Adults

Adult mosquitoes are usually 3-6 mm long, but some toxorhynchitines, which are the largest mosquitoes, are nearly 20 mm long. As in other insects the body has three principal regions, the head, thorax and abdomen. The head is the anterior section of the body bearing the compound eyes, antennae and mouthparts. It is separated from the second or intermediate division of the body, the thorax, by a membranous tube called the cervix, or neck. The thorax is comprised of three segments, each bearing a pair of legs. The largest or middle segment of the thorax bears the wings and the reduced third or posterior segment bears the halteres. The abdomen is the third, posterior division of the body. It consists of ten segments and the terminal genital structures involved in reproduction. The male genitalia are extremely important in species identification in most genera. Except for sclerotized elements of the alimentary canal and the reproductive system, the internal anatomy is not used in mosquito taxonomy. The presence or absence of setae and the placement and color of scales provide important characters for species identification.

Mosquitoes are much more extensively clothed with scales than other Diptera which have scales. The scales are either pale or dark. Dark-colored scales vary from reddish brown to black depending on the species and to a lesser extent on the age of the specimen. Some mosquitoes have metallic-colored scales which appear blue, green, gold, red or violet in certain light. Pale-colored scales are white, silver, yellow or gold. Scales may be broad and flat, narrow and curved, or erect and forked apically; those comprising the wing fringe are fusiform in shape. Scales are easily rubbed from the body; hence, important taxonomic characters may be missing in rubbed specimens. The color of the integument also varies interspecifically, and this may influence the overall color of the scales.

Abdomen Adult Mosquito

The abdomen consists of 10 segments of which only eight are distinctly visible. Segments IX and X are reduced and more or less modified as part of the external structures involved in reproduction, the genitalia. In some species segment VIII is sexually differentiated from the preceding segments, and in such cases it is also treated as part of the genitalia. Each segment consists of a dorsal sclerite, the tergum (pl. terga), which is joined by an elastic pleural membrane to a ventral sclerite, the sternum (pl. sterna). Intersegmental membrane connects the terga and sterna of adjacent segments. It is the elasticity of the pleural and intersegmental membranes that allows the abdomen of the female mosquito to become distended when she is taking a blood meal or when she is gravid and full of eggs. The terga are collectively referred to as the dorsum; likewise the sterna as the venter. Both the dorsum and venter are clothed with scales in culicines and toxorhynchitines, but are usually bare or have few scales in anophelines. The terga may have basal, apical or lateral patches of pale scales which provide useful taxonomic characters. Scale patterns are generally not so evident on the sterna.

The male genitalia of mosquitoes (segments VIII-X) undergo a clockwise or counter clockwise rotation of 180° shortly after emergence which results in a reversal of the dorsal and ventral structures. To avoid confusion, these structures are referred to in their original positions before rotation (dorsal and ventral, or tergal and sternal) even thought they appear to occupy the opposite positions in the mature specimen. Modifications of the genital parts are more or less complex and varied according to the genera and species, thus providing important taxonomic characters. Female genitalia have been largely neglected in mosquito taxonomy, but certain prominent structures have been used to a limited degree in recognising species of certain genera.

Head Adult Mosquito

Mosquitoes, both males and females, can be distinguished from all other Diptera by their long, slender, scaled proboscis. A few other flies also have an elongate proboscis, but it is not covered with scales.

Males of most mosquitoes can be distinguished from females by their verticillate (bushy, not plumose) antennae (sing. antenna). The sections, flagellomeres, of the third antennal segment, the flagellum, bear whorls of setae in both sexes, but in females these setae are relatively short. In the males of most species (not all), the setae on all but the last flagellomere are several times longer and more numerous than in the females.

The head is globular and largely comprised of a pair of prominent compound eyes. The eyes may be touching or separated both dorsally and ventrally. The dorsal surface of the head behind the compound eyes consists of the indistinguishable vertex (anteriorly) and occiput (posteriorly), and is more or less covered by erect and/or decumbent scales. The erect scales are usually truncate or forked apically and often differ in colour from the decumbent scales. They may be numerous, occurring on the vertex and the occiput, or they may be restricted to the occiput, usually in a transverse row. The decumbent scales may be narrow and curved or broad and flat. A row of anteriorly directed, ocular setae arise along the margin of each eye. One or more pairs of interocular setae located at the junction of the eyes (interocular space) are usually longer and project more strongly cephalad than the other ocular setae. The narrow anterior part of the vertex between the eyes bears the frontal tuft characteristic of most Anopheles species. The tuft composed of long setiform scales curving forwards over the frons and clypeus, together with setae and slender but shorter scales.

