28 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 



the metathorax is not visible at all from above. These lateral angles appear 

 as small humps, the humeri (fig;. 7, Hv, in side view). Each bears from one 

 to three bristles and a number of hairs. Beneath the humerus, and ex- 

 tending to the base of the coxa of the front leg, lies the propleura (fig. 7, Pp). 

 This is usually bare, but in a few forms has a single bristle. Several small 

 prothoracic sclerites are visible on tli« anterior surface of the thorax when 

 the head is removed. 



Mesothorax: The greater portion of the dorsal surface of the thorax is 

 made up of the mesonoium (fig. 7, Mn). On each side of this sclerite is an 

 incomplete transverse suture (Ts), sometimes referred to simply as the 

 suture. Behind the mesonotum lies the scutellum {Sc), which is roughly 

 triangular in shape as viewed from above, and which overhangs the dorsal 

 surface of the metathorax. The scutellum nearly always bears four mar- 

 ginal bristles, but among the Drosophilinse is usually without hairs (except 

 in Curtonotum). Below the mesonotum and behind the propleura lies the 

 large subquadrate mesopleura (Ms). This plate is bare, except in such forms 

 as Curtonotum and Camilla. The suture separating it from the mesonotum 

 is the notopleural suture. Behind the mesopleura and below the base of the 

 wing lies the pteropleura {Pt). It is always bare. Below the mesopleura 

 and pteropleura, and above the coxa of the middle leg, lies the sternopleura 

 (St). This plate always bears a few bristles on its upper part and a few 

 hairs below. Near the base of the wing there are a number of complex 

 smaller mesothoracic sclerites that I have not worked out in detail. 



Metathorax: The dorsal part of the metathorax, as far down as the level 

 of the spiracle, is known as the metanotum (fig. 7, Mt). ]\Iorphologically 

 this is reall}'- a complex of several sclerites; but the sutures separating them 

 are obscure, and it is convenient to treat the structure as a unit. Below 

 this plate and above the hind coxa lies the hypopleura (Hp). These two 

 regions are both usually bare, the only metathoracic bristles being on the 

 hind legs. 



The thorax bears two large pairs of spiracles. The anterior thoracic 

 spiracle (/Si) lies just below the humerus, between the propleura and 

 the mesopleura. The posterior thoracic spiracle (»S2) lies below the 

 haltere, between the metanotum and the hypopleura. 



The thoracic bristles and hairs of taxonomic importance, other than 

 those named for the parts on which they occur, are the following: 



Dorsocentrals (fig. 7, ds) : On the median posterior part of the mesonotum, 

 in front of the scutellum. There are usually two pairs, and they are the 

 conspicuous backward-pointing bristles seen in a dorsal view of the thorax. 



Notopleurals {np): Two bristles on the mesonotum, just above the noto- 

 pleural suture. 



Presutural (ps) : Behind the inner angle of the humerus, in front of the 

 transverse suture, and nearer the median line than is the anterior noto- 

 pleural. 



Supra-alars (sa): Behind the transverse suture; above and near or an- 

 terior to the anterior portion of the base of the wing. Usually a small 

 anterior one and a large posterior one. 



Postalars (pa): Above the middle of the wing-base, just in front of the 

 scutellum, and lateral to the dorsocentrals. A large anterior one and a 

 small posterior one. 



Prescutellars: Between the members of the posterior dorsocentral pair, 

 and at the same level with them. Absent in most members of the genus 



