No. 421.) .SKELETON OF THE HEAD OF INSECTS. 23 
prevented a correct homologizing of the sclerites of the head. 
It is evident that the old belief is still held by many ; thus Hey- 
mons ('95) designates what he believes to be the first segment 
the primäres Kopfsegment oder Oralstück. 
The earliest suggestion of a different position of the mouth 
that we have met was by E. Ray Lancaster (73), who refers 
to an *adaptational shifting of the oral aperture." The later 
writings contain many references bearing upon this, although 
the full force of them is evidently not appreciated by the writers. 
Thus it has been said by many that although the antenna were 
doubtless originally postoral they have become preoral. The 
facts would be more accurately stated by saying that although 
the mouth was doubtless originally preantennal it has become 
postantennal. This, however, would only partially indicate the 
change that has taken place; for, as will be shown later, the 
antennz have moved cephalad at the same time that the mouth 
has moved caudad. 
THE STRUCTURE OF A TYPICAL SEGMENT. 
In order to determine the homologies of the sclerites of the 
head, it is necessary to decide what sclerites were probably 
present before the consolidation and reduction of its segments 
took place. We have decided that the head is composed of 
seven segments; let us now determine the elements in the 
skeleton of a single segment. This necessitates a study of 
segments in other regions of the body. 
In the abdomen it is evident that a reduction of certain parts 
has taken place, correlated with the loss of the abdominal 
appendages; it is to the thorax, therefore, that we must look 
for the more typical insectean segment. 
The parts of a thoracic segment that are commonly recog- 
nized are those described by Audouin (24): a ventral part, 
sternite; two lateral parts, p/eurites; and a dorsal part, tergite. 
- These are most easily seen in the wing-bearing segments ; 
but they can be recognized also in the prothorax of certain 
generalized insects. This is especialy the case in many 
Orthoptera, as cockroaches and walking-sticks, where the 
