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 chordee tendinez 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 chordee tendinez; 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, 17.) It probably assists in dilating the tubular mouth. 
b. The Oral Lobes and the Structure of the Pseudo-Traches. 
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-trachez 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-trachez 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 (Pl. 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. 
