We have referred above to the respiratory openings, and, indeed, these are the key to the whole situation. Close these openings— as they can be closed by floating petrol or other oil on the surface of the water—and ‘the trick is done.’ The larvae and the pupae can no longer breathe, and there is thus no imago to “carry on.” In Culex (the gnat), these respiratory orifices are borne on a long tube directing dorsalwards—a tube which is larger and longer than a segment of the body, and whose presence gives the larva the appearance of a Y with slightly unequal limbs. These breathing-openings are of the greatest complexity, but the outstanding fact is that these stigmata pierce through the watery film and put the respiratory system of the larva into communication with the atmosphere of the whole cosmos. If anything frightens the larva, certain side-pieces and flaps fold suddenly backwards and over the stigmata, the connexion through the surface-film is broken, and the little larva, like a German submarine when it sights an English battleship, darts below, frequently carrying with it the drop of air attached to the rim of the respiratory recess which surrounds the openings of the two stigmata.
Not infrequently the larva ceases to lie parallel to the surface of the water, its palmate hairs are put out of action, and then its body hangs down into the water, but it still maintains its respiratory connexion with the outer air through these breathing-pores. From time to time the hairs mentioned above are brushed over by the mouth parts and cleaned of any débris.
The larvae, when they leave the surface-film sink by their own weight; but they not infrequently swim actively downwards, their swimming action being very like that of an eel. When returning to the surface they are entirely dependent upon their powers of swimming, being slightly heavier than water. When the tail reaches the surface-film the larvae are at once arrested, and immediately cease their swimming-movements. They invariably move tail forwards, and the hairs which we have mentioned above at the posterior end of the body undoubtedly act as ‘buffers’ or ‘fenders.’ As a rule, when they are above, they are actively engaged in feeding; but at the bottom they lay inert, as though feigning to be dead. Kept in a glass beaker they are apt to lie with their respiratory apparatus attached to the concave film, which capillary attraction draws up on the surface of the glass. Their heads then point towards the surface of the beaker. If forcibly kept below—say, by submerging them under a watch-glass—they are frequently enabled to breathe by attaching the openings of their respiratory apparatus to an air-bubble.
The general colour of the larva is a mottled brown, darkening where the chitin thickens. The older larvae are to some extent green, possibly due to their food; but this green colour is not by any means confined to the alimentary tract. After moulting, the issuing larva is a uniform light lavender colour, which, however, very soon darkens.
A strong wind passing over a pool where Anopheles eggs, larvae, or pupae are floating, will gradually pile them all up on the side towards which it is blowing. The Anopheles larvae undoubtedly are braver than those of the Culex—that is to say, a disturbance which will send all the Culex larvae scurrying to the bottom will leave the Anopheles larvae unmoved.
When first hatched the larvae measure somewhere about 0·7 mm. to 0·95 mm., but when ready to pupate they have attained the length of 7 mm. The rate of development is greatly influenced by the temperature, and a few cold days will markedly retard the larval growth. In warm sunny weather, larvae will pupate between the second and third week, but larvae taken in August (if the autumn be cold) do not attain their full growth until November. The young larvae undoubtedly die in considerable numbers, and the act of pupating is also attended with certain and varying dangers. Out of 834 larvae and pupae caught in Cambridgeshire, 636 were small larvae, measuring less than 4 mm.; 181 were large larvae, measuring up to 7 mm. But only 17 pupae were taken. There are other facts which show that the larvae under natural conditions succumb in very considerable numbers.
Fig. 23.—Side view of late pupal stage of Anopheles maculipennis. f, The stigma opening at end of trumpet-like projections. (From Nuttall and Shipley.)
When the larva is about to turn into a pupa it comes to rest, and now the thoracic regions are more swollen than ever. Soon a dorsal slit appears along the larval cuticle and the pupa slowly, but gradually, emerges through this slit and leaves the larval chitinous cuticle behind it. On first emerging, the pupa measures about 6·5 mm., the head and thorax making up one-third of this. During the last larval stage many of the pupal organs have been re-forming and are more or less visible through the cuticle. The mouth parts and limbs of the third stage—the future imago—show no relation to those of the larva. They are there enclosed in their respective sheaths, but these are quite independent of the larval ‘appendages.’ The respiratory trumpets, which, as in the larva, pierce the surface-film, are ready to act as breathing-organs. Whereas the larvae breathe through two stigmata at the posterior end of the abdomen, the pupae breathe through two respiratory trumpets issuing from the anterior dorsal surface, and it is these trumpets, together with certain palmate hairs, which support the pupae in the right position and put the respiratory organs at this stage into communication with the outer atmosphere. During the pupa stage Anopheles, like the pupa of other insects, takes no food.
The pupa is something like a tadpole, with its tail bent under its body and flapping up and down, instead of from side to side. The whole pupa is enclosed in a thin semitransparent membrane, through which the organs of the adult can readily be seen. As it grows older its colour darkens. Until about the time when it will give rise to the fly, the pupa floats quietly at the surface, breathing through its respiratory trumpets. When disturbed it shows considerable activity, and it is by no means always easy to capture by means of a pipette. At the least sign of danger it darts below with a series of intermittent strokes and rests at the bottom of the water. Its own buoyancy brings it back to the surface, as, unlike the larva, it is lighter than water. Not only has it a certain amount of air in its tracheae, but there is a reservoir of air at the posterior end of the thorax which acts as a very efficient float. When retreating below the surface the respiratory trumpets usually carry down with them two minute air-bubbles.