OF THE MOUTH PARTS OF CERTAIN INSECTS. 179 
Attached and articulated to the mentum anteriorly are the central ligula, a pair of 
paraglossa bounding it, and a pair of palpigers, one at each outer edge, bearing the 
labial palpi. 
The ligula or glossa, marked gl in all the figures, is a paired organ only in the 
more generalized orders, and is usually present as a single, central structure, which may 
be either chitinous and rigid or membranous and flexible. It is the most persistent of 
all the labial structures, is never attached except to the mentum, and always has asso- 
ciated with it the hypopharynx where that is present. We always find at its base the 
opening into the alimentary canal, or cesophagus, as this part of it is termed, and this 
must eyer be the test of labial structures—that they are attached to the mentum and 
have at their base the opening into the alimentary canal. The association is never 
broken, and the base of the ligula, whatever its form or however it is modified, always 
marks this point. On the other hand, by tracing the alimentary canal to its external 
opening, we can always recognize the ligula by its position, however little it may re- 
semble normal types. 
The paraglossx are sometimes intimately united with the ligula, sometimes com- 
pletely separated from it: they may be of the same or a different texture; but they 
always arise from the mentum on each side of and close to the central structure. Their 
tendency is to obsolescence, but they may become united and form a bed for the ligula 
which remains the inner organ. Their range of variation is not great; they are never 
jointed, and never become mechanical structures. 
The palpi are tactile in function under all circumstances, though they may lose 
this function in great part and may, by coalescence, form a sheathing to the ligula. 
They are never, under any circumstances, attached anywhere except to the mentum, 
directly or indirectly, and their location must be constantly the same. They cannot, 
without losing their essential character, become disassociated from the mentum, 
nor can they ever form an envelope or covering for it, or for the submentum, with- 
out a change entirely at variance with any reasonable theory of development. ‘To 
accomplish this they would first lose their character as labial appendages. In 
brief, the labium is the external beginning of the alimentary canal, and none of the 
parts ever lose this association. Whatever their modification, no labial structures 
can eyer be joined to the sides of the head outside of mandibular or maxillary 
structures. 
As an illustration of the most generalized form of labium at present known to 
me, the roach (Periplaneta orientalis, Pl. II, Fig. 16) may be selected. Here we find 
the mentum with a well-defined impression resembling a suture, and bearing a broad 
paired structure, from which arise the slender, two-jointed ligula, the broad, fleshy 
