272 BENJAMIN P. Youne 
species as Borborus equinus (Plate XXVI, 57), Rhopalomera flaviceps 
(Plate XX VII, 61), and Loxocera pleuritica (Plate XXIX, 66). These and 
other examples that might be cited seem to warrant the consideration of 
this first definitive tergite as the fused first and second tergites. In 
unbleached specimens these indications of a suture would hardly be 
noticed, and so it is not to be wondered at that systematists of the order 
do not always agree as to the number of abdominal segments. 
Another possible source of confusion to the systematist is to be found. 
in the division of tergites into secondary parts, either by sutures or a strip 
of membrane. This may occur in either the first or the second segment. 
As an example of a tergite being divided into two subdivisions by a suture, 
one need only refer to the drawings of Szmulium hirtipes (Plate XIV, 16, 
lt, and 1t,) and Midas clavatus (Plate XVII, 27, 1t, and 1t,). Examples 
of division by membrane may be seen in Sciara ochrolabis (Plate XIII, 13, 
lt, and 1t,) and in Plecia heteroptera (Plate XIII, 15, 2t, and 2t,). An 
example of the separation of the first and second tergites by a narrow 
band of membrane or weak chitin is given in Allognosta fuscitarsis (Plate 
XV, 19). 
In several species the first abdominal tergite has so overgrown the thorax 
that on superficial examination it appears as a part of this region. This 
condition is most evident in such species as Chrysops indus (Plate XV, 
20, 1t) and Anthrax alternata (Plate XVI, 24, 1t). 
A very noticeable character, and one that was first found in Myopa 
vesiculosa (Plate XX, 36), is an adventitious suture arising from the 
latero-cephalic margin of the first abdominal tergite and running caudo- 
dorsad through this tergite often to the cephalic margin of the second 
tergite. This varies considerably in its degree of development in different 
species, but was found in all the species examined among both the 
Calyptratae and the Acalyptratae. Generally speaking, this suture is 
less highly developed in the calyptrate than in the acalyptrate muscids, 
as is seen on comparing Thelaira nigripes (Plate X XI, 39) and Muscina 
stabulans (Plate XXII, 41) with Scatophaga stercoraria (Plate XXV, 
54) and Sepsis violacea (Plate XXVIII, 64). 
The sternites 
The greater demand made upon the tergites for the attachment of 
muscles than upon the sternites is clearly reflected in the larger number 
