No. 421.] SKELETON OF THE HEAD OF INSECTS. 37 
with the mandible, joining it near the insertion of the flexor 
muscle. But in the present state of our knowledge little stress 
can be laid on supposed homologies of the parts of the pharyn- 
geal skeleton; for it is evident that in this region sclerites are 
frequently developed secondarily. 
The articulation of the mandible with the postgenz is its 
pleural articulation ; the ventral articulation appears to be 
with the clypeus in Periplaneta (Fig. 2), Ze, with the sternite 
of the second segment in front of the mandibular segment. 
It is difficult to imagine the steps by which, in the course of 
the phylogenetic development of cockroaches, this condyle of 
the mandible could pass from one segment to another with- 
out interfering with the usefulness of the mandible during 
the transition period. This was for us a perplexing problem 
for a long time. 
Later it was found that in Gryllus (Fig. 1) the mandible 
articulates with a small sclerite which is sometimes distinct 
from the clypeus. The same thing was found in the larva of 
Corydalis, except that here (Fig. 3, ac) the sclerite is large 
and clearly distinct. 
As the ventral articulation of a coxa is with an antecoxal 
piece (see page 27), we regard this sclerite as the antecoxal 
piece of the mandible. In the course of the consolidation of 
the segments of the head and of the dorsal flexure of the 
sternites of the first three segments (the procephalon), the 
antecoxal piece of the mandible has been pushed out of its 
own segment past the labrum (the sternite of the second 
antennal segment) and, in Gryllus, nearly past the clypeus. 
Such a migration of the antecoxal piece could take place 
without interfering with the action of the mandible. 
Doubtless a factor in bringing about these changes is the 
fact that while there has been a marked reduction in the length 
of the head segments (the entire head composed of seven seg- 
ments being approximately of the same length as a single seg- 
ment elsewhere), there has been no reduction in the width of 
the base of the mandible in biting insects. It is not strange, 
therefore, that the least firmly fixed point of articulation, the 
antecoxal piece, should be pushed out of its primitive position. 
