28 THE VOYAGE OF H.M.S. CHALLENGER. 



then bent backwards. Between the outer and inner line on each side, is a series, 

 beginning at the apex, and running back for a considerable way, of somewhat semilunar- 

 shaped tubercles (PI. III. fig. 6) ; while between the two inner lines is a close series of 

 fine curved transverse lines, below which may be seen — by altering the focus — another 

 series of similar lines, whose direction from side to side is a little different. If the organ 

 is viewed in profile, the appearance is altogether different, as each maxilla exhibits on one 

 side a series of comb-like teeth, which, beginning at the curved apex, runs back for a 

 considerable way (PI. III. fig. 6, o). What I conceive (perhaps erroneously) to be the 

 real structure of the organ is this. Each maxilla consists of a fine tube, longitudinally 

 open on the inner side and fringed on the outer (from the apex for some distance), with 

 fine back-pointing hairs. The fine curved transverse lines which are seen in the central 

 lono'itudinal space of the organ are fine curved hairs, which cross from one maxilla to the 

 other, and of which there are on each maxilla a series both above and below (PI. III. fig. 

 5, a). The semilunar-shaped tubercles are the comb-like teeth viewed dii-ectly from above, 

 and which can only be properly made out when the organ is seen partly in profile. On 

 tracing the organ further back, but while it is still included in the rostrum, both the back- 

 pointing lateral hairs and the comb-like teeth disappear, but the inner series of curved 

 hairs are continued. On tracing them backwards into the head the maxillae can be 

 seen to gradually diverge, and each of them then appears as if finely transversely striate 

 in the middle line. Gradually and slightly dilating, and continuing to diverge, they 

 can be traced backwards into the mesothorax for some distance. The breadth of the 

 maxillae a little behind the apex is lo. sericeus about '025 mm. and in the larva oihayanus 

 about '014 mm. 



The under side of the head is fiatly convex, with a concave hind margin. The gula 

 is broad, reaching to the hind margin. In form it varies somewhat in difi"erent species ; 

 sometimes the sides are slightly elevated, and sometimes there is a central longitudinal 

 obtuse keel. 



The Thorax. 



The thorax constitutes by much the largest part of the body, and increases in width 

 more or less from the front to beyond the middle. 



Of the three segments which compose it, two only can be easily distinguished, since 

 the mesothorax and metathorax are coalesced, while the prothorax and mesothorax are 

 quite distinct from each other. 



The Prothorax. 



The Pronotum is transverse and collar-like, the length in the middle being two to 

 three times less than the breadth, which is less than the head with the eyes. The 

 front margin is usually rather strongly concave between the eyes to receive the head ; 



