No. 421.] SKELETON OF THE HEAD OF INSECTS. 31 
support the view that the antenna are postoral in the early 
embryo support the opposite view. 
A migration cephalad of the antennz has been noted by many 
observers ; and it is obvious that the position of the antennz in 
adult insects is more or less remote from the primitive position. 
In the Plecoptera is to be found the most generalized condi- 
tion of the antennal sclerites yet observed in postembryonic 
stages of insects (Fig. 19) ; here they are distinct sclerites, 
and are only slightly removed from the clypeus, compared with 
their position in the more specialized insects. Even here, 
however, they are opposite the front, having been pushed out 
of place by a migration of the mandibles and the antecoxal 
pieces, to be described later. 
In most insects each antenna has migrated along the suture 
between the front and the gena, and occupies a position on the 
lateral border of the front remote from the clypeus. Even in 
so generalized an insect as a cockroach (Fig. 2), the antenna 
has reached a point opposite the cephalo-lateral angle of the 
vertex. Here it is remote from the anterior arm of the ten- 
torium ; while in Pteronarcys it is very close to it. 
In the case of the second antennal segment, the reduction 
has been so great that we have been able to find in the Ptery- 
gota no trace of the parts derived from the lateral fields of the 
segment ; the labrum is the only well- 
marked remnant of this segment rep- 
resented in the skeleton ; it is possible 
that the lateral elements are fused with 
the gena. It is probable that a study 
of those Apterygota in which the second 
antennze are retained will reveal the pres- 
ence of distinct lateral sclerites pertain- 
ing to this segment. Fic. r9. — Head of a nymph 
In taking up the study of the postoral E rim 
segments of the head, the subject can be treated most easily 
by beginning with the labial segment and proceeding forward 
to the point reached in the above discussion. 
In the early embryonic stages the labial segment is obviously 
a body segment (Fig. 6, 2°mx); but in the course of the 


