270 Z BrenJAMIN P. YoOuNG 
as in Scatophaga stercoraria (Plate XXV, 54, cx*), Dictya umbrarum (Plate 
XXVI, 58, cx’), and Sapromyza lupulina (Plate XX VI, 59, cx*). 
The abdomen 
General description 
In taking up the discussion of the morphology of the abdomen in its 
relation to the thorax, the tergites, the sternites, and the spiracles are 
considered separately. There is little to be said in regard to the pleura 
aside from the fact that they are commonly taken as the membranous - 
areas between chitinized tergites and sternites. Needless to say, this 
region shows a considerable variation in width in species showing some 
deposition of chitin in the sternites and the tergites. A comparison of 
Calobata albiceps (Plate XXVIII, 63) with one of the calyptrate muscids, as 
Thelaira nigripes (Plate X XI, 39), will at once show a wide difference. In 
the latter example the pleura consist only of narrow inflexed areas between 
the greatly enlarged tergites and the small sternites which they have over- 
grown. In the case of species showing no deposition of chitin in the sternal 
region, as Phora concinna (Plate XIX, 32) and Chlorops sp. (Plate XXX, 
69), and in species showing no deposition of chitin in either the tergal or 
the sternal region, as Orphnephila americana (Plate XIV, 18), Melophagus 
ovinus (Plate XXXI, 73), and Olfersia americana (Plate XXXI, 74), it 
is impossible to speak of a definitive pleuron. 
In homologizing the sclerites in the abdomen of Diptera the position of 
the spiracles is of invaluable aid. In fact, it would be next to impossible 
to be sure of the sclerites of the first segment without the location of the 
first abdominal spiracle. An internal study is often necessary to establish 
the position of this opening to the tracheal system. 
In this order, usually five more or less definite abdominal segments may 
be seen. Beyond the fifth the segments are variously modified to form 
the genitalia. For this reason it is very easy to overlook spiracles beyond 
the fifth, as these are often small and show a tendency toward cephaliza- 
tion. It is not unusual to find those of successive segments crowded 
close together in the same segment in this region. 
The tergites 
There are many interesting features in the structure of the abdominal 
tergites. In looking at the group as a whole, it is quite evident that 
there is a tendency for the first tergite, as well as for the first sternite, 
