202 THE ENTOMOLOGIST. 



he might. The female meantime remained lying as though dead, 

 on the bottom of the bowl, for a quarter of an hour, and had by 

 this time been almost an hour under water. She then, however, 

 stirred slightly, and appeared to have some life yet remaining.* 



The sexes are very easily separable in this genus. The 

 females are nearly all a little more robust, but there are also 

 well-marked external structural characters. In the females the 

 apical margin of the last "abdominal" segment (that is to say, 

 the last segment of the abdomen, with a "connexivum," or more 

 or less flattened lateral piece, often apparently but, not actually, 

 sutured off from the abdomen) is almost straight ; while in the 

 males it is usually more or less profoundly emarginate. This 

 emargination varies more or less in form and extent in the 

 different species. Seen from below there are three " genital " 

 segments (that is to say, abdominal segments modified for 

 sexual purposes, apical to the true "abdominal" segments, and 

 not furnished with a connexivum) visible in the males, and only 

 two in the females. 



The apical segments of the venter are represented on plate 

 iii.t fig. 15 $ , fig. 16 ? , &c. 



(a) Aquarius, Schell., 1800 = Hygrotrechus, Stal, 1868. 

 There are two British species, generally known as G. najas 

 (De Geer) and G. pallidum, Fabr., the connexival spines in the 

 former not projecting so far as, in the latter projecting as far as, 

 or farther than, the apex of the abdomen. Now, as may be seen 

 at once by anyone who will take the trouble to refer to the original 

 description, f it is the latter that is najas of De Geer, najas of 

 modern authors being distinctly referred to by the sagacious 

 Swede as being a very similar but distinct species. His de- 

 scription (p. 312, &c.) refers to PL 16, fig. 7, which is undoubtedly 

 pallidum, auctt. He then describes (p. 314) the wingless form, 

 referring to figs. 8 and 9 (certainly najas, auctt.), and concludes 

 by saying (p. 319) : " Je crois en attendant que les non-ailees, 

 qu'on pourroit distinguer par le nom de Pundise aquatique tres- 

 allongee non-ailee d'un brun noirdtre, a pattes anterieures courtcs, 

 sont une espece particuliere." In the explanation of the plates 

 the distinction is as strongly drawn (pp. 649-51) : "La fig. 7, 

 represente la Punaise (nayade) . . . ." [certainly pallidum auctt.] 

 "La fig. 8, est celle d'une Punaise aquatique, qui, quoique tres 

 semblable a la precedente paroit cependant d'une espece difi'erente 

 et n'ayant point d'ailes, ni dans l'un ni dans l'autre sexe." " La 

 fig. 9 fait voir en grand la meme Punaise . . . ." Figs. 10 and 

 11, magnified portions of no. 8 [figs. 8-11 undoubtedly represent 

 najas auctt. J Figs. 12-19 do not concern the point at issue. It 



" ;: Further biological notices will be found further on, when treating of 

 the earlier stages in the life-history of the Gerrida;. 



f De Geer, 1773, ' Menioires,' iii. pp. 311-22 and 649-51, PL 16, figs. 7-19. 



