No. 421.] SKELETON OF THE HEAD OF INSECTS. 39 
THE ENDOSKELETON OF THE HEAD. 
The endoskeleton of the head, like that of the thorax, con. 
sists of invaginations of the body-wall and of chitinized tendons. 
Some of these invaginations may be homodynamous with tho- 
racic trachez, but others are obviously apodemes. 
In many insects the mouths of some of the invaginations of 
the wall of the head remain open in the adult ; in Corydalis, for 
example, they are very conspicuous. 
Some of the apodemes remain separate, and are comparatively 
simple ; but in the case of two or three pairs of invaginations, 
they meet and coalesce. In this way there is formed in the 
interior of the head a complicated structure which is known as 
the Zentorium (Burmeister, '82, Vol. I, p. 25). 
The three pairs of invaginations which may enter into the for- 
mation of the tentorium are known as the anterior, the posterior, 
and the dorsal arms of the tentorium respectively. The coalesced 
and more or less expanded tips of these invaginations constitute 
the central portion of the tentorium, and may be designated as 
the body of the tentorium. From the body of the tentorium there 
extends a variable number of processes or chitinized tendons. 
The Posterior Arms. — The posterior arms of the tentorium 
(Figs. 26, 28, 29, pr) are the lateral apodemes of the maxillary 
segment. In many Orthoptera the open mouth of the apodeme 
can be seen on the lateral aspect of the head, just above the 
articulation of the maxilla (Fig. 4). In the Acrididz (Fig. 28) 
these apodemes bear a striking resemblance to the lateral apo- 
demes of the thorax (Fig. 16), except that the ventral process 
of the maxillary apodeme is much more prominent, and the two 
from the opposite sides of the head meet and coalesce, thus 
forming the caudal part of the body of the tentorium. 
The Anterior Arms. — Each anterior arm of the tentorium 
(Figs. 26, 27, 29, a£) is an invagination of the body-wall which 
opens on the cephalic margin of the antecoxal piece of the 
mandible (Fig. 2, a¢),! or, when the antecoxal piece is not distinct, 
1 Note that owing to the flexure of the head in those insects, like Corydalis, in 
which the mouth is at the anterior end of the body, the opening of the anterior 
usi apte t OU O e e T T 

