1911 Encyclopædia Britannica/Mosquito

by Ernest Edward Austen

MOSQUITO (Span. mosquito, a gnat, diminutive of mosca, a fly), a term originally applied to many species of small blood-sucking Diptera (q.v.), belonging to various families, but now by common consent restricted to those known to naturalists as Culicidae, or gnats. Before the year 1899 mosquitoes had never been collected systematically, and had received little notice from entomologists, so that but few genera and comparatively few species were known. Although it had long been suspected that these insects were in some way connected with malaria and other diseases, While that the species now called Stegomyia calopus was the carrier of yellow fever had been asserted by Finlay as early as 1881, it was not until the closing years of the 19th century that the brilliant researches of Ross in India, and of Grassi and others in Italy, directed the attention of the whole civilized world to mosquitoes as the exclusive agents in the dissemination of malarial fever. The result has been that in subsequent years mosquitoes have been collected, studied and described by naturalists and medical men in all parts of the globe. Nearly 100 genera and about 700 species of mosquitoes are now recognized, but in all probability the total number of species is not less than 1000.

Fig. 1.
A,	Larva of Anopheles.	F,	Female Anopheles costalis, Loew.
B,	Pupa of Anopheles.	G,	Head of ♀ Culex.
C,	Larva of Stegomyia.	H,	Head of ♂ Anopheles.
D,	Pupa of Culex.	J,	Head of ♂ Culex.
E,	Egg-float and further enlarged   detached egg of Culex.	K,	Eggs of Anopheles.

In general appearance mosquitoes resemble many harmless midges (Chironomidae), but may be distinguished by the following characters. (1) The prolongation of the lower lip or labium into a prominent proboscis, which in the female sex contains the full complement of piercing organs found in blood-sucking Diptera, namely paired mandibles, paired maxillae, a tubular hypopharynx (the common outlet of the salivary glands), and an upper lip or labrum. (2) The presence of variously formed scales on the body and its appendages: the head is clothed with scales, the thorax with hairs or scales, and the abdomen with either hairs or scales, or both; the legs and veins of the wings are always covered with scales, and the palpi are often (as in some Anophelinae) conspicuously scaly. (3) The fact that the costal or marginal vein runs completely round the wing. The wings exhibit six longitudinal veins (seven in Heptaphlebomyia), two of which are characteristically forked. The antennae, usually bottle-brush shaped (plumose) in the male sex, are less hairy in the female. The palpi vary in form and in the number of their component segments, and the proboscis, though usually straight, may be curved (as in Megarhinus) or otherwise modified in shape.

In dividing the Culicidae into genera reliance is placed chiefly upon characters derived from the scales on the three divisions of the body and on the wings. A fairly satisfactory attempt at grouping the genera has been made by Lutz (1904), who divides the family in the first place into the Euculicidae, with a piercing proboscis (i.e. all ordinary mosquitoes), and the Culicimorphae or forms without a piercing proboscis (Mochlonyx, Corethra, &c.). It has since been proposed to treat the Culicimorphae as a distinct family under the title Corethridae, and it is probable that with this modification Lutz’s scheme will meet with general acceptance. The Euculicidae are divided into the Asiphonatae (＝Anophelinae), the larvae of which have no respiratory siphon, and the Siphonatae, or forms in which a respiratory siphon is present in the larval state. The divisions of the Siphonatae are the Ankylorhynchae (genera with curved proboscis, e.g. Megarhinus and Toxorhynchites) and Orthorhynchae (genera with straight proboscis). The latter again are divided into Metanopsilae (in which the metanotum or posterior region of the thorax is bare) and Metanotrichae (in which the metanotum is clothed with bristles or scales). The Metanopsilae are made up of the Heteropalpae [palpi long in the male, short in the female; sub-families Culicinae (Culex, &c.) and Heptaphlebomyinae (Heptaphlebomyia)] and Micropalpae [palpi short in both sexes; sub-families Aedinae (Aedes, &c.) and Haemagoginae (Haemagogus, Uranotaenia, &c.)]. The Metanotrichae are similarly divided on the basis of the palpal characters into two groups, the Heteropalpae or Hyloconopinae (Joblotia, Rhynchomyia, &c.) and Micropalpae or Dendromyinae (Wyeomyia, Sabethes, Limatus, &c.).

The old genus Anopheles (characterized by the palpi being long in both sexes) is now divided into a number of genera according to the character and shape of the scales on the different regions of the body and on the wings. These genera make up the sub-family Anophelinae, and together include over 100 species. The genus Culex, from which the family takes its name, though it has been similarly split up, is still in its restricted sense larger than any other, and some 200 species are comprised in it alone.

