No. 421.) SKELETON OF THE HEAD OF INSECTS. 17 
maxillz (Fig. 5, Gz). In the more generalized orders, the sclerite 
corresponding to the gula does not form a part of the skull. 
The Cervical Sclerites (cervical sclerites, Huxley). — Small 
sclerites found in the neck of many insects. Of these there 
are dorsal, lateral, and ventral sclerites ; the lateral cervical 
sclerites have been termed the jugular sclerites (pièces jugu- 
laires, Straus-Durckheim) (Fig. 4, es, em). 
Other Sclerites. — 1n addition to the areas and sclerites 
named above, the following sclerites will be described in later 
pages of this essay: the ocular sclerite, 
the antennal sclerite, the trochantin of the 
mandible, and the maxillary pleurites. 
These terms should be added to the list 
of those available for the purposes of 
systematic entomology. 


THE SEGMENTS OF THE HEAD. 
The determination of the number of X P. UN 
segments in the head of an insect is a 
problem that has been much discussed 
since the early days of entomology. The 
first important step towards its solution was made by Savigny 
(1816), who suggested that the movable appendages of the head 
were homodynamous with legs. This conclusion has been 
accepted by all; and as each segment in the body of an insect 
bears only a single pair of appendages, there are at least four 
segments in the head; z.e., the antennal, the mandibular, the 
maxillary, and the second maxillary or labial. 
As the compound eyes are borne on movable stalks in cer- 
tain Crustacea, it was held by Milne-Edwards that they represent 
another pair of appendages; but this view has not been gen- 
erally accepted. It is not necessary, however, to discuss in 
this place whether the eyes represent appendages or not ; the 
existence of an ocular segment has been demonstrated in 
another way, to be discussed later. 
This is the point to which the solution of the problem was 
carried by the methods of comparative anatomy. The existence 



Fic. 5. — Head of Corydalis, 
adult, ventral aspect. 
