PROCEEDINGS OF SOCIETIES. 
17 
that with which a comparison is here instituted, whether we regard the general 
form, the insertion of the legs, the segmentation of the abdomen, the embry¬ 
ology, the concentration of the nervous masses, or the convolutions of the elon¬ 
gated alimentary canal.* * * § In regard to internal organization, the Pulicidse agree 
with the Diptera, and with them alone, among the anelytrous orders, in the 
normal number of the malpighian vessels, f The reproductive organs show no 
differences to forbid the conjunction, and this is perhaps all the evidence we can 
look for from a system, the variations of which are correlative to the characters 
of minor groups or species, more obviously, than to the broad lines of demarca¬ 
tion between the orders. It may be observed that the triplicate system of ap¬ 
pendages to the oviduct, so prevalent, under one or other form, in the Diptera, 
does not reappear, so far as has been ascertained, in the present group ; but we 
know the same to fail in other extreme groups of Diptera. The like may be ob¬ 
served of the absence of the petiolated stomach or fluid-reservoir in the perfect 
insect, where we find only a slight tendency to coecal appendage behind the 
crop. The nervous system offers a much more serious discrepancy in the bipartite 
internodes of the rachidian chain. The anomalous condition so singularly pre¬ 
sented in the females alone of the family ConopidaeJ does not seem much to 
lessen the weight of this argument. More important in this light is the obser¬ 
vation of Siebold, that some of the Nemocera \Culex, Chironomus , Simulia , Sfc.) 
in the larva state have a double chord for the internodes.§ 
Reverting to the external organization, the main differences which I would put 
forward are the singular relative position of the eye and antenna, the plan 
of articulation between the head and thorax (the hind margin of the former in 
great part embracing the front of the prothorax in its full amplitude), the re¬ 
markable displacement of the fore epimera, giving to the fore-legs the appear¬ 
ance of being attached to the head, and the disproportionate enlargement of the 
metathorax which becomes the largest segment of the trunk. Among all these, 
perhaps the articulation between the head and thorax deserves the most serious 
consideration, not merely as tending to separate the Pulicidae from the order 
Diptera, but as suggesting the chief difficulty which affects their position, as a 
distinct group, in the series of the orders. Irrespective of this, the embryology 
and the tarsal formula would concur to mark out for the group a place some¬ 
where among the typically winged— i. e., anelytrous orders. But, on the other 
hand, the complete separation and even versatile freedom of the head seems to 
stand in a direct relation to the general perfection of the organization for flight, 
even though the actual function and the immediate instruments of it should 
become abortive within the limits of species, genus, or even family. Still, 
having the concurrent data of complete metamorphosis in respect of the 
mouth and legs, and the acme of tarsal articulation attained in the final state, 
it would be going very far to nullify this compound expression of affinity because 
of the contrary value of a single term. So, leaving to these considerations their 
full weight as to place in the scale, I single out that particular of the segmenta¬ 
tion, and the relative position of the eye and the antenna, as the strongest 
arguments for the independent rank that has long been so generally attributed 
to the Suctoria or Aphaniptera. 
* Burmeister has placed Pulex at the point of divarication of a double series 
of Dipterous families, and thus made it a connecting link between those ex¬ 
treme forms; De Insectorum systemate naturali. a.d. 1829. 
f Six is the rule in Lepidoptera. In Hymenoptera the number varies from 
two (g. Anaphe s), up to an indefinite multitude, differing even in the species of 
one genus (g. Megaspilus , 3, 5, 7), and passing through the odd as well as even 
numbers, as three in Polynema ovulorum , Diapria verticillata , Microps rubi , 
Inostemma boscii , etc. In Diptera we uniformly find four malpighian vessels, 
with the exception of the families Culicidae and Phlebotomidse, which regularly 
have five. 
t Memoires presents al’Academie de France. Tome 11, p. 185, pi. 1. fig. 8. 
§ Handbuch der Vergleichenden Anatomie, ler Theil, p. 572. 
