No. 421.] SKELETON OF THE HEAD OF INSECTS. 29 
or rather as a part of this sternite; for, as the invagination of 
the stomodzeum is surrounded by the labral rudiment, the labrum 
represents only that part of this sternite that lies cephalad of 
the mouth. 
The clypeus (clypeus anterior) isa sclerite between the front 
and the labrum ; for this reason, we believe it to be the sternite 
of the intermediate of the three preoral segments, the deuto- 
cerebral. 
We have described the sclerites derived from the procephalon 
as representing the sternites of the preoral segments. But 
strictly speaking, we believe that each represents only the 
median field of a sternite (Fig. 15, wf), and that the lateral 
elements of the sternites have not been separated from the 
pleural portions of the lateral fields of the segments; in other 
words, that the early embryonic divisions of the segments have 
been retained, and that those parts derived from the lateral 
fields of the segments form a single sclerite on each side of 
each segment. 
In the ocular segment each lateral sclerite constitutes one-half 
of the vertex and the corresponding gena, the line of union of the 
lateral sclerites being the stem of the Y-shaped epicranial suture. 
Each lateral sclerite of this segment bears a compound eye, 
except in cases where they have been lost and except in the 
larvae of metabolous insects, in which the development of these 
organs is retarded; this is obviously a secondary condition, 
like the internal development of the wings in the same forms. 
The position of the compound eye, in the lateral sclerite 
slightly removed from the middle field of the sternite (the 
front), is that in which one would expect to find an appendage, 
and it seems to us that the question whether or not the com- 
pound eyes represent the appendages of the ocular segment is 
still an open one. 
Heretofore the chief reason for regarding the compound eyes 
as representatives of appendages has been the stalked condition 
of them in certain Crustacea ; but later writers are inclined 
to regard the eye-stalks **as secondarily abstricted lateral por- 
tions of the head which have.become independently nuosable a 
gosc and Heider, '99a, p. 165). 
