64 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 139 



THE THORAX 



The thorax of a winged insect may truly be said to be the most re- 

 markable anatomical mechanism developed anywhere in the animal 

 kingdom. It is remarkable both for its efficiency as a flight mechanism 

 and for its structural simplicity. In insects with two pairs of wings 

 the two wing-bearing segments have essentially the same structure, 

 and are equipped with duplicating sets of muscles. In the Diptera, 

 however, in which the flight function has been taken over entirely by 

 the first pair of wings, the mesothoracic wing muscles have to do the 

 work of the muscles of both winged segments in four-winged insects. 

 Consequently, the mesothorax of the flies has been greatly enlarged 

 and the metathorax much reduced. The knobbed stalks known as 

 halteres borne on the metathorax are undoubtedly reduced wings, 

 since, as seen in the mosquito pupa (fig. 17 E), they are developed in 

 flat wing lobes of the metanotum. They are still important accessories 

 of flight, being vibratory organs for maintaining the equilibrium of 

 the flying insect, but their musculature is very simple, and the usual 

 wing musculature of the segment has been eliminated. 



In the adult mosquito (fig. 25) the mesothorax appears as a great 

 wedge inserted between the narrow prothorax and metathorax. It 

 alone retains the structure typical of a thoracic segment. Two princi- 

 pal plates, an anterior notum (AN 2 ) and a posterior postnotum 

 (PN 2 ), cover almost the entire dorsum of the thorax. The strongly 

 convex postnotum, furthermore, is deeply infolded posteriorly be- 

 neath the narrow metanotum (N s ) and extends into the first ab- 

 dominal segment as a bilobed phragma (fig. 27 D, Ph). A narrow 

 paranotal fold (pnf) borders the edge of the notum between the first 

 spiracle and the wing. The pleural area tapers downward and becomes 

 continuous with the sternum (S 2 ) between the first and second legs. 

 A typical pleural sulcus (PIS 2 ) extends from the base of the middle 

 leg to the wing fulcrum at the base of the wing (W). The area before 

 the groove is episternal, that behind it epimeral. The episternal area 

 includes a major episternal plate (Eps 2 ) continuous below with the 

 sternum, and a smaller preepisternum (eps 2 ). The epimeron (Epm 2 ) 

 is a simple quadrate plate. Below it is a small triangular plate (S s ), 

 which in the mosquito appears to be a postcoxal lobe of the sternum ; 

 but a plate in the same position in higher flies is the detached meron of 

 the coxa. In some species the episternum is divided into an upper 

 and a lower part (fig. 27 A). 



The prothorax is so reduced and modified that it is difficult to 

 interpret its parts. The notum (fig. 25 Ni) includes a narrow plate 



