ii>02.] 259 



with whitish under-side, and in an undescribed freshwater-genus from 

 the island of Aldabra. hi the species of Gerris with black under- 

 side the tubercle is very difficult to find, if it be eoncolorous with the 

 surrounding surface or pubescence, but in various forms with black 

 under-side it is of a rusty-red colour, and then at once visible. The 

 tubercle is comparatively large and conspicuous, for instance, in the 

 European Velia rivuloritm, and its place is always on the segment which 

 in the typical Oerridce is unquestionably the metasternum. "What 

 Buchanan White describes as the first genital segment of Halobates is 

 really the last abdominal segment, and his " first abdominal segment " 

 is, as stated above, the metasternum. This being evident, the perforated 

 tubercle cannot be bomologized with the ventral tube of the Collem- 

 bola. The use of this tubercle is unknown, but as its position and 

 colour is sometimes of specific importance, I propose to call it the 

 omphalium (from o/xtftaXiov) , until its true nature is ascertained. Is it 

 possible that in the Oerridce and Velindce there is but one unpaired 

 secretory opening to the stink-gland, instead of the two as in other 

 families? In several cases I have found traces of a viscid fluid round 

 the omphalium, and the reddish colour of it in some species is per- 

 haps due to discoloration of the surrounding black pubescence by 

 evaporation from the stink-gland. 



In the genus Oylindrostetliux, Fieb. (and possibly in a few species 

 of Gerris), the metasternum apparently consists of two segments, it 

 being divided by a more or less distinct, transverse, impressed line a 

 little before the posterior margin. That this additional pseudo- 

 segment is not the first ventral segment, but belongs to the metaster- 

 num, is shown by its coalescence at the sides with the hind acetabula, 

 and by the impressed line terminating it anteriorly not reaching the 

 lateral margin. This is clearly seen only in those species of Cylin- 

 drostelhus having the posterior acetabula somewhat distant from 

 the middle acetabula (I have examined G. productus, Spin., and two 

 new species). 



In his description of the new Gerrid Thaumastometra Montandoni 

 (Rev. d'Ent., xviii, p. 87), Mr. Kirkaldy says: "segments abdo- 

 minaux 2e — 7e possedent un connexivum." It is evident that what 

 Mr. Kirkaldy has considered to be the first dorsal abdominal segment 

 is really the metaphragma of the metanotum. The metaphragma in 

 the apterous Gerridce often has quite the form of an abdominal seg- 

 ment ; the connexivum of the first abdominal segment in certain 

 species being prolonged to the metascutum, so as to embrace also the 

 sides of the metaphragma. The exact mutual position of the posterior 



