264 BengAMInN P. YouNG 
meso- and the metathorax are clearly set off from each other, while the 
beginning in the development of phragmas is seen in both the anterior 
and the posterior infolding of the walls of the terga of these two thoracic 
segments. The maximum development of phragmas can be seen in the 
posterior mesothoracic and the anterior metathoracic one in some dip- 
terous species. 
For convenience in comparing, a copy of Crampton’s (1914 b) “ ground 
plan of a typical thoracic segment in winged insects” is here inserted 
(fig. 21). In the author’s own words, this figure 
represents an hypothetical composite type, to which the thoracic segments of any insect 
can be referred as a basis of comparison, rather than an attempted reconstruction of the 
original condition of the thoracic sclerites in the ancestors of winged insects. Most of the 
primitive features, however, are included in the figure, and to these have been added condi- 
tions found in the more specialized insects. 
DISCUSSION OF PARTS 
The mesothorax 
The teryum 
The two plates of the tergum as described by Verhoeff (1902) and by 
Snodgrass (1909-a) are readily distinguished in all forms figrred. The 
large anterior plate, extending backward and including the axillary cord 
of Snodgrass (1909 b) and the post-tergite of Crampton (1914 b), composed 
largely of the scutum and the scutellum, has been called the scuto-scutel- 
lum when there are no visible divisions. Audouin (1824) has been fol- 
lowed herein in his divisions of the scuto-scutellum. The most anterior 
median region is termed the prescutum. The large central region, which 
is very often fused with the prescutum and also often subdivided by second- 
ary sutures, is called the scutum. Laterally this division carries both the 
anterior and the posterior notal wing processes of Snodgrass, both of 
which are usually well defined in Diptera. ‘The third division, known as 
the scutellum, is fairly constant as a narrow elevated median plate extend- 
ing laterally into a narrow band bearing the axillary cord, which is con- 
tinuous with the anal margin of the wing. 
The small caudal division of the tergum when not divided is termed the 
postscutellum. This plate is often divided by sutures into a median and 
one or two lateral sclerites. When such is the case, only the abbreviations 
for these divisions appear on the drawings. For the middle plate the 
term meditergite (mt), and for the lateral plate or plates the term pleuro- 
