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.