The antennae, maxillary palpi (sing. maxillary palpus) and the proboscis arise from a frontal area more or less transcribed by the compound eyes. Each antenna is divided into three segments. The basal segment, the scape, is hidden behind the greatly enlarged second segment, the pedicel. The pedicel often bears a group of minute setae or a small patch of scales on its mesal surface. The third segment, the flagellum, is divided into 13 or 14 false segments called flagellomeres. The first flagellomere usually also bears a mesal patch of scales. The remaining flagellomeres are usually without scales. In females, the flagellomeres are about equal in size, but in males of most species the apical two flagellomeres are longer. In genera of tribe Sabethini (e.g. Wyeomyia and Sabethes) and certain other genera (e.g. Hodgesia) the antennae of males resemble those of the females in not having whorls of long setae.

The maxillary palpi are composed primitively of five false segments or sections called palpomeres. In both sexes of anophelines, and in the males of nearly all other genera, the palpus is elongate and all five palpomeres are distinct. A small lobe (palpifer) at the base of the maxillary palpus gives the palpus of males a six-segmented appearance. Because of reduction of the apical palpomeres, the palpus of females appears to be composed of two or three palpomeres. The basal half of the first palpomere in both sexes lacks scales and setae. The rest of the palpus is covered with scales, and the three distal palpomeres (3-5) of males usually have setae.

The proboscis is covered with appressed scales and terminates in a pair of lobe-like labella (sing. labellum). The visible part of the proboscis is largely composed of the labium, which encircles the piercing stylets of the mouthparts. The proboscis is usually more or less straight or slightly curved, but in Toxorhynchites it is bent strongly backwards (reflexed). The labium is closed dorsally by the labrum. The distal part of the proboscis is distinctly swollen in some mosquitoes (e.g. Malaya and males of Ficalbia and Mimomyia).

Thorax Adult Mosquito

The thorax is comprised of three segments. Most of the thorax belongs to the second or middle segment, the mesothorax. The prothorax and metathorax are much reduced, especially dorsally. Each segment bears a pair of legs, while the mesothorax carries the wings and the metathorax bears the knob-like halteres (sing. halter). The dorsum of the prothorax is represented by laterally displaced halves, which are divided transversely into a lobe-like antepronotum and a flattened postpronotum. Dorsally the mesothorax occupies almost the entire thorax in the form of the large sclerite, the scutum, which is covered with scales and usually bears rows of setae. Those along the midline are called the acrostichal setae while the rows on either side are referred to as the dorsocentral setae. The arrangement of the scales and the patterns they form on the scutum are important in species identification. A small oblong or triangular sclerite called the paratergite lies at the edge of the scutum between the mesothoracic spiracle and the base of the wing. The presence or absence of scales and setae on the paratergite are often important in generic and species identification. Behind the scutum is the crescent-shaped scutellum. The scutellum is usually tri-lobed (except in Anopheles, Bironella and Toxorhynchites), covered with scales and bears groups of setae along the posterior edge. A dome-shaped mesopostnotum lies below and behind the scutellum. This area is usually bare, but scales and/or setae are present in some species and taxa. The dorsum of the metathorax is represented by an inconspicuous sclerite behind the mesopostnotum.

The lateral side of each thoracic segment is known as a pleuron (pl. pleura). In winged insects, a vertical ridge divides the pleuron into an anterior episternum and a posterior epimeron, and each of these may be divided longitudinally into a dorsal anepisternum and a ventral katepisternum. In mosquitoes, only the mesopleuron has the structure of a more generalised thoracic segment. The propleuron is represented by the partially fused, V-shaped proepisterna (sing. proepisternum) located between the forecoxae and the neck (cervix). Scales and setae borne on the proepisterna are often of taxonomic importance. The upper proepisternal scales and upper proepisternal setae are located above the base of each forecoxa while the lower proepisternal scales and lower proepisternal setae are borne in a mesal position below the neck and between the coxae. The upper and lower patches of scales may be contiguous. The anteprocoxal membrane situated between the forecoxa and the proepisternum and the postprocoxal membrane borne between the forecoxa and the mesothorax may also bear scales.

