EXTERNAL ANATOMY OF INSECTS. 46'7 



sents ; but in all, the theca or sheath is present, which re- 

 presents the labium (including the mentum) of the man- 

 dibulate Orders a . It consists of three joints, the last of 

 which is formed by the liplets {Lobelia). Those in the 

 Muscidce are large, turgid, vesiculose, and capable of 

 dilatation ; in the Bombylidce and other tribes they are 

 small, slender, long and leathery, and sometimes re- 

 curved. The second joint or stalk, which may be said 

 to represent the mentum, the liplets being properly in a 

 restricted sense the analogue of the labium , its sides being 

 turned up, forms a longitudinal cavity, which contains 

 the haustellum. The upper piece of this, the valvula, is 

 long, rigid, and very sharp, representing the labrum b . 

 Beneath this cover, in the above cavity, are the lancets ; 

 which, as far as they are at present known, vary in num- 

 ber and form: sometimes there are Jive of them, some- 

 times four, sometimes two, and sometimes, it should seem, 

 only one c . In the gnat {Culex) they are finer than a 

 hair, very sharp, and barbed occasionally on one side d ; 

 in the horse-fly {Tabanus L.) they are flat and sharp like 

 the blade of a knife or lancet e . In this tribe the upper 

 pair, or the knives [Ctdtelli), represent the mandibles; 

 the lower pair, or the lancets (Scalpella), usually palpi- 

 gerous, the maxillce ,• and the central one the tongue. In 

 the horse-fly Reaumur has figured only four, exclusive 

 of the lalrrum and labium ; but in a specimen I have pre- 



3 Plate VII. Fig. 5, 6. a'. b Ibid. 



c Reaum. iv. t. XVI. Fig. 13. z. 



d Authors are not agreed as to the precise number of lancets con- 

 tained in a gnat's proboscis. Swammerdam affirms there are six, in- 

 cluding the labrum. i. 156. b. t. xxxii./. 3. Reaumur could find only 

 five. iv. 597 — . t. xlii./. 10. And Leeuwenhoeck only four. 



e Plate VII. Fig. 5. 



2 H 2 



