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.