INTRODUCTION. 
possible to get two mandibles into the same position for compari- 
son. When seen from slightly different angles, the form of the 
mandibles and the comparative size of the teeth, or even the num- 
ber of teeth, appear very different. Often the mandibles are 
asymmetrical, differing in their armature of teeth, hairs, and 
brushes. The labrum is separated from the fused frons and cly- 
peus by a membranous hinge by means of which the labrum folds 
back within the capsule of the head, often becoming invisible from 
above without skilled manipulation. The labrum is usually chiti- 
nous, but in some families is membranous. It is provided with a 
few large sete, often with dense brushes of hairs, and peculiar, 
cycle-shaped, marginal sete, which offer good taxonomic charac- 
ters. The labium and lower mouth parts differ in different fami- 
lies, but only slight study has been given them and they are used 
but little in classification. 
The thorax consists of three distinct segments. The dorsal 
surface of the first thoracic segment, or pro-thorax, is termed the 
pronotum, and is always covered by a heavily chitinized plate, 
which extends over the sides of the segment. On the ventral sur- 
face of the prothorax there is often a somewhat curved, finger-like 
process, of doubtful function, which is found in no other order of 
insects. It is termed the “prosternal horn” or the “horn.” The 
dorsal surface of the two succeeding segments, the meso- and the 
meta-thorax, are termed the meso- and the meta-nota. The meso- 
and meta-notum may each bear from one to four chitinous plates, 
or they may be entirely membranous. On the sides of the three 
thoracic segments, forming supports for the legs, are chitinous 
plates which probably represent the episternum and the epimeron. 
They offer most excellent taxonomic characters, but as yet are 
but slightly studied and their use has been almost entirely neg- 
lected. All of the plates of the thorax are armed with character- 
istic spines and sete. 
A pair of jointed legs is borne by each thoracic segment. These 
often arise from bulbous swellings of the integument of the body 
which closely resemble the coxe of certain insects and have some- 
times been confused with the basal joints of the legs. Commencing 
at the basal end of the leg, its segments are as follows: (1) The 
Coxa, a large, entirely chitinized segment. (2) The Trochanter, 
usually a short, somewhat triangular segment, that is divided by a 
suture into two pieces of almost equal length. (In the Lepto- 
II 
