42 THE AMERICAN NATURALIST. [VoL. XXXVI. 
resemblance to an ocellus (Fig. 19, dt). In the Orthoptera the 
peripheral end is more strongly chitinized than in the Plecoptera 
and firmly attached to the skull between the front and the 
antennal sclerite. It is small in the cockroaches, but is larger 
and easily seen in Gryllus (Fig. 27). In this case it bears some 
resemblance to an apodeme. 
The resemblance to an apodeme is much more marked in 
some other insects. Thus in the Hymenoptera each dorsal 
arm is firmly attached to the skull near the antennal sclerite, 
and, in Cimbex for example, it is hollow and has an open 
mouth, appearing exactly like an apodeme. 
It remains to be determined whether or not the dorsal arms 
in the Plecoptera are homologous with the apodeme-like dor- 
sal arms in other insects, and, if so, which type is the more 
generalized. 
The Body of the Tentorium. — This is the part to which the 
name ¢entorium was originally applied. It is the prominent 
bridge which divides the occipital foramen into two parts. The 
alimentary canal in entering the head passes above it, and the 
nervous system below it. It is formed of the coalesced and 
expanded tips of the arms of the tentorium (Fig. 28, 2). 
The Frontal Plate of the Tentorium. — In the cockroaches 
the anterior arms of the tentorium meet and fuse, forming a 
broad plate situated between the crura cerebri and the mouth; 
this plate may be termed the frontal plate of the tentorium 
(Fig. 26, ff). On each side, an extension of this plate con- 
nects it with the body of the tentorium; these enclose a cir- 
cular opening through which pass the crura cerebri. 
The Tendons of the Esophageal Muscles. — Muscles extend 
from the body of the tentorium to the cesophagus, passing 
between the crura cerebri. The tendons connecting these 
muscles with the body of the tentorium are frequently chiti- 
nized. In Pteronarcys there is a single chitinized tendon; in 
Periplaneta there are two (Fig. 26, toe). 
_ The Tendons of the Flexors of the Head. — These arise from 
"e bed lower surface of the body of the tentorium and extend 

Sometimes, as in dtt tron eel are e and 
chitinized. 


