or THE MOUTH PARTS OF CERTAIX INSECTS. 179 



Attached and articulated to the mentum anteriorly are the central ligula, a pair of 

 l)araglo.s.sa 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 sti-ucture, 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 oesophagus, as this part of it is termed, and this 

 must ever 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 ba-^e 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 paraglossiL' are sometimes intimately united with the ligula, sometimes com- 

 pletely separated from it : they may be of the same or a dillerent 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. Tiieir range of variation is not great; they are nev'cr 

 jointed, and never become mechanical structures. 



Tlie 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 camiot, 

 without losing their essential chai'acter, 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 

 accomi)lish this they would first lose their character as labial appendages. In 

 brief, the labium is tlie external beginning of the alimentary canal, and none of the 

 parts ever lose this association. Whatever their modification, no hibial >tructui-es 

 can ever be joined to the sides of the head outside of mandibular or maxillary 

 structures. 



As an illustration of the mo>t generalized form of labium at present kutnvn to 

 me, the roach (/'criphnieta orienUdis, Pi. II, Fig. 16) may be selected. Here we find 

 the mentum with a well-defined impression resembling a suture, and bearing a broatl 

 paired structure, Croni whicii arise the slender, two-jointed ligula, the broad, lleshy 



