68 Annals Entomological Society of America  [Vol. XIV, 
As the study of embryology indicates, six segments enter into the 
composition of an insect’s head. These are the protocephalon, antennal 
segment, intercalary segment, mandibular segment, first maxillary 
segment, and second maxillary or labial segment. Janet, 1899, Berlese, 
1909, and others who profess to be able to mark off the head capsule of 
adult insects into rings of sclerites corresponding to the embryonic 
segments entering into the composition of the head, have simply let 
their imagination run away with them, and the diagrams which they 
have constructed do not correspond to the facts at all, for, as embryo- 
logists such as Riley, 1904, have pointed out, ‘‘the definitive sclerites 
can afford us little or no evidence as to the primary segmentation of 
the insects.’’ It is therefore preferable to describe the sclerites of the 
head with reference to certain “‘landmarks’’ which occur in most 
insects, rather than to attempt to divide the head capsule into rings 
corresponding to the original segments entering into its composition. 
It must be borne in mind, however, that these convenient ‘‘landmarks’”’ 
for establishing the boundaries of the head regions are not present in all 
insects; and in some forms, the sutures, etc., which occur in the lower 
insects may become obliterated and new ones may be formed, thereby 
tending to mask the original condition, and making it very difficult to 
define the regions with any great accuracy throughout the series of 
insects. In such cases it is often impossible to do more than state that 
the secondarily formed region is equivalent to the original one only in a 
general way. 
The upper lip or Jabrum, (labeled ‘‘1”’ in all Figures) together with 
the clypeus, frons, “vertex,’’ and gene, all arise from the first or proto- 
cerebral segment of the embryo, according to Riley, 1904; and the 
labrum has been compared to the prostomium of annelids by scme 
investigators, although there is some doubt as to the exactness of the 
homology in the latter case. The anterior (or lower) margin of the 
labrum is so deeply incised in some insects, as to give the labrum a - 
pronounced bilobed appearance, which has led certain entomologists 
to conclude that the labrum is the fusion produce of two appendages; 
but a study of the embryological development of the labrum has shown 
that this view is entirely fanciful and unfounded. The labrum extends 
posteriorly to the transverse labral suture which usually demarks the 
labrum from the clypeal region behind it, although the labral suture is 
obsolete in some insects. When the labrum and clypeus are viewed 
from the ‘“‘inner’’ or buccal surface, a small hinge-like thickening 
called the torma by Peterson, 1916, may be observed near either end 
of the labral suture, at the junction of the labral and clypeal regions; 
and these torme are frequently of value in determining the posterior 
boundaries of the labrum. | 
The labrum may be long and narrow as in certain Diptera, etc., or 
it may be broader than long, as in the orthopteroid insects. In some 
Hymenoptera, the labrum is not distinct, and a membranous lobe-like 
region called the epipharynx may project from the anterior region of 
the mouth to close the base of the tube for sucking honey, etc. In 
