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ATTACHMENT OF THE ABDOMEN TO THE THORAX IN DIPTERA 269 
The course of the metathoracic apodeme (Plate XI, 8, ap*), together 
with the locations of the metathoracic spiracles (sp*) and the first abdominal 
spiracle (Isp), has been a great aid in deciding on these relationships. 
Besides the episternum and the epimeron, there is a third pleural 
sclerite in the metathorax of Diptera which seems to correspond to Cramp- 
ton’s (1914 b) compound sclerite, the pleurotrochantin (fig. 21, ptn.) This 
compound sclerite is described as being composed of the antecoxale (ac) — 
which is a narrow marginal strip of the basisternite — the trochantinelle 
(tnl), the trochantin (tn), and the lower part of the episternum. ‘This 
sclerite is present in a large number of. the higher Diptera, and especially 
is well marked in the acalyptrate muscids, as Leria serrata (Plate XXV, 
56, ptn®), Rivellia viridulans (Plate X XVII, 60, ptn’), and Sphyr acephala 
brevicornis (Plate X XIX, 67, ptn’). 
The sternum 
As in the mesothorax, the furcasternite — the ster nal region which bears 
the internal diapophyses, or furcae (Plate IX, 1, apys*) — seems to be the 
only sternite remaining in the order. A glance at the ventral view of 
Olfersia americana (Plate XXXII, 75) will show the marginal lips of the 
two sclerites termed pleurotrochantin (ptn*®) meeting in the mid-ventral 
line but separating caudad to form the furcasternite (fs’). 
The legs 
In no case was the metathoracic coxa found divided by a suture into 
the true coxa and the meron. If muscular tension causes the ridges of 
the body wall to be drawn inward for the formation of phragmas, apodemes, 
and apophyses, this may give a partial explanation of the disappearance 
of a suture dividing the coxa into two subdivisions, as the metathoracic 
muscles are greatly reduced in size due to the replacement of the wings 
by the halteres. 
Nothing further need be said concerning the metathoracic legs, except 
that the coxae show the greatest variation. Leia winthemi (Plate XII, 
12, ex*) and Sciara ochrolabis (Plate XIII, 13, cx*) show a considerably 
elongated condition. In Blepharocera tenuipes (Plate XIV, 17, ex*) the coxae 
assume a diagonal position between the thorax and the trochanter, while 
innumerable variations in shape exist. A number of the Acalyptratae show 
this part of the leg fairly well sculptured with secondary external sutures, 
