SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 393 



process of the furca. The contraction of these muscles opens the oral 

 disc, which is folded by the elasticity of the thyroid cornua acting on the 

 furca (see p. 143). 



2. The Paraphysal Muscles arise from the paraphyses, and are inserted 

 into the chords tendineae of the oral disc. Kraepelin described these 

 muscles as having an insertion into the discal sclerite. I have been unable 

 to discover any such insertion. I believe they render the disc concave by 

 increasing the tension of the chorda; tendineae; but I am by no means con- 

 fident with regard to their function, nor is it easy to make out their exact 

 insertion. 



3. The Transverse Muscle of the Haustellum. This consists of several 

 muscular slips, which arise from the thyroid sclerite, near the median plane, 

 and diverge and are inserted into the paraphyses. Kraepelin regarded it as 

 an erector of the oral lobes, and supposed that its contraction drives the 

 blood from the cavity of the haustellum into the lobes. I believe its real 

 function is to approximate the paraphyses. 



The last muscle I have to mention is the prelabral muscle. 

 The Prelabral Muscle consists of radiating fibres in the cavity of the 

 prelabrum (Fig. 29, r m.) It probably assists in dilating the tubular mouth. 



b. The Oral Lobes and the Structure of the Pseudo-Trachese. 



Much has been written on the structure of the oral surface of 

 the suctorial disc, which has been for. years past a favourite 

 study with microscopists, and mere surface views present 

 appearances the interpretation of which is by no means easy. 



The cuticle of the oral surface of the disc is perfectly 

 transparent and colourless, but the chitinous rings of the 

 pseudo-tracheae are deeply coloured, frequently almost black in 

 the mature insect. The edges of the pseudo-tracheal channels 

 are also coloured and thickened, so that in surface views they 

 have the appearance of small scales ; these scales are more 

 marked towards the margins of the disc. 



Between each pair of pseudo-tracheae two wavy lines are seen 

 parallel with them ; between these there are several nipple- 

 like projections, which I regard as the orifices of the ducts of 

 the labial glands ; there are usually five or six in each inter- 

 space. Two, with the papillae invaginated, are seen in the 

 figure (PI. XXIII., Fig. 2). 



The pseudo-tracheal rings have the form represented in 

 Fig- 31, 5, and the forks are placed alternately right and left of 

 the median channel by which the groove opens on the surface. 



