254 The American Naturalist. [March, 
by a faint longitudinal median carina, and its posterior border 
presents a reéntrant angle into which the forward-projecting 
apex of the scutellum fits. The scutellum of the mesothorax 
is subtriangular in outline with rounding angles. The dor- 
sum of the metathorax is less than one-third as long as the 
meso-notum. The scutum is chiefly apparent in its two lateral 
portions, the median portion being reduced to a narrow trans- — 
verse bar; the scutellum is a small transversal sclerite with a 
curving anterior margin. The pleural aspect of the meso- 
thorax is markedly greater than that of the metathorax, but 
the relation between them does not show such a preponderance 
of the mesothorax as is shown on the dorsum. 
With the specialization of the flight function indicated by a 
cephalization of flight, including a specialization of the fore- 
wings and a reduction of the hind-wings, there comes a cor- 
relative change in thoracic structure. This may be shown in 
Hemaris thysbe, one of the swift-flying sphinges, with a highly 
specialized venation. The mesonotum (see fig. 3, plate XVII) 
constitutes almost the entire dorsal aspect of the thorax, the 
metanotum being limited to a very narrow transverse bar, 
dilating laterally to more conspicuous dimensions. The scutum 
of the mesothorax is almost as broad as long. 
' Turning now to the Jugate, the venation of whose wings, 
according to Professor Comstock, is the most generalized of any 
among the Lepidoptera, an examination of the thorax reveals 
a distinctly generalized condition. In Micropteryx unimaculella 
(see fig. 4, plate XVII) the mesonotum does not exceed the 
metanotum in length by more than one-half the length of 
the latter. The general outlines of the dorsum of both seg- 
ments are much alike, and the scutum and scutellum of the 
mesonotum resemble the equivalent sclerites of the meta- 
‘notum in shape. In each segment the scutellum projects for- 
ward into an angular emargination of the hind border of the 
scutum. In the morespecialized forms of the Lepidoptera the 
reduction of the metanotum is accompanied by the narrowing 
of the median portion of the scutum, until, in many cases, the 
scutum is divided, apparently, by the scutellum into two 
lateral pieces. 
