No. 421.) SKELETON OF THE HEAD OF INSECTS. 25 
between the coxa; the former pertains to the first subsegment, 
the latter, to the second. 
In the furrow or suture between these two sclerites are the 
invaginations forming the furca of that segment. The position 
of the furca is, therefore, a good landmark for determining the 
line of union of the two scle- 
rites forming a sternite, or, in 
other words, the division be- 
tween the subsegments. 
The second of the sclerites 
forming a sternite is smaller 
than the first, even in those in- 
sects where it is best developed; 
and in most insects it is greatly — 
reduced or obsolete, so that the vote ita Viris ug alamo 
furca appears to arise from the pne — Hie body sre amined 
caudal margin of the segment 
(Fig. 14). It is for this reason that the sternite of the 
second subsegment has been overlooked heretofore. The ster- 
nite of the first subsegment is retained in all insects, and is 
the sclerite to which the term sternum has been universally 
applied. The smaller sternite of the second subsegment may 
be termed the sternellum (Fig. 12, S2; Fig. 13, S2). 
Sometimes, as in Pteronarcys, the cephalic portion of the 
sternum is more or less separate from the main part; this 
detached portion may be termed the presternum (Fig. 12, je 
A poststernellum, corresponding to the 
postscutellum, has not been observed. 
It has been shown by Haase (89) and 
= | Heymons (95) that in a comparatively 
| | early embryonic stage each segment of 
M iei the body is composed of three parts, — 
Frc. 15.— Diagram ofa segment a median field and two lateral fields 
aan embryo, air Heymons- (Fig. r5). and that the appendages 
are developed as evaginations of the lateral fields. Each ster- 
nite is therefore composed of three parts, — the portion derived 
from the median field of the segment, and, on each side, a 
portion derived from that part of. the lateral field which lies 








