OF THE MOUTH PARTS OF CERTAIN INSECTS. 181 
and paraglosse of this same Polistes. The structures are here membranous, some- 
what bladder-like, and well adapted for lapping by means of flattened, bent processes, 
set in series on the entire inner surface. The paraglossze are completely separated and 
the mouth opening is shown at the base of the figure, as well as the chitinous ring 
marking the beginning of the cesophagus. 
In Andrena vicina (Pl. I, Fig. 9) we find a similar yet quite different structure, 
v. é., the same parts, used for much the same purpose, yet considerably modified in de- 
tail. The mentum is here much longer, more shallow, but similarly bears the epiphar- 
ynx on chitinous rods. The ligula is more inflated and the paraglosse are much 
reduced, but the palpi originate as before, and we have simply an illustration of the 
variation in form found in this united mentum and submentum. It is important to 
note here that in Polistes, Andrena, and indeed the Hymenoptera generally, the labial 
structures are free from all lateral attachments to the head and may sometimes be pro- 
jected forward quite a distance. The attachment to the head, indeed, is muscular and 
membranous entirely, and there is no direct articulation to any point by chitinous or 
rigid processes. There is nothing therefore to prevent the growth of the head sclerites 
around the mentum, which would thus become an internal structure—as has actually 
happened in the Diptera. 
Another feature upon which Dr. Packard rightly places great stress is that a 
salivary duct opens into the hypopharynx at the base of the ligula, which he thereby 
identifies. As this ligula is always attached to the mentum, it follows that this struc- 
ture may be identified in the same way, while no structures not originating from the 
same point can be labial in character. 
Before studying further the specializations of the labial structures, it may be well 
to say that they sometimes tend to become useless or obsolete, or so much reduced that 
they are difficult of recognition ; and, curiously enough, in such cases the palpi seem 
to be the persistent organs, Thus in some species of Scolzide among the Hymenop- 
tera the mentum bears only little, feebly developed palpi. A striking case is in the 
Panorpide, where on Pl. III, Fig. 4’, the mouth structures of Bittacus strigosus are 
shown. Here ligula and paragloss have disappeared entirely; but the palpi are dis- 
tinct and the curiously developed hypopharynx marks the beginning of the opening 
into the cesophagus. 
A modification of this type is to be found in the Lepidoptera, where practically in 
all cases the palpi alone, attached to a plate of variable size and shape, represent the 
labial structures. 
It seems a long jump from the reduced type in Panorpide to the fully developed 
labium of the Apide ; yet, except for the fact that all the parts are much elongated, 
