184. AN ESSAY ON THE DEVELOPMENT 
chitinous rod a little to one side of the middle, and just this sort of structure we find 
everywhere in the 'Tabanids, lying outside of the ligula at base, articulated to the 
outer edge of the mentum. This, in fact, first led me to suspect the true nature of 
the structure. If now we section Bombus and Tabanus near base, the cuts will be 
alike, save that the palpi in the latter are united at one margin. If the cuts are made 
toward the tip, the sections are alike—ligula and hypopharynx alone appearing in both 
cases. We have then, in Chrysops also, a complete labium, save that the paraglossze 
are absent and the palpi are united on one edge. 
In the Simulide are many interesting species with generalized mouth structures, 
and of these I have studied the “ Buffalo gnat,” from material kindly furnished by 
Dr. Riley, an undetermined Simuliwm sent me in numbers by Prof. Aldrich, and an 
undetermined little midge collected by me at Anglesea, N. J. The species are prac- 
tically identical in the labial structures, and here again the mentum and submentum 
strongly recall Polistes and other Hymenoptera. The hypopharynx is well developed 
and the ligula are nearly divided; but I have no satisfactory sections of this insect 
and the relations of the parts are not clear to me. At Pl. I, Fig. 1°, the labium of 
the “ Buffalo gnat” is shown. In the species sent by Prof. Aldrich I succeeded in 
getting a dissection illustrating the connection of the epipharynx with the mentum, 
and this is illustrated at Pl. I, Fig. 1% This is really an exceedingly interesting speci- 
men and it clears up the relation of the frontal prolongation of the mouth. That the 
structure so labeled is really the epipharynx there is little room for doubt, and the 
location of the little, chitinous, toothed processes, and their character, leaves no doubt 
in my mind that they are mandibular rudiments—exactly as I claimed in my firet 
paper. ‘That they can be dermal appendages, as has been claimed, does not seem rea- 
sonable to me. They are too highly chitinized in comparison with their surroundings, 
and why should they so completely resemble miniature mandibles? I do not know of 
any case of dermal appendages of a similar character, and it is at least passing strange 
that such should be developed exactly where, normally, mandibular rudiments might 
be reasonably expected. 
The tendency in the piercing Diptera is constantly in the direction of simplicity 
of labial structures, and so we gradually note the loss of all trace of accessory labial 
structures, leaving the ligula and hypopharynx as sole representatives. In the As- 
ilide there are no other attachments to the mentum, as shown in PI. III, Fig. 1’. 
These apparently single structures are sometimes interesting in section, as appears 
in Stomoxys calcitrans, Pl. I, Fig. 11. Here the cut shows two crescent-shaped struc- 
tures connected at one edge by the thinnest kind of a chitinous shell, and closed oppo- 
site by a hypopharynx, which is almost tubular in structure. 
