushes. Likewise, while hanging at the top of the water, the larva can in the same manner swing itself about on its point of suspension, or glide rapidly across the surface.

The larvae of Culex mosquitoes reach maturity in about a week after hatching, during the middle of summer; but the larval period is prolonged during the cooler seasons of spring and fall. The larva passes through three stages, and then becomes a pupa.