: More Minor Horrors by Shipley A E Arthur Everett Sir - Insects as carriers of disease; Insect pests; Animals as carriers of disease; Pests

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n our paper Mr. Wilson and I thought it well to figure the upper surface of the halter as seen under a high magnification. The drawing showed the hinge on which the halter quivers--and certain basal papillae, as Weinland calls them. There is little doubt that the main function of the halteres is that of balancing and orientating the insect. They may, however, have a secondary function; in some flies they are known to vibrate with extreme rapidity. It is just possible that in these rapid vibrations the papillae of the concave surface rubbing against those of the convex basal plate may produce a note. As long ago as 1764 von Gleichen-Russworm observed that when the halteres of the common house-fly are removed the volume of the buzzing diminished. This, however, in all probability is due to the diminished activity of the wings. On the other hand, Professor J. Stanley Gardiner informs us that he has noticed that mosquitos still continue to give forth a faint note even when their wings are quite at rest, and this note may possibly be caused by the halteres.

THE MOSQUITO

PART IV

Gnats are unnoted wheresoe'er they fly, But eagles gazed upon with every eye.

The eggs of the mosquito are deposited in fresh water, and at first they are white, but they very rapidly darken until they assume a polished black appearance. Each egg is 0?72 mm. in length, and its greatest breadth, which is somewhere about its middle, is 0?16 mm. The egg is boat-shaped, and one end, as is usual in boats, is slightly deeper and fuller than the other. The under surface is fluted, and is marked by a minute network. The upper surface has a coarser reticulation which divides the surface into nearly equal hexagonal areas. The rim of the 'boat' is thickened, and these thickenings are regularly ribbed; they extend over above the median third of the egg, and recall the rounded float which runs along the edge of a life-boat: and indeed they serve the same purpose, for they are composed of air-cells, and their function is to keep the boat-shaped eggs right side upward. Soon after the egg has been laid it is of a greyish-black colour, but after a certain amount of attrition an outer membrane splits off--the membrane which has given the egg its reticulated appearance. This membrane scales off in fragments, and is of a grey colour. The egg beneath it is glistening black--as shiny and as black as patent leather.

One curious fact that Professor Nuttall and I noticed in the life-history of the egg is that when it is drawn by capillary forces a little way out of the water on to the leaf of a water-plant or some other half-submerged object, the blunt end always points downwards. Now the blunt end is the head end, and thus, should hatching take place whilst the egg is suspended half in the water and half in the air, the larva will emerge into its proper element and not into the atmosphere.

On the second or third day after oviposition , 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 the head, the thorax, and the abdomen.

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.

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 d?bris 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; 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.

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.

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