OF WASHINGTON. 145 
In a longitudinal section, these parts appear as stout, chitin- 
ous hooks, resembling somewhat the mandibular hooks in 
dipterous larvae thus they may be referred to as labral hooks. 
This structure presents some striking variations or modifications 
in different families, groups, genera, and even in allied species 
of the same genus. In Scolytidge there is a striking difference in 
the structure and form of the clypeus, labrum, and labral hooks 
in representatives of different genera, primary groups, and sub- 
families. In Dendroctonus (Plates IV and V), the clypeus and 
labrum are separated by a distinct suture, and the latter appears 
to be capable of articulation. The labral hooks are prominent in 
some species and extend back to the base of the clypeus, while 
in others they are very short, and do not extend to the base 
of the labrum. In Platypus and Crasso tarsus (the two principal 
genera of the sub-family Platypodinae) the clypeus appears to 
be completely divided and widely separated by the labrum, 
which occupies a median position and is firmly attached to the 
epistoma, or second clypeus (Plate V) ; the apex scarcely extends 
beyond the apical angles of the first clypeus, from which it 
is separated by a deep, narrow emargination. The labral 
hooks appear to be modified into parallel sclerites between the 
labrum and clypeus, and firmly attached to the second clypeus. 
The ventral portion of this combination structure is chitinous, 
strongly convex in the middle, and closely, obliquely sulcate 
each side, while towards the base it is deeply concave. 
In the Corthyli group (sub-family Scolytinae), including Cor- 
thylus, Pterocyclon (Monarthrum), Tripodendron (Xyloterus), 
the structure of the clypeus and labrum is intermediate between 
the Hylesinae and Platypodinae, in the fact that the anterior 
margin of clypeus is obscure or completely coalesced with the 
labrum, and the labral hooks are parallel. 
In Scolytus muticus (which, at present, represents another 
sub-family) the clypeus and labrum are separated by a distinct 
suture, and the labral hooks extend from the middle of the 
latter to the middle of the former, curving outward towards 
the base (Plate V). 
There is another feature of the labral hooks exhibited in 
some species, where the anterior ends appear to be jointed. 
In Dendroctonus terebrans they are also articulated with another 
pair of similar structures, which appear to belong to the 
pharynx. 
As previously noted, there is a remarkable resemblance 
between these labral hooks and the mandibular hooks in certain 
dipterous larvae. They also appear to be representative of 
primitive appendages. Perhaps, after all, the labrum may 
represent a primary first segment or head, At any rate, the 