The mesopleuron is divided into unequal anterior and posterior sclerites by a mesopleural suture. The larger anterior sclerite, the mesepisternum is subdivided by a longitudinal anapleural suture into a dorsal mesanepisternum and a ventral mesokatepisternum. The anepisternum is further divided by a diagonal anepisternal cleft into anterior and posterior sections. The anapleural suture is indistinct in most mosquitoes, except Uranotaenia, so that the posterior section of the anepisternum (prealar area), which bears the prealar setae, appears to be a dorsal extension of the mesokatepisternum. The anterior portion of the anepisternum bears the mesothoracic spiracle. An important group of setae, the prespiracular setae, arise from a small sclerite (prespiracular area) on the anterior side of the spiracle. These setae are usually small and inconspicuous and arise immediately posterior to the postpronotal setae, which can be mistaken for the prespiracular setae. The presence or absence of prespiracular setae in combination with other characters is important in distinguishing mosquito genera. The remainder of the anterior section of the anepisternum is arbitrarily divided into three areas, the hypostigmal area, subspiracular area; and postspiracular area, which may bear taxonomically important setae and/or scales. The mesokatepisternum is well developed and bears two important groups of setae, the upper mesokatepisternal setae and lower mesokatepisternal setae. Scales are also associated with these groups of setae, termed the upper mesokatepisternal scales and lower mesokatepisternal scales by association. The mesepimeron, a rectangular sclerite located behind the mesopleural suture, is also divided transversely. The dorsal mesanepimeron, or simply the mesepimeron, is quite large and bears patches of setae and scales of taxonomic importance, while the ventral mesokatepimeron is an insignificant strip of bare cuticle. A small triangular sclerite known as the mesomeron is located below the mesepimeron and between the mid- and hindcoxae. The positional relationship of the mesomeron to the base of the hindcoxa is important in the higher classification and generic recognition of mosquitoes.

The metathorax has limited value in mosquito taxonomy. It is greatly reduced and only the metapleuron and the metameron are readily distinguishable. Metepisternal scales are present in Toxorhynchites and a few sabethine genera (Topomyia and some species of Malaya and Tripteroides). The metameron sometimes bears a patch of scales.

Wings Adult Mosquito

The wings of mosquitoes are long and slender, and the wing veins are covered with scales. The terminology currently in use for the veins is adopted from Colless & McAlpine (1970). The principal longitudinal veins of mosquitoes (from anterior to posterior) are the costa, subcosta, radius, media, cubitus and anal vein. The branches of these veins are indicated by numbers, thus the anterior branch of the media is media-one (vein M1) and the posterior branch is media-two (vein M2). Short, usually transverse veins between two longitudinal veins are known as crossveins. In mosquitoes, the usual crossveins are the humeral crossvein between the costa and subcosta, the radiomedial crossvein between the radius and media, and the mediocubital crossvein between the media and cubitus. Areas of wing membrane delimited by veins or by veins and the wing margin are termed cells. A cell is named after the vein immediately anterior to it or after the posterior element if the vein is formed by the fusion of two vein branches, for example, cell R5 is posterior to radius-four-plus-five (vein R4+5). The cell membranes are covered with minute spicules called microtrichia (sing. microtrichium). They are especially minute in species of Uranotaenia.

The veins are clothed with scales on both the dorsal and ventral surfaces, and the apex and posterior margin of the wing bear a fringe of scales wing fringe. The scales vary considerably in size and shape on different parts of the wing. The scales are usually dark, but many species have some pale scales either intermixed or in patches. Distinct patches of pale and dark scales known as wing spots occur in most Anopheles species and species of Aedeomyia, Finlaya, Orthopodomyia, the Mimeticus Group of Culex (Culex), Orthopodomyia, Psorophora and Uranotaenia, and these play a major role in species identification. A standardised nomenclature for the wing spots of these mosquitoes was developed by Wilkerson & Peyton (1990). The alula, a small lobe located posteriorly at the base of the wing, usually bears a line of marginal or submarginal scales of different shapes or posture in various taxa, and the upper calypter, one of two lobes connecting the alula to the thorax, sometimes bears setae or hair-like scales on its margin.

