28 THE NORTH AMERICAN SPECIES OF DROSOPHILA. 
the metathorax is not visible at all from above. These lateral angles appear 
as small humps, the hwmeri (fig. 7, Hu, 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 the 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 mesonotum (fig. 7, Mn). On each side of this sclerite is an 
incomplete transverse suture (T's), 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 Drosophiline 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). Morphologically 
this is really 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 (S;) lies just below the humerus, between the propleura and 
the mesopleura. The posterior thoracic spiracle (S.) 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 
ere 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 
