No. 421.] SKELETON OF THE HEAD OF INSECTS. AI 
In the cockroach (Fig. 26) the buttress of the anterior arm 
that extends along the suture between the front and the gena 
is twisted in a way that suggests that it has been pulled out of 
place by the migration of the antenna. If we imagine the 
antennal sclerite pushed into a position which would untwist 
this buttress, it would bring this sclerite opposite the clypeus, 
that is, in its assumed primitive position. 
In the forms upon which the study is 
chiefly based, — the Plecoptera, the Orthop- 
tera, and the Neuroptera (Sialidz), — the 
antecoxal piece of the mandible is joined to 
the clypeus near its cephalo-lateral angle as 
described above. But in the Hymenoptera 
that we have studied it is joined to the 
.caudo-lateral angle of the clypeus very near 
the labrum. A result of this in some forms 
is that the point of invagination of the 
anterior arm is remote from the front, as in 
the ants, although even here the buttress- 
like extension follows the margin of the 
clypeus and extends along the suture be- 
tween the clypeus and the front. In other 
Hymenoptera the opening of the invagina- 
tion has migrated to the cephalo-lateral angle 
of the clypeus and is thus remote from the | 
articulation of the mandible ; this is the case Frc. 29.— Teniorium of 





Fic. 28. — Head of Mela- 
noplus, caudal aspect. 



. Melanoplus, cephalic 
in Bombus. cei 
The Dorsal Arms. — Each dorsal arm of yay dorsal arms 
the tentorium arises from the side of the 
body of the tentorium between the anterior and posterior arms 
and extends either to the front or to the margin of the antennal 
:sclerite (Figs. 26, 27, 29, dz). 
In the Plecoptera it appears to be merely a chitinized tendon, 
the peripheral end of which is less chitinized than the base and 
is only loosely attached to the skull. Here the point of attach- 
ment is on the front, some distance from the antennal sclerite, 
beneath a spot in the cuticle, which is marked by numerous 
polygonal areas. This spot is very distinct and bears some 
i 