Legs Adult Mosquito

Each thoracic segment bears a pair of long, slender legs, which are almost entirely clothed in scales. Individual legs and leg segments are denoted by prefixing fore-, mid- or hind- as appropriate. Each leg is composed of six segments, the coxa (pl. coxae), trochanter, femur (pl. femora), tibia (pl. tibiae), tarsus (pl. tarsi) and posttarsus (pl. posttarsi). The tarsus consists of five false segments termed tarsomeres. The terminal posttarsus bears a pair of ungues (sing. unguis), a pad-like or spiculose empodium, and sometimes (Culex, Deinocerites, Galindomyia and Lutzia) a conspicuous pair of pulvilli (sing. pulvillus). Widespread dissatisfaction with the term ungues has resulted in the common usage of the term claws for these structures. The various parts of the legs often bear patterns of pale and dark scaling, especially bands and stripes, that are taxonomically useful. The ungues are usually simple in females, but in most species of tribe Aedini they bear tooth-like basal processes that are useful in species identification.

Eggs

Mosquito eggs are usually 1 mm or less in length. Eggs are laid on the surface of the water, either individually (e.g. Anopheles) or in rafts (e.g. Culex), or in sites subject to inundation by rain water (e.g. Psorophora). In the latter case the eggs hatch when flooded.

The shell, or chorion, of the mosquito egg is formed of two principal layers, an opaque inner chorion and a transparent outer chorion. In newly laid eggs, the inner chorion is also transparent, thus the eggs are white when laid and gradually become dark brown or black. The anterior end of the egg (the end opposite the head of the developing larva) bears the micropylar disc surrounding a minute opening, the micropyle, which permits the spermatozoa to enter during oviposition. The eggs of culicine mosquitoes are usually elongate-oval in shape. The shape of the eggs is generally characteristic of the various genera. The pattern of impressed and raised markings on the outer chorion provides useful taxonomic characters. The eggs of anopheline mosquitoes are generally boat-shaped. Dorsally, the outer chorion is modified to form a projecting frill, which partly or completely surrounds a flattened deck, and a pair of hollow lateral floats. Variations of these structures, along with the lobed tubercles at either end of the deck, provide characters for distinguishing closely related species.

Larvae

Descriptions and keys for identifying mosquito larvae are almost always based on fourth-instar larvae. Identification of earlier instars is generally not possible with such keys. The presence of well-developed mouth brushes (see palatal brush), fusion and enlargement of the thoracic segments and the tubular siphon borne on the dorsum of abdominal segment VIII in all genera except Anopheles, Bironella and Chagasia distinguish mosquito larvae from all other dipteran larvae.

Mosquito larvae have three principal body regions, the head, thorax and abdomen, which bear numerous setae (sing. seta) and surface features used in classification and identification. The head is a sclerotised capsule (cranium) bearing the mouthparts, eyes and antennae (sing. antenna). It is connected to the thorax by a membranous cervix (neck). The thorax is composed of the fused prothorax, mesothorax and metathorax, which are distinguished by series of bilaterally paired, circumferentially arranged setae. The abdomen tapers posteriorly and is composed of nine apparent segments. Since segment VIII actually comprises the embryonic eighth and ninth abdominal segments, the last or terminal segment is correctly recognised as abdominal segment X rather than IX. Segment VIII bears a dorsal siphon with an apical spiracular apparatus, which encompasses the spiracles. Anopheline larvae lack a siphon tube and the spiracular apparatus is borne on the dorsum of segment VIII.