Mosquitoes are found in all parts of the world. Even within the Arctic Circle they are in many localities abundant and excessively bloodthirsty during the short summer. Under such conditions the deeply-rooted nature of the blood-sucking instinct is most remarkable; for insects whose ancestors for many generations may not have tasted blood will seek for it with the utmost keenness and pertinacity so soon as an opportunity presents itself. Some species are normally phytophagous, and the vast majority, at any rate, appear to be capable of continuing to exist and reproducing their kind upon a purely vegetarian diet. As a rule the blood-sucking habit is confined to the females, but in the case of a few species it is said to be common to both sexes. The thirst for blood is stimulated by heat, and in temperate climates it is only during hot weather that mosquitoes are troublesome. Some species of mosquitoes, such as the common gnat (Culex pipiens), are rarely found away from human habitations; others seldom or never enter houses, but are met with either in more or less open country, or in the recesses of forests and woods. In Europe and North America the continued existence of species is ensured by the hibernation of impregnated females, or else the winter is passed in the egg or occasionally in the larval state. In tropical climates with a well-marked dry season mosquitoes pass into a semi-dormant condition during the period when there is little Water in which to deposit their eggs. Culicidae are by no means confined to low-lying districts, and have even been met with in the Himalayas at an altitude of 13,000 feet. The wide distribution of certain species is undoubtedly attributable to the agency of ships and trains; under natural conditions mosquitoes seldom travel far from their breeding grounds, although the powers of flight of some species are greater than has been supposed.

(Redrawn by permission from Farmers Bulletin 155, Bureau of Ent.,
U.S. Dept. of Agriculture.)

Fig. 2.—Stegomyia calopus (Culex fasciatus, Stegomyia fasciata).

The preliminary stages of all mosquitoes are passed in water, either fresh or salt, stagnant or slightly moving. The nature of the breeding-place varies greatly according to the species, and while many of the mosquitoes that infest houses will breed even in the smallest accidental accumulation of water such as may have collected in a discarded bottle or tin, the larvae of other species less closely associated with man are found in natural pools or ditches, at the margins of slow-moving streams, in collections of water in hollow trees and bamboo-stumps, or even in the water-receptacles of certain plants. The eggs are usually deposited on the water itself, and while in the case of certain species, such as Culex pipiens or the widely distributed C. fatigans, they are agglutinated together in masses known as “boats” or “rafts” containing from 50 to 400 ova, those of others, such as the Anophelinae and many Culicinae (e.g. Stegomyia calopus), are laid separately. The larvae are active and voracious little grub-like creatures (known in the United States as “wrigglers”), with large heads and jaws provided with a pair of brushes, which sweep food-particles into the mouth. Their food consists of minute animal and vegetable organisms, algae, and probably decaying vegetable matter; they are often cannibals, and feed on their own species. The larvae of species belonging to the Culicinae have a prominent breathing tube, or respiratory siphon, on the penultimate (eighth) abdominal segment, and when taking in air hang head downwards (often nearly vertically) from the surface film. Larvae of Anophelinae, on the other hand—which are grey, green or brown in colour, and often extremely difficult to see—have no respiratory siphon and lie almost horizontally at the surface; they frequently appear as though anchored by the tail to a weed or other object, and possess the curious faculty of completely rotating the head so as to browse on the surface film. Mosquito pupae are comma-shaped (see fig. 1), and breathe by means of a pair of respiratory trumpets on the thorax.

The majority of mosquitoes are dull in hue, but certain species are brilliantly coloured or conspicuously banded or spotted with white. The Anophelinae have narrow bodies, and generally spotted wings, and when at rest keep body and proboscis in a straight line, often at a considerable angle with the supporting surface; in this way they can be distinguished from Culicinae, which have a humped-up thorax with which the proboscis forms an angle, and in the resting position keep the body parallel to the support.

The disseminators of malaria are exclusively Anophelinae, but even among these it is only certain species that are dangerous, since the others appear to be incapable of acting as hosts of the parasites. Stegomyia calopus, on the other hand, a very widely distributed species and the almost certain carrier of yellow fever, belongs to the Culicinae. In the case of filariasis due to Filaria bancrofti, which is common throughout the Tropics, the embryos of the parasite are disseminated by various Culicinae and Anophelinae (Culex pipiens in Queensland; C. fatigans in the West Indies; Myzomyia rossii in India; Pyretophorus costalis in a large portion of tropical Africa; &c.). Six or seven species of mosquitoes are also the intermediate hosts of Filaria immitis, which infests the right auricle and pulmonary artery of the dog, and occurs throughout the tropics, in southern Europe, the United States of America, and elsewhere. There is reason to believe that malaria, yellow fever and filariasis are not the only diseases disseminated by mosquitoes.  (E. E. A.)