As far as one can judge, the currents set in motion by the action of all these forces extend in an area equal to twice the length of the larva, or even more. The currents are in the plane just below the surface-film, and any organic matter lighter than water is swept towards the mouth. In fact the larva sweeps the lower side of the surface-film of the pond or puddle just as a careful housemaid might sweep spiders and flies off a ceiling with a hand-brush.
The principal food-supply of the larva consists of the spores of fresh-water algae, diatoms, particles of Spirogyra, and any other organisms which do not penetrate the surface-film. Occasionally the larvae devour the decaying leaves of duck-weed (Lemna), and sometimes they attack their dead fellows.
Grassi found the intestine of the larva to contain protozoa, unicellular algae, and other organic detritus. In course of time some object too big for the larva to swallow is brought to its mouth by the currents, but after a very short struggle this is rejected. The minuter particles accumulate in the chamber for a certain time, and then are swallowed by a gulp-like motion and thus pass into the oesophagus.
Fig. 22.—A comparison between the various stages in the life-history of the mosquito (Anopheles), on the left, and the gnat (Culex), on the right. (including Imago, Pupa, Larva and Ova)
CHAPTER VIII
THE MOSQUITO (Anopheles maculipennis)
Part V
Amongst aquatic larvae, the most beautiful and delicate are those of numerous species of gnat.—(Goring and Pritchard’s Micrographia, 1837.)
In the young larva of Anopheles the head is broader and deeper than the thorax, but in the older larvae the segments that succeed the head have at least twice its diameter. It is a characteristic of true flies, or Diptera, that the thorax should not exhibit that separation into three divisions which is so usual in the less specialised insects—such as the cockroach and this is peculiarly true of the larva of the mosquito—at any rate, so far as its external structure goes. The abdomen of the larva consists of nine free segments; the third, fourth, fifth, sixth, and seventh of these bear palmate hairs on the dorsal or upper surface, something like hands with fourteen ‘fingers’ spread out. These hairs adhere to the under layer of the surface-film of the water, and help to maintain the animal in a horizontal position just below that film. When the larva relaxes its hold and sinks into the water, it not infrequently carries with it air-bubbles enclosed by these fourteen ‘fingers.’
The eighth abdominal segment bears the stigmata or the openings of the respiratory apparatus, and the ninth segment has abandoned the flattened and square cross-section of its predecessors, and is cylindrical and tapering. The posterior end of the body is cut off sharply. Round the posterior opening of the alimentary canal are four white, soft papillae, which are well supplied with tracheae and are capable of contracting and expanding. Above these are four very prominent hairs, two median and two lateral, and ventrally to the ninth abdominal segment is a fan-shaped arrangement of hairs springing from two pieces of very complicated structures. These hairs seem to act to some extent as a rudder, and they probably serve as an accessory organ of locomotion. Possibly they have also a sensory or tactile function, and act, as so many posterior filaments do in insects, as antennae ‘from behind.’