EXTERNAL ANATOMY OF INSECTS. 425 



divided into two, three, or more lobes a ; the intermediate 

 one more directly answering to the labium of other in- 

 sects, and the basal one or mentum. This organ in the 

 Libellulina is of a different structure : it has only two 

 articulations representing labium and mentum; but the 

 former consists of three parallel pieces, the two exterior 

 ones rising higher than the intermediate one, and at their 

 inner angle having an acute sinus from which the palpi 

 emerge ; and the intermediate piece, which is longitudi- 

 nally channelled, lapping over the inner side of the lateral 

 pieces. From the angle of the covered part of these 

 pieces, a subulate short horizontal horn points inwards 

 towards the tongue, which it must keep from closing 

 with the labium^. 



3. Palpi Labiales c . — The last-mentioned organs, the 

 labial palpi, next claim our attention ; but before I advert 

 particularly to them, it will be proper to premise a few 

 words upon palpi, or feelers, in general. These are usual- 

 ly jointed moveable organs, of a corneous or coriaceous 

 substance, attached by ligaments to the labium and 

 maxillae, and in the Crustacea even to the mandibulce. 

 Their joints, which are usually more or less obconical, 

 articulate also in each other by ligaments, with perhaps 

 some little mixture of the ball and socket. Their ends, 

 the last joint especially, seem furnished with nervous 

 papillce which indicate some peculiar sense, of which they 

 are the instrument. What that sense is has not been 

 clearly ascertained, and concerning which I shall enter 

 more into detail in another place. Their motion seems 



» Plate VI. Fig. 6. b'. b Ibid. Fig. 12. b". 



« Plates VI. VII. XXVI. b". 