The identification of mosquito larvae to species is largely dependent on the arrangement and numbers of branches of the setae (chaetotaxy). As many as 193 pairs of symmetrically arranged setae (excluding those of the mouth and mouthparts) are recognised on fourth-instar larvae. A set of six setae occurs on the antenna, a maximum of 19 pairs are found on the cranium, 15 on the prothorax, 14 on the mesothorax, 13 on the metathorax, 12 on abdominal segment I, 15 on abdominal segments II-VII, seven on abdominal segment VIII, 13 on the siphon (pecten plate in anophelines) and spiracular apparatus, and four on abdominal segment X. Individual setae are given a number followed with a hyphen and a capital letter or a Roman numeral to indicate the body area involved. This system uses the following symbols: A - antenna; C - cranium (head capsule); Lp - labropalatum; Mo - mouth; Mn - mandible; Mx - maxilla; P - prothorax; M - mesothorax; T - metathorax; I-VIII, X - abdominal segments 1-8, 10; S - siphon and spiracular apparatus.

The characters mostly used in mosquito identification are found on the head and terminal abdominal segments (segments VIII and X and the siphon), but certain thoracic and abdominal setae, usually the more prominent lateral ones, are also needed for identification.

Abdomen of Mosquito Larvae

The abdomen of mosquito larvae consists of nine apparent segments. The first seven segments are similar in appearance. In anopheline larvae, seta 1 may be palmate on some or all of these segments, but this seta is never palmate in non-anophelines. Seta 12 of segment I (seta 12-I) is absent in sabethine and some aedine genera, and this character has some value in distinguishing them from others. Abdominal segment VIII and abdominal segment X, the apparent ninth segment, differ significantly from the preceding segments. Segment VIII structurally comprises the embryonic abdominal segments VIII and IX. It bears a five-lobed valvular structure, the spiracular apparatus, which encompasses the postabdominal spiracles on the dorsal surface. The spiracular apparatus is borne at the apex of a siphon in culicine and toxorhynchitine larvae, and is highly modified in Mansonia, Coquillettidia and some Mimomyia larvae for piercing plant tissues. Segment X is generally smaller than the preceding segments and bears a sclerotised dorsal saddle, which may completely encircle the segment.

Segment VIII bears a wealth of taxonomic information. In most culicine and first-instar anopheline larvae, this segment bears a lateral patch of specialised spicules known as the comb. The individual spicules are called comb scales (comb scale) irrespective of their shape. The arrangement and character of the scales have significant taxonomic value. The scales may be thin and flattened and bear a fringe of denticles, they may be large and spine-like or the comb may consist of scales of different forms. The scales may arise directly from the integument or they may be borne on a sclerite termed the comb plate (e.g. Aedeomyia and most Uranotaenia). A comb is absent in Toxorhynchites, some Trichoprosopon and the second, third and fourth instars of anophelines.

The siphon provides a variety of diagnostic and differential features in toxorhynchitine and culicine larvae. Features of value include its shape, the relationship of its length to its basal diameter (siphon index), nature of the pecten and characteristics of the siphonal setae. The pecten, when present, consists of a varying number of pecten spines (pecten spine) that extend a variable distance from near the base of the siphon. The number, size and shape of the spines are important taxonomic characters. The pecten is much reduced in some mosquitoes and entirely absent in others. A pecten is absent in all species of 14 generic-level taxa, including Aedeomyia, Armigeres, Coquillettidia, Johnbelkinia, Kimia, Limatus, Mansonia, Orthopodomyia, Phoniomyia, Runchomyia, Sabethes, Shannoniana, Toxorhynchites and Trichoprosopon. Seta 1-S may be present or absent, single or duplicated (as in Culex), or variable in position. The position of seta 1-S in relation to the pecten spines is taxonomically important. In Culiseta, Ficalbia and Hodgesia, seta 1-S is inserted very near the base of the siphon.

