184 AN ESSAY ON THE DEVELOPMENT 



chitinous rod a little to one side of the middle, and just this sort of structure we tind 

 everywhere in the Tabanids, lying outside of the ligula at base, articulated to the 

 outer edge of the mentuni. This, in fact, first led me to suspect the true nature of 

 the structure. If now we section Bomhus and Tahanus 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 paragloss;e 

 are absent and the palpi are united on one edge. 



In the Simulndo' 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 Shnulium 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 Polistf.'^ and othei" 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 " Butfalo 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 PI. 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 

 structuie so labeled is really the epipharynx there is little room for doubt, and the 

 location of the little, chitinous, toothed jirocesses, 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 highlj^ chitinized in comparison with their surroundings, 

 and why should they so completely resemble miniatui'c 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 ])iercing 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- 

 ilidiB thei-e arc no other attachments to the mentum, as shown in PI. Ill, Fig. 1-. 



These apparently single structures are sometimes interesting in section, as ai)pears 

 in StomoxTjs calcitrans, PI. 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, Avhich is almost tubular in structure. 



