1921] Crampton: Sclerites of the Head 89 
Crustacea) forms the so-called mola or molar region in insects, the molar 
or grinding region of certain coleopterous larve being a region of some 
interest in the classification of the larval forms. In the mandibles of 
Crustacea, a fringe of flattened seta-like hairs occurs below the incisor 
region, between the latter and the molar region of the mandible. Some 
of the flattened seta-like structures fuse to form a movable appendage 
called the ‘‘lacinia mobilis”’ by carcinologists, and some entomologists 
have thought that a similar “lacinia mobilis” in the mandibles of 
insects represents the lacinia of an insect’s maxilla. The idea that the 
various parts of the maxilla are repeated in the mandible (proposed by 
Packard, Smith, and other entomologists), which has received a rather 
widespread acceptance, is wholly unfounded and misleading, since 
the mandible represents only one segment (the coxopodite) of a limb; 
while the body of the maxilla (i. e., the part at the base of the maxillary 
palpus) is formed of more than one segment of such a limb (i. e., the 
basipodite and ischiopodite) and the parts are not at all comparable in 
the mandible and maxilla, since they are formed in different ways in 
the two structures. Furthermore, the palpus of the mandibular limb 
is lost in many of the higher Crustacea, and a structure homologous 
with the mandibular palpus of the Crustacea has not been found in 
any insects, despite the statement to the contrary made by several 
persons who have not properly studied the evolution of the mandibular 
appendage in insects and related Crustacea. 
With regard to the portions of the head capsule formed by the 
mandibular segment, there is a considerable difference of opinion on 
the part of embryologists in this matter. Thus Riley, 1904, states 
that a part of the postgena is formed by the pleural region of the 
mandible, while a portion of the hypopharynx is formed by the sternum 
of the mandibular segment. Holmgren, 1909, on the other hand, 
ascribes the “vertex’”’ and genz to the mandibular segment, in addition 
to the upper portion of the hypopharynx. It is difficult to believe that 
the gene which are situated immediately above the mandibles and 
bear the mandibles in most insects, are not portions of the mandibular 
segment, rather than parts of the protocerebral segment to which 
Riley, 1904, assigns the gene. I have, therefore, followed Holmgren 
in ascribing the gene to the mandibular segment. As far as the basi- 
mandibula ‘“‘bmd”’ of Fig. 39 is concerned, this sclerite is apparently 
formed by a chitinization of the articulating membrane at the base of 
the mandible and is not homologous with the trochantin or plate at the 
base of the leg in the thorax, as Comstock and others have maintained. 
Since the sclerite ‘““bmd”’ of Fig. 39 is formed by the chitinization of the 
basimandibular membrane, or the articulatory membrane at the base 
of the mandible, it is also a part of the mandibular segment, as Riley, 
1904, states, although it is strange that Riley did not also consider 
that the gena ‘‘ge”’ of Fig. 39, which is so closely associated with the 
region ‘“‘bmd”’ is not also a portion of the mandibular segment. 
The mandibles have two principal articulations with the head 
capsule. The dorsal articulation of the mandible occurs near the postero- 
lateral angles of the clypeus, and the condyle of the mandible which 
