58 MOSQUITOES OF NORTH AMERICA 
bases, both of the notal wing processes, 7. ¢., the lateral lobes of the notum to 
which the wings are articulated, arise near the posterior end of this plate. The 
first belongs to the scutum, the second to the scutellum. The scutum bears also 
a small lateral lobe (w) between the front spiracle and the base of the wing. The 
axillary cords (AxC), which form the corrugated cord-like thickenings of the 
posterior margins of the basal or axillary membranes of the wings, arise from the 
posterior margin of the scutellum. They thus mark the posterior limit of the 
true notum as distinguished from the postnotum, though the two plates are 
separated from each other by a membranous suture. 
The mesopleurum is triangular, its corners being marked by the anterior 
spiracle (1Sp), the base of the wing (W,) and the base of the middle coxa 
(Cz,). Its two main plates, the episternum (Hps,) and the epimerum (Hpm,) 
are separated by the pleural suture (PS,) which extends from the coxal articu- 
lation into the pleural wing process (WP,). In front of the main episternal 
plate (Hps,) is a smaller one (eps,) evidently derived from the first but sepa- 
rated from it, except ventrally, by a wide membranous suture. Its front edge 
is united along a weak suture with the prothoracic epimerum (Hpm,) and the 
plate itself, therefore, may appear to be a part of the latter, though its dorsal 
end projects upward behind the first spiracle. In Tipula there is no separate 
sclerite corresponding with this plate, the mesoepisternum being continuous 
from the pleural suture to the first spiracle. In Bibio there is a suggestion of 
the separation of the anterior plate from the rest of the episternum, while in all 
higher flies it forms a conspicuous sclerite on the anterior part of the side of the 
thorax. Some authors have regarded it as a part of the mesosternum, but this 
would involve the unseemly condition of having the sternum reaching to the 
tergum in front of the episternum. 
The principal episternal plates (Hps,) of the opposite sides meet on the 
ventral surface and fuse with each other along the mid line. In most of the 
Diptera a distinct longitudinal suture cuts off a ventral plate from the lateral 
episternal plate, which is usually regarded as a mesosternal sclerite, but no such 
suture is present in Psorophora. 
The epimerum (H#pm,) of the mesothorax is a large plate lying behind the 
pleural suture (PS,). Below itis asclerite (S,) lying posterior to the mesocoxa 
(Czx,) and connected ventrally with the mesosternum. It may be regarded, in 
fact, as a postcoxal wing of the sternum, though there is no positive evidence that 
it does. not belong to the epimerum. Its upper end is overlapped by a small 
marginal sclerite (6) on the edge of the epimerum. 
The articulation of the coxa is against a small triangular plate (a) wedged in 
between the episternum (Hps,) and the postcoxal sclerite (S,), beneath the 
epimerum (Zpm,). The internal pleural ridge extends upward from this plate 
along the line of the pleural suture (PS,) to the pleural wing process (WP,). 
The latter is formed by the adjacent parts of both the episternum and the 
epimerum, and is the support of the wing from below. 
The mesothoracic paraptera are not as well developed in the mosquito as in 
some other flies. The first is absent entirely and the second (2P,) is fused with 
