Like other objects floating on the surface, the mosquito-egg slightly indents the surface. The number of eggs seems to vary. According to Grassi, each female deposits about one hundred eggs, whilst Howard puts the number as varying from forty to one hundred. We, however, found in captivity the female laid about one hundred and fifty. According to Grassi, the eggs of A. maculipennis lie side by side like the bridges of boats which span the Rhine, whilst those of A. bifurcatus arrange themselves with their ends in contact, forming starlike patterns. Unlike the eggs of the gnat (Culex) the eggs of Anopheles do not adhere together, and the result is they are very readily scattered by the wind. But in sheltered places, like a laboratory aquarium, if undisturbed, the Italian Professor found that they tended to congregate together, as indeed do most minute objects floating on the surface of the water. Our observations did not entirely confirm those of Grassi. In Cambridge, at any rate, we found the eggs in our aquaria always scattered. Very frequently empty egg-shells were met with, but they too were scattered. As a rule, in nature, the ova are deposited in water rich with algae or other vegetable life, and they are more frequently in shallow than in deep water, the temperature of shallow water being naturally somewhat higher.
On the second or third day after oviposition (and this depends upon the temperature), the young larva leaves the egg and begins to swim in the water. The egg hatches by the detachment of a cap-like portion of the anterior end of the egg-shell. There is no visible ring indicating the limits of this operculum, but the cap is usually more or less of the same size. Opinions differ as to how far desiccation interferes with development of the larva in the egg-shell. They do not seem to be able to stand more than forty-eight hours of drought. There is no evidence that they can survive throughout the winter period. Everything that we know indicates that the egg must pass this period within the mother’s body, and that they only attain maturity in early spring, when the weather becomes warmer.
The larva of the mosquito is one of the most fascinating objects one can watch under the microscope. It is very complex, and consists of the usual arthropod regions of (1) the head, (2) the thorax, and (3) the abdomen.
Fig. 20.—Side view of head of a fully grown larva of Anopheles maculipennis. b, Brush; c, antenna; d, palp of maxilla; m, hooked hairs at edge of maxilla; p, median tuft of hairs; r, thickened rim of chitinous covering to head; s, large, feathered hairs which overhang head; t, mandible; u, larval eye; v, eye of adult, forming above and behind u. (From Nuttall and Shipley.)
Without going into the question of how many typical somites make up the head, we must state that the thorax has the typical number of three, much fused together, and the abdomen nine. The first seven of these are very much alike; the eighth, however, bears the large stigmata or orifices of the breathing system, and the ninth a number of beautifully arranged hairs, by means of which the larva to a great extent steers itself. The head resembles two-thirds of a sphere, and is covered with a complete and clearly defined brown, chitinous case. The eyes are lateral, and on each side we have both a simple and a compound eye. In front of each eye is a little protuberance, which carries the antenna, and between these two eminences a band of pigment runs across the head, from which six symmetrically placed immovable feathered hairs project, wreathing the head, as it were, with a halo. There are many other hairs on the head, whose number and shape are of great systematic importance. The anterior edge of the head carries on each side of its under surface a conspicuous brush, very like a shaving-brush, the constituent hairs of which are arranged in a spiral, and it is these brushes which sweep the food into the mouth of the young and voracious larva. The base of this brush is so arranged that when depressed and bent towards the mouth the two brushes approximate, but each brush can move independently and often does so: one may be depressed towards the mouth, whilst the other remains erect.
Fig. 21.—Ventral view of head of a fully grown larva of Anopheles maculipennis. b, Brush; c, antenna; d, palp of maxilla; j, stout hairs of mandible, which arrange the brush; k, teeth of mandible; m, hooked hairs at edge of maxilla; p, median tuft of hairs; q, the ‘underlip’ of Meinert, or metastoma; r, thickened rim which passes into the soft tissues of the neck. (From Nuttall and Shipley.)
The larva passes its life hanging on to the under surface of the surface-film of the water, its dorsal surface being uppermost. In fact, as Sidney Smith pointed out about the sloth, ‘it passes its life in a state of suspense, like a young curate distantly related to a bishop.’ But, since these larvae feed on any kind of organic débris that floats up to the top and is there arrested by the surface-film, it is obviously important that the brushes which sweep together these organic particles and carry them to the mouth should be next the surface, and to effect this the head must rotate through an angle of 180°; and the head does in fact turn upside-down on the neck so sharply and accurately that, as it comes into position, you almost think, as you are watching it, that you hear a click, just as you do when you rotate the diaphragm of a microscope.
The mouth parts now begin to vibrate upwards and forwards, and the brushes are bent downwards, backwards, and inwards. Round the mouth is a small space, the walls of which are completed by the mandibles, and into this space the brushes are suddenly bent back, at the same time the mandibles and maxillae move forward to meet them. This movement may take place as many as 180 times a minute, and it produces a current converging in concentric curves towards the above-mentioned chamber. The water filters out between the sides, and any particle of food is retained by the hairs or by the mouth appendages; from time to time the mandibles are brought together, and their stiff bristles are run through the brushes as one’s fingers run through a beard;[11] at other times the brushes disappear far into the mouth, and then are slowly withdrawn, passing through the comb-like bristles on the mandibles. The brushes are frequently swallowed, and are withdrawn in little jerks, so that the maxillae have every opportunity of combing any nutritive particles out of them. The whole operation is a most fascinating one to watch.