MAN AND HIS ENEMIES.

New Zealand Herald, Volume LXVI, Issue 20162, 24 January 1929, Page 8

MAN AND HIS ENEMIES.

(THE MOSQUITO PEST.

SUITABLE BREEDING PLACES.

f>OTENT AGENTS OF DISEASE.

'ALL TYPES UNDER SUSPICION.

No. IX. BT W. IL M'GBEGOR. The investigation of the life-histories ef the various species of mosquito is of more than ordinary interest, as & knowledge of the peculiar conditions under ;which the early stages of development are passed often provides a definite clue to to practicable control measures. In the course of its development the insect undergoes a remarkable transformation, known as a complete metamorphosis. From the egg issues a larva, or, as it is generally termed, a "wriggler." This, in due time, passes into a quiescent stage known as the pupa, which is transformed by a remarkable process of concentrated growth into the adult insect. The mosquito deposits its eggs in water, in which element all the earlier stages of the life-history are passed, the larva and pupa both having special structural adaptations that equip them for aquatic life. This alternation between air and water, though a most remarkable expedient, is, however, seen elsewhere among insects. Tho egg, which is enclosed in a tough capsule of extremely resistant, chitinous material, is roughly cylindrical in form, pointed at one end and somewhat founded at the other. It is fitted with a tiny lid or cap, which eventually lifts off to permit the larva to emerge. From two to three hundred eggs are usually deposited in a batch and these are arranged in a variety of characteristic fashions according to .the species concerned. Several different ingenious devices are adopted to keep the eggs floating at the surface ot the water, where the higher temperature promotes more rapid development Where Eggs Ara Placed. For many reasons running water does not provide a suitable habitat for the young mosquito. Consequently the eggs are usually placed m still, fresh water, the stagnant pools of swamps being especially suitable on account of the abundance of microscopic plants and animals that afford a plentiful supply of food for the voracious larvae. Some species prefer the clean water of tanks and waterbutts. Others tolerate distinctly brackish conditions, while, in the Egyptian desert, certain species have been observed to breed in salt pools of very great concentration. On the whole, then, the larva has a wide choice of suitable habitats. Ponds and the stagnant waters of swamps, coastal marshes and the ground-puddles in forests, tanks and the water trapped in tins and old receptacles, horse troughs and drain-traps all afford suitable breeding places for the hosts of the mosquito army. Under suitable temperature conditions the eggs hatch in from one to three days when the larva immediately commences its principal business in life. Feeding greedily and rapidly growing it casts its skin periodically to accommodate its everincreasing bulk. It has a markedly segmented body with head, squarish thorax and jointed abdomen all clearly discernible. The head bears a pair of prominent feelers and the mouth parts are provided with a number of jointed plates thickly beset with long, thick spines and hairs. •The rapid, vibratory movement of these "mouth brashes" causes a current of water to set in toward the month cavity, carrying with it those miscroscopic organisms, plants and animals on which the Creature principally feeds. Differing Diets,

But the nature of the food differs somewhat with the species. Some are quite content with a mixed diet of microscopic particles. Others are carrion-eaters, while others are fiercely carnivorous and frequently indulge in cannibalism. The rapacity of these larvae may be realised by confining a number in a glass vessel with a limited food supply. It is observed that the numbers rapidly diminish as the larger ones proceed to devour the smaller and weaker members of their own kind. •These larvae progress through the water to a limited extent by means of spasmodic, jerking movements, assisted by the tufts of stiff bristles that project, paddlewise, from the sides of the body segments. In about ten or twelve days the larva is full-grown and ready to pupate. Its form changes and it assumes a shape not nnlike that of a comma. Head uppermost it leads a still, quiet life awash at the surface of the water, its two little breathing tubes projecting into the air like the periscope of a submarine. The last larval cuticle forms a closely-fitting pupa case, and within this a most wonderful transformation is effected. The tissues of the larva undergo complete degeneration, and an entirely new set of tissues with a vastly different arrangement are rapidly elaborated. When, at the end of two or three days, this amazing process of growth is complete, the pupa comes slightly above the surface of the water. Walks on the Water. As it dries the skin commences to split 'down the back, and the crack, gradually extending, reveals at last the fully developed mosquito floating, as it were, in a tiny boat. Slowly it commences to 'draw itself out, the head first, then the thorax with its long legs, and finally the slender abdomen. It remains seated for a time rocking to and fro on the remains of its own pupa case, and quietly surveying this new world of bright skies and fresh breezes into which it is born. Meanwhile sun and air rapidly harden its protective cuticle and render its wings rigid for flight. It can walk with ease npon the surface film of the water, which it lightly touches with the tips of its feet, as for a time it stretches its slender legs. At length, climbing on to its discarded shell, it spreads its flashing win;;n. and lifting itself high into the air it flits away from its watery cradle, perchance to seek its first meal of the blood of A beast. More Research Necessary. In all several thousand different species of mosquitoes or gnats are known to Ecience and new forms are continually being described. There are at least five well-defined species in New Zealand, but St is highly likely that further research will reveal still others. No adequate survey of the mosquitoes of New Zealand has yet been made and relatively little is known about the local varieties. In this land, favoured by nature above all others, ma'.aria has never appeared in epidemic form, research has never been •timulated by necessity and as a result we know considerably less about our mosquitoes than enables us to guarantee continued immunity. It must always be remembered that the whole great tribe of mosquitoes, together with their numerous relatives, the hordes of midges and sandflies of diverse type that share the bloodsucking habit, are potential agents for u ®' sse m' na tion of disease germs and should therefore be regarded with marked suspicion.