12 DRAGONFLIES OF NORTH AMERICA 
the lips (Fig. 1) are of a strictly flesh-eating type. Above the upper 
lip (labrum) the clypeus (in two pieces ante- and post clypeus) covers 
the flat central portion of the face. The front rises above it in a more 
or less shelf-like or angulate prominence. The vertex, bearing the 
ocelli, and elevated, often into a high prominence at the front, extends 
backward between the compound eyes; and at the rear the angulate 
hind margin of the head is formed by the occiput. 
The prothorax is small, more or less deeply divided by transverse 
furrows on the dorsal side into a succession of lobes, the hindmost of 
which is often hair-fringed or sculptured. The synthorax is large and 
quite remarkable for the great development of its side plates. By 
their expansion the tergum and wings are pushed far upward and 

Fia. 1. Face view of the head of the club-tail Gomphus graslinellus. occ, 
occiput; v, vertex; f, frons; c, postclypeus; c’, anteclypeus; lJ, labrum. 
backward, and the sternum and legs, far downward and forward; the 
episterna meet on the middle line at the front above; and the epimera 
meet at the back below. By this arrangement the very large muscles 
of flight are accommodated within. Their pull is vertical, and to 
withstand it, strong braces are developed along the edges of these 
chitinous plates: at the lateral sutures infolded edges form internal 
ridges (not externally visible); and at the front the episterna meet on 
the median line in a strong and conspicuous ridge that is called the 
middorsal thoracic carina. It ends below in a transverse ridge called 
the collar, and above in a forking ridge, that runs out around the wing 
roots, called the crest (see fig. 27). These are landmarks much used 
in descriptions. 
The backward slant of the side pieces of the synthorax is one of the 
striking peculiarities of the structure of dragonflies. This slant has 
been measured in a series of forms (Needham and Anthony, ’03) and 
