1921] Crampton: Sclerites of the Head 77 
The pleural region of the labial segment is included in the region 
about the occipital foramen referred to as the “‘trophiger’’ or trophi- 
bearing area, and forms the narrow marginal area shown in Figs. 13, 10, 
etc. It sometimes bears an occipital condyle or process, with which the 
anterior process of the lateral neck plate articulates to aid in supporting 
the head capsule, while allowing for the freedom of movement of the 
head. 
Holmgren, 1909, states that ‘‘an embryological study (of the second 
maxille or labial appendages) further indicates that the submentum 
is formed by a portion of the articulatory membrane between the 
second maxillary segment (or labial segment) and the first thoracic 
segment, and therefore has nothing to do with the second maxille 
(labial appendages).’’ Holmgren, Heymons, and other embryologists 
likewise agree that the hypopharynx or ‘“‘tongue’’ is formed in part 
by the sternum of the labial segment, and Boerner, 1903, states that he 
considers it quite impossible to regard the mentum, submentum and 
gula as the sternites of the labial segment, “‘since in some insects, 
traces of a true (labial) sternum are retained between the bases of the 
coxe of the posterior mouthpart-limbs.’’ In other words, Boerner 
regards the median triangular area between the bases of the structures 
labeled ‘‘pgr”’ in Fig. 10, as the representative of the sternum of the 
labial segment. Whether these views are correct or not, I cannot say, 
although I have a strong feeling that either the mentum, or a portion 
of the submentum may be formed from the sternal area of the labial 
segment (in addition to the basal portion of the hypopharynx which is 
formed from the sternal area of the labial segment). What proof there 
is to be drawn from embryology, however, would not bear cut the latter 
view—at least so far as is at present known; and under these conditions, 
it would be wholly unjustifiable to claim that the mentum, or a part of 
the submentum are formed from a portion of the sternal region of the 
labial segment. On this account, I have provisionally accepted the 
view that the submentum and mentum are secondarily formed sclerites 
situated behind the true labium, in the following discussion. It has 
been found convenient, however, to treat the underlip as though it 
were composed of three principal plates or areas, in comparing this region 
in the different types of insects; and for the sake of convenience, I have 
referred to these three plates or areas as the gulamentum (1. e., the 
region bearing the labels ‘‘gu’”’ and ‘‘sm’”’ in Fig. 23), the mentum, 
labeled ‘“mn”’ in Fig. 23, and the true labium, or eulabium, which bears 
the labels “‘prm,”’ “‘lg”’ and ‘‘Ip,” in Fig. 23. 
The gulamentum (‘“‘gu’’ and ‘‘sm” of Figs. 23, 13, etc.) becomes 
divided into a gular region and a submental region in some insects, 
while the gular portion becomes ‘‘soldered’’ onto the head capsule in 
some insects (e. g., ‘‘gu’”’ of Fig. 16) as was stated above; and the gular 
region may be further augmented by the addition of portions of the 
integument behind the gulamentum. The gular region ‘“‘gu”’ is sepa- 
rated from the submentum ‘‘sm”’ by a pregular cleft, or suture, in such 
insects as the caddice fly larva shown in Fig. 24, and the occurrence of 
such a pregular suture is of value in marking off the posterior extent of 
