Fig. 24.—A, Side view, B, ventral view, of the pupa of a male Anopheles maculipennis; C and D, the same views of the female pupa.
The sex of the pupa can be determined by the lobes at the posterior end of the tail: A and B (Fig. 24) representing the male, and C and D the corresponding parts of the female. The duration of the pupal life is generally three to four days, but conditions of temperature and the state of the natural surroundings exert considerable influence upon the rate of development. Howard has pointed out that the addition of creosote or creosote-oil to the water in which the larvae are living hastens the metamorphosis into pupae, and the pupa stage is passed through in as short a time as fifteen hours instead of the normal forty-eight hours of the warm waters of the Southern States in America. It has also been observed that showery weather hastens the rate of development.
When the adult mosquito is about to emerge, a certain amount of air is secreted under the chitinous casing of the pupa. A fine streak containing air appears along the back, extending between the respiratory trumpets and the base of the head. This central streak gradually passes backwards until it reaches the seventh abdominal segment, and then suddenly the pupa extends its abdomen so that it floats parallel to the surface of the water instead of being under the rest of the pupa’s body. The chitinous integument now splits along the median dorsal line, and through the slit thus made the thorax of the adult mosquito now protrudes. By gradually pressing its abdomen against the pupa-case, the body of the perfect insect is slowly but gradually raised above the surface of the water. The head is pulled backwards and upwards, and millimetre by millimetre the mouth parts, the palps, and antennae are withdrawn, and at first remain bent backwards beneath the body of the insect. Gradually the bases of the wings and the abdomen emerge, and soon the wings are freed and immediately flatten out and begin to harden. The legs and the tip of the abdomen alone now remain to be dealt with. At this stage the insect projects far beyond the anterior end of the pupa encasement, and somewhat resembles an exaggerated figurehead on a ship. The pupa-case is still filled with air, and acts as a float to support the emerging insect. At last the front legs are being freed, the second and third pair of legs soon follow, and now the insect is standing on the surface of the water raised on its tarsal joints, the tip of the abdomen being the last part to free itself from the pupa-case.
Fig. 25.—Imago of a mosquito extracting itself from the pupa-case, which floats on the surface of the water. Magnified. (From Guiart.)
The exit of the fly is naturally a very critical period in its life-history, and in many cases it is fatal. The freeing process takes between five and ten minutes. When undisturbed the emergent fly rests for a time until its wings and limbs are sufficiently hardened to enable it to fly, or at least to walk about. Sometimes the mosquito takes its first flight straight from the pupa-case; at other times it rests awhile before taking to the air. The young imago is pale in colour, the thorax being brown and the abdomen transparent, with a greenish tinge. At first the abdomen is much longer than it is later, for, almost immediately after the mosquito’s exit from the pupa-case its abdomen begins to contract, and from its hinder end four or five drops of a glistening, greenish-white fluid are exuded.
The newly born imagines generally take to flight between five and ten minutes after they have emerged, and they at once begin to darken in colour, and in two hours assume the normal dusky colour of the adult. If anything hinders the insect from properly extending its limbs immediately on issuing from the pupa-case, the parts harden and remain distorted throughout life.
Anyone who has spent a day or two in Lille or Bruges, or other towns in Picardy and in Southern Belgium, will understand why, as my Uncle Toby has it, ‘Our army swore terribly in Flanders.’ The incessant and tireless biting of mosquitos would make any army swear, even though they were ignorant—as my Uncle Toby’s army certainly was ignorant—that the gnats, as they called them, conveyed tertian and quartan ague. In Europe the trouble is a summer or early autumn trouble; but our troops are fighting in many tropical and sub-tropical countries, where the mosquitos—like the poor—are always with them.