THE LIFE-HISTORY OF HYDROBIUS FUSCIPES, L. 333 



Helochares, where, however, it takes a slightly different form, and is perhaps to be found 

 elsewhere. There seems to be no reason for such irregularity, which is quite constant, 

 and it is difficult to see how it can be of any advantage to the larva. 



On the posterior region of the head, on that part which telescopes under the tergite 

 of the first thoracic segment, are two small sclerites. 



The prothoracic segment is entirely chitinised, and has no characters calling for 

 remark. The mesothoracic segment is much shorter, and has a large chitinised tergite. 

 On either side of it and at its anterior border a small projection exists into which runs 

 a tracheal tube. Packard (I.e., p. 471) mentions that the American species of Hydro- 

 bius possess seven pairs of short " setose gills." The larva of H. fuscipes has, on either 

 side of the mesothorax, this setose structure, and on either side of each of the first seven 

 abdominal segments is a similar structure also containing a tracheal tube communicat- 

 ing with the main lateral trunk. Hydrophilus also possesses the abdominal " setae," but 

 1 am- not sure as to whether the mesothoracic pair are present. The larvae of both 

 Hydrophilus and Hydrobius drown comparatively easily ; they are never long away 

 from the surface of the water, and are constantly to be seen renewing their air-supply 

 by the posterior spiracles, and I do not see why these structures, which may be aborted 

 or closed stigmata, should be described as "gills." 



The dorsum of the metathoracic segment is less strongly chitinised than that of the 

 mesothorax, and on either side of the median line are two minute papillae each giving off 

 a long fine stiff hair. Both meso- and meta-thorax tend to be bilobed at the sides, the 

 one lobe lying above the other. 



The first abdominal segment is as short as the meso- or meta-thorax, but is distin- 

 guished from these — apart from the absence of legs — by possessing two well-marked, 

 though small, dorsal sclerites. The tracheal papillae or setose structures already referred 

 to project on either side of the dorsum and near the anterior margin, and below this the 

 sides of the segment are distinctly bilobed, the lobes being one above the other and 

 somewhat resembling parapodia in appearance. Immediately behind the two sclerites 

 and nearer the posterior margin of the segment are two papillae each with a long hair, 

 and a similar pair lie, one on either side, just behind the tracheal papillae. The other 

 seven abdominal segments are all very much longer from in front backwards, and are 

 apparently composed of at least two annuli. The first annulus of each segment is 

 plain, possesses no sclerites and no papillae. There is a slight constriction at its junction 

 with the posterior part of the segment. The posterior part of each segment from 2 

 to 7 is exactly similar to the anterior abdominal segment, since it possesses two 

 distinct sclerites, smaller, however, than those of the first abdominal segment, a pair of 

 tracheal papillae, and two pairs of papillae bearing long hairs. The anterior annulus of 

 each of these segments is apparently the one which is specially contractile. Each of 

 the papillae is apparently slightly chitinised, and perhaps their bases are correctly 

 described as additional sclerites. 



The second part of segment 8 is somewhat peculiar. It extends posteriorly into 

 TRANS. ROY. SOC. EDIN., VOL. XLVII. PART£lI.;(NO. 14). 51 