Segment X is commonly known as the anal segment. In addition to the saddle, it bears two pairs of anal papillae, which have an osmoregulatory function, and four pairs of setae. The saddle covers the dorsal and lateral surfaces of the segment, sometimes completely encircling it, and other times it is much smaller. It is usually smooth, but can be spiculate posteriorly or over much of its surface, or bear spines just along the posterior border (e.g. Zeugnomyia). Sometimes the length of the siphon is divided by the dorsal length of the saddle to derive the saddle/siphon index, which may be of some use in distinguishing certain species. Seta 2-X and seta 3-X together comprise the dorsal brush, and seta 4-X comprise the ventral brush. The ventral brush serves as a rudder during swimming. It consists of a more or less linear series of irregularly paired setae that are often divided into two groups, a posterior group of cratal setae borne on a sclerotised grid and an anterior group of precratal setae borne before the grid, sometimes inserted on the ventral midline of a complete saddle. Certain larvae possess an elevated sclerotised boss in place of a grid. A grid or boss may be weakly developed or entirely absent. The number and development of setae 4-X provide important taxonomic characters. All species of tribe Sabethini, except species of subgenus Sabethinus of Sabethes, bear a single pair of seta 4-X, which may not be homologous with the ventral brush of other mosquito larvae. Species of subgenus Sabethinus bear an accessory pair of seta 4-X.

Head of Mosquito Larvae

The larval head capsule is dorsoventrally flattened and heavily sclerotised. It consists of three principal regions, the large dorsal apotome, the bulging lateralia and the midventral labiogula. The lateralia are separated on either side of the dorsal apotome by a frontal ecdysial line; they are separated from the ventral labiogula by a complete or incomplete hypostomal suture, or an imaginary line extending from the maxillary articulation to the posterior tentorial pit when the hypostomal suture is absent. A dorsal plate of the labrum, the median labral plate, is either fused with the anterior margin of the dorsal apotome or separated from it by a clypeolabral suture. The posterior opening of the head (occipital foramen) is usually partially or completely surrounded by a thickened rim-like collar.

The head of the mosquito larva is basically prognathous. The mouthparts, therefore, are considered as anterior appendages. The mandibles and maxillae exhibit a wealth of taxonomic characters that, unfortunately, are not extensively used in mosquito taxonomy. A lateral palatal brush (mouth[/no-lexcion] brush) is borne anteriorly on either side of the mouth. In most larvae, the brushes consist of numerous slender setae with fine pectination on one side, but in predatory larvae these setae are much thicker and greatly reduced in number. The antennae (sing. antenna) may be finely or coarsely spiculate, or lack spicules. The development and placement of seta 1-A is taxonomically important. In Coquillettidia, Mansonia and Mimomyia, the distal part of the [no-lexicon]antenna is elongate and distinctly articulated beyond setae 2,3-A. Only about six setae of the head capsule are used regularly for identification.

A dark triangular structure called the dorsomentum is situated ventrally behind the mouth. Its size and shape and the number and arrangement of its teeth provide characters for distinguishing some species. There are two eyes on either side of the head. The larger kidney-shaped one is the developing compound eye of the adult; the smaller posterior one is the  simple larval eye, the stemma.

Thorax of Mosquito Larvae

The thorax is generally globular and wider than the head. The setae of the prothorax are numbered 0-P to 14-P. Seta 0-P is generally small and borne laterally behind setae 4,5-P. Setae 1-3-P are frequently used in classifying anophelines, Culex and certain Uranotaenia. Setae 1-7-P have been used to distinguish certain species of tribe Aedini. The pleural setal group, setae 9-12-P,M,T, on all three of the thoracic segments are useful in classifying Anopheles. The metathoracic setal group is unusually small and useful in distinguishing larvae of genus Opifex. Other setae of importance include the presence or absence of seta 8-M and the development of setae 11-P,M,T and seta 13-T. The positional relationships of setae are sometimes important as well. The thoracic integument sometimes bears a covering of small spicules. A feature peculiar to anopheline larvae is the pectinate branching of most of the larger setae. In many anophelines seta 3-T is palmate. Seta 1-M is also palmate in species of Chagasia.

Pupae

Unlike other insect pupae (sing. pupa), the pupae of mosquitoes are remarkably active and suddenly swim with a tumbling motion when disturbed. The body of the pupa consists of an enlarged cephalothorax, representing the combined head and thorax, and the abdomen. The cephalothorax bears a pair of dorsolateral appendages known as trumpets. Each trumpet contains a mesothoracic spiracle. The abdomen consists of nine segments (the ninth segment is reduced and indistinct) and terminates in a pair of flattened paddles that enable the pupa to swim in a shrimp like fashion by jerking the abdomen. Despite an abundance of taxonomic characters, the pupae of most species have received little attention and are sometimes unjustly regarded as being taxonomically unimportant.