OF THE MOUTH PARTS OF CERTAIN INSECTS. 183 
tion. First let me say that I have already shown that a change from flexible to rigid 
ligula is not uncommon, and the suggested union of the palpi is a much less violent 
requirement than that imposed by the current explanation of the Dipterous mouth. 
Referring for a moment to Pl. I, Fig. 3, we see the entire mouth structure of Hristalis 
tenax. Above is the mentum and submentum, very like the structure already de- 
scribed for Polistes and entirely homologous with it, and at its tip we find arising in a 
group the structures further enlarged at Pl. IIT, Fig. 5. Centrally we find the now 
rigid ligula, deeply grooved in the middle, the channel closed by a flattened, also rigid 
and chitinized hypopharynx. Loosely enveloping this central ligula is a more mem- 
branous cylinder, evidently made up of two lateral halves, two-jointed, and the ter- 
minal joints separated or paired except at the base. As in Bombus the mouth of Hris- 
talis is hinged, and the joint is also at the base of the ligula. The latter organ is so 
articulated as to allow of the flexion; but in the palpi we find again the provision 
already noted in Bombus—a flexible, membranous, pseudo-segment. Now if we sec- 
tion the Bombus and Hristalis at the middle, we find the cuts alike, except that in 
Eristalis the palpi are completely united over the hypopharynx and closely approxi- 
mated at the opposite side. If we section near the tip, the cuts in both cases are 
identical. That this united structure in Hristalis is the united labial palpi seems to 
me beyond doubt. In the first place, the point of origin is normal, next to the ligula 
and at the tip of the mentum; and, secondly, it is a jointed organ and therefore can- 
not be paraglossa. It is in all points the structure of Bombus, with the terminal joints 
lost and the two halves united for the greatest part of the distance. That the parts 
named mentum and submentum are really such, is proved by the fact that the hypo- 
pharynx, which is not in dispute, originates from and that the cesophagus originates 
within it. 
In Bombus fervidus the ligula is unusually developed and much longer than the 
labial palpi, while the paraglosse are wanting. In PL. III, Fig. 12, is a camera lucida 
sketch of the labial parts of a carefully mounted specimen. The structures here are 
exactly as normally held when at rest, and only the mentum is a little crushed by the 
cover glass on the shallow cell. Now chitinize this whole structure thoroughly, and 
then compare with the drawing of Chrysops vittatus (Pl. III, Fig. 13) made in the 
same way. The magnifications are different, of course, the Bombus being drawn at 
short range with a four-inch lens while the Chrysops was drawn at long range under a 
one-inch objective. The object was to get the two of approximately the same size for 
convenience of comparison. In the Tabanids the mouth parts are rigid and not flexed, 
and no sort of joint or hinge is required ; hence the structures are all rigidly united at 
the base to the mentum. In Bombus fervidus the palpi are reinforced by a heavier 
