No. 421.] SKELETON OF THE HEAD OF INSECTS. 35 
the thinner band in front of the invagination is the epister- 
num, and the invagination itself is the lateral apodeme of this 
segment. 
From this it will be seen that the posterior arms of the 
tentorium are to be homologized with lateral apodemes instead 
of with spiracles, as is often done. It should be borne in mind, 
however, that the spiracles are lateral invaginations between 
segments, and that the lateral apodemes are invaginations in 
a similar position between subsegments (Fig. 16). If each 
segment consists of two consolidated metameres, the lateral 
apodemes and spiracles are homodynamous structures. The 
solution of this question must wait, however, the solution of 
the larger question, the structure of the segment in air- 
breathing arthropods ; we need not dwell upon it here. 
The superlingual segment is so greatly reduced that we are 
able to find no trace of the lateral elements of it in the skele- 
n; if they exist, they are inseparably united with the mandib- 
ular pleurites. The sternal elements are represented by that 
part of the floor of the mouth cavity that bears the superlinguz ; 
and the appendages of the 
segment, by the superlinguz 
themselves. In the Ptery- 
gota these parts are greatly 
reduced and have received 
but little attention. Fig. 24 
represents the hypopharynx 
of Melanoplus, in which the 
lingua is very prominent, and what we regard as the superlinguze 
are reduced to a pair of small sclerites. As the superlinguz 
have been carried into the mouth cavity by its invagination, 
they lie behind the lingua, although they originated in front 
of it. The superlinguz are connected with the skull by a 
membranous portion of the body-wall, which, on each side, 
extends between the attachments of the maxilla and of the 
mandible. 
In the mandibular segment the pleurites are represented by 
the postgenz, which, excepting the vertex and genz, are the 
most prominent sclerites of the head in many Orthoptera. 





Fic. 24. — Hypopharynx of Melanoplus. 
