On Aquatic Carnivorous Coleoptera or DytiscidcB. 211 



times so as to form two or more fine pencils, but sometimes extending along the whole 

 margin in an equal manner : the outer surface of the ligula is impressed on each 

 side for the accommodation of the labial palpi, so that along the middle it is more or 

 less bulged or prominent : on the inner face the ligula is membranous, except that 

 there is a transverse horny strip at the extremity. The paraglossse are closely united 

 to the ligula, and do not project beyond it ; on the outer face of the ligula they are 

 difiicult to detect, but are nevertheless visible on each side, as forming a more 

 transparent corneous outer edge to the ligula ; but on the inner face of the ligula 

 they are very conspicuous, as each of them has its inner margin, fringed with an 

 elongate series of very fine, dense cilise. The supports of the labial palpi are 

 conspicuous and at their base are attached to the outer face of the ligula, and covered 

 by the front portion of the central piece of the mentum ; their extremity is on the 

 contrary ver}^ conspicuous, and appears in the form as it were of a stout basal joint 

 to each palpus ; this joint bears a deep notch on its antero-external face and thus the 

 palpus is capable of being bent back over the mentum as well as moved in a forward 

 and dependent manner. The labial palpi are of moderate length and are three- 

 jointed : the basal joint is considerably shorter than either of the others, while the 

 second and third differ but little from one another in length (except in the Noterides) ; 

 the second joint frequently bears a minute prominence on the middle of its inner face 

 causing it to appear faintly bi-emarginate on this fiice : the apical joint is usually 

 simple and cylindrical, but may be dilated, and bear a notch (Coptotomus). 



In the Noterides the labial palpi are peculiar, the basal joints being short, 

 while the apical one is large, more or less dilated, and bears a notch : in 

 Hydrocanthus this dilatation of the terminal joint of the labial palpus becomes 

 extreme. 



The labial palpi and the outer face of the ligula, are, like the maxillary palpi, polished 

 and shining and destitute of any exserted setae. In Pelobius the ligula shows much 

 diff"erence from that of the Dytiscidte, it is short, and its anterior portion is flexed 

 upwards so as to assume an obliquely vertical direction, this semi-vertical portion is 

 rather coarsely punctate. I can detect no paraglossse, and the supports of the labial 

 palpi are not emarginate. In Amphizoa the ligula does not project beyond the 

 lateral wings of the mentum, owing to the great development of these latter, but 

 its structure is quite that of the Dytiscidse. 



The parts of the mouth in the Dy tiscidfB offer two points of interest — first, the 

 great uniformity they show throughout the family, and second, the way in which the 

 different pieces are coadapted so as to close together quite completely and exclude 

 the entry of water : the mandibles are curved so that they close under the labrum, 

 and of the maxillse only the very hard, hooked, extremities are at all exposed, the 

 ligula and mentum completely closing the underside of the mouth ; notwithstanding 

 this close packing together of the parts of the mouth the whole of the maxillary 

 and labial palpi are entirely exposed — more completely, in fact, than is usual. 



TBANS. ROT. DUB. SOC, N.S., VOL. II. 2 F 



