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.