ATTACHMENT OF THE ABDOMEN TO THE THORAX IN DIPTERA 267 
the mesothorax to take care of the powerful wing muscles seems to have 
taken place at tie expense of the following thoracic segment. Especially 
is this evident in the tergal region, which usually consists of a narrow band 
of integument connecting the two halteres and providing a single lamella 
cephalad as its share in the formation of the mesothoracic postscutellar 
phragma (phg?,). It is impossible to make out the four principal sub- 
divisions of the tergum in the metathorax of Diptera. This plate reaches 
its greatest development in Psychoda slossoni (Plate X, 6), so far as the 
writer’s studies have gone. It may be continuous with the epimeron of 
this segment, asin Plecia heteroptera (Plate XIII, 15), Scenopinus fenestralis 
(Plate XVII, 26), Leptogaster loewi (Plate XVIII, 28), and many other 
species, or there may be a suture separating these two sclerites, as in 
Anthrax alternata (Plate XVI, 24), Platypeza velutina (Plate XIX, 33), 
Macrorchis ausoba (Plate XXII, 42), and many others; but practically 
in every case the tergum is separated from the episternum in this 
segment by the pleural suture. 
The pleuron 
Although comparatively small sclerites, the episternum (es*) and the 
epimeron (em’) are separated in the metathorax, as in the mesothorax, 
by the pleural suture. This infolding of the body wall is fairly constant 
in its extent from the coxae to the base of the halteres, and, studied from 
the inside, affords one of the best landmarks for homologizing these 
sclerites. Occasionally in the more generalized species the presence of 
secondary sutures in the episternal sclerite makes necessary the use of 
the terms anepisternum (aes*) and katepisternum (kes*), as in Pachyrrhina 
ferruginea (Plate IX, 3) and Leia winthemi (Plate XII, 12). Secondary 
sutures are present also in the posterior pleural sclerite, as in Dixa modesta 
(Plate X, 5) and Culex canadensis (Plate XI, 9), but because of the 
uncertainty as to the line of demarcation between the postscutellum and 
the epimeron the single term epimeron (em*) is used here. 
A large number of peculiarities or variations in the shape of the pleural 
sclerites are to be found in the various families of this order. Chief 
among these might be mentioned the following. Commonest of all, 
perhaps, is a greatly developed episternum with a resulting small epimeron, 
often nothing more than a narrow strip of chitin and in some cases result- 
ing in the lower part of the epimeron becoming membranous throughout. 
