K. Jordan and N. C. Rothschild 
21 
genal area, which is the portion of head situated beneath the eye and 
extending from the mouth to the antennal groove. This so-called genal 
process is in the various Siphonaptera either prolonged so far backwards 
as to meet the hind-edge of the post-antennal portion of the head and 
therefore closing the antennal groove behind, or it is short, being widely 
separated from the occipital (or posterior) portion of the head by the 
antennal groove, the latter therefore being open behind. The closed 
antennal groove (PI. II, figs. 1, 5, 13) is found in Pulex, Pariodontis, 
Moeopsylla and Loemopsyllci and a number of combed fleas, while the 
American non-combed allies of Pulex have the antennal groove open 
(PI. II, figs. 3, 12), which is also the case in Goptopsylla lamellifer from 
Transcaspia (PI. II, fig. 4). Two stages therefore in the phyletic develop¬ 
ment of the head can be observed in this group of fleas, and the question 
presents itself which of them is the earlier and which the later stage; or 
in other words, had the ancestors of these insects an open antennal groove 
or a closed one ? From whatever order of insects the Siphonaptera may 
be a derivative, the antennal groove is a specialization and hence also 
the division of the head from the crown to the lower posterior corner. 
Further, the closed antennal groove is not merely a lateral impression 
surrounded by the solid skeleton of the caputal capsule, but the groove 
is closed, because the genal process of the prae-antennal portion of the 
head reaches to the post-antennal portion, there being a suture (or even 
a small gap underneath the genal process) which separates the apex of 
the genal process from the occiput. This suture (or gap) proves that 
there was here at one time a disconnection. The same conclusion is 
arrived at if we consider the antenna itself. The closed antennal groove 
of Pulex, Loemopsyllci and allies corresponds to a reduction in the 
antennae. The Pulicidae with long and well-segmented antennae have 
also an open antennal groove extending on to the prosternite. The 
complete segmentation of the antennal club being regarded as an earlier 
phyletic stage than the short club, which is segmented on one side 
only, necessitates the conclusion that similarly the closed antennal 
groove is a derivation from the open groove, the closing of the groove 
being perhaps a consequence of the reduction in the size of the 
antenna. 
The sexes of Loemopsyllci differ considerably in the upward extension 
of the antennal groove, as they do in most other Siphonaptera. The 
groove reaches nearly to the vertex of the head in the </, there being, 
moreover, an internal incrassation from the groove upwards, this being 
the suture, or the remnant of it, which in insects generally limits the 
