OF THE MOUTH PARTS OF CERTAIN INSECTS. 195 
has written, see any reason why the rudimentary structures at the tip of the labral 
extension in Semuliwm are not mandibles. 
If we refer back again for an instant to the Panorpids we note (Pl. III, Fig. 4”) 
that in Bettacus strigosus the origin of the mandibles form an extension of a lateral 
head sclerite, with the labrum-epipharynx between them. In Panorpa the mouth 
structures are much shorter, set on an immensely elongated stipes, and at the tip of 
the frontal extension of the head we again have the mandibles, much reduced, with a 
small, lappet-like labrum-epipharynx between them. Now the situation of the rudi- 
ments in Simulium corresponds almost exactly with that of the undoubted mandibles 
in Panorpa rufescens (Pl. III, Fig. 4°); but in the Hmpzide we find a yet more closely 
allied structure. I have already called attention to the peculiar elongation of the front 
of the head in this family, and now if we examine this at tip, in Hmpis spectabilis 
(Pl. I, Fig. 13") its very close resemblance to Panorpa is at once evident. We find 
a central lappet-like structure with a sensitive surface, which looks like and logically 
should be the epipharynx, and moving below it is a pair of appendages which, in my 
opinion, represent mandibles. They are membranous and probably not functional; but 
this is no argument against their character. I believe that the similarity in the appear- 
ance between PI. III, Fig. 4°, and Pl. II, Fig. 13%, is the expression of a true homol- 
ogy, and that mandibles in the Diptera exist in no other form or situation. It is likely 
that other species, showing them much more perfectly, will yet be discovered ; but so 
indeed do I believe that labial palpi, properly connected with the mentum, will yet be 
found, so distinct in character that, even if not functional, their homology cannot be 
mistaken. 
Labrum and epipharynx have been frequently referred to in the course of this 
paper, and in the introduction the general relation of these two parts has been ex- 
plained. Both structures occur in many families of the Diptera. As in the case of 
the hypopharynx, the epipharynx has always connected with it a salivary duct. In its 
intimate connection with the labrum it is shown on PI. I, Fig. 10°, illustrating the 
epipharynx of Zzbellula. Here the chitinous tube giving passage to the duct is fully 
shown. As an example of a highly developed structure, the epipharynx of Copris 
carolina is shown (Pl. I, Fig. 4), and here the salivary duct opens among the dense 
central mass of spinous processes. The epipharynx of Polistes was referred to in the 
description of the labium, as was that of Andrena in the connection. In the Hemip- 
tera the labrum and epipharynx are usually well developed and the salivary duct is in 
many cases very well marked. 
Among the Diptera some of the larger Syrphide have the labrum quite distinct, 
and on the under surface is a sensitive surface into which an obvious duct, with chit- 
