74 THE CANADIAN ENTOMOLOGIST. 



[Male?] Body blue-black ; thorax with a pale line; antenn* and feet 

 dark honey yellow ; tergum and venter without lines." 



" Van a. Australis. Head beyond the eyes a little longer and a 

 little more dilated at tip ; second joint of the antennae a little more 

 dilated at tip ; abdomen with five lateral whitish points. Inhabits New 



Orleans." 



It will be noted in the above description that Say could find no very 

 specific difference between H. lijieata and H. stagnoriim ; by looking 

 carefully at the genitalia of each, however, it is seen that there is a wide 

 difference between them. It is in these fundamental structures that we 

 find the variations which are best adapted for separating the species 

 referred to in the above. Just what these differences are may best be 

 seen by reference to Plate III., where Figs, i and 2 show lateral and 

 dorsal views of the genitalia of the female H. Imeata, and Figs. 3 and 4 

 show lateral and dorsal views of the male genitalia of the same species ; 

 Figs. 5 and 6 are the genitalia lateral and dorsal of female H. stagnoruin ; 

 Figs. 7 and S are the same for the male of this latter species. The male 

 is darker in colour than the female, and much smaller, the average length 

 of twenty males being 8.8 mm., while twenty females averaged 9.7 mm. 

 in length. 



The peculiar habitat of Hydrometra, combined with its elongate 

 form, has given rise to a secondary sexual character, which occurs in both 

 II. lineata and // stagnorum. This consists of two notched projections 

 on the inner side of the sixth abdominal segment, close to the incisure be- 

 tween the sixth and seventh segments. The object of these notched 

 elevations of the abdominal walls is to fit over the lateral keels of the 

 female abdomen, thus steadying the abdomen of the male during copula- 

 tion. This is rendered necessary not only by the elongate abdomen, but 

 also by the fact that it is necessary for the insects to maintain their balance 

 upon the water or run the risk of breaking through the treacherous surface 

 film, an accident very likely to cause death. The abdomen in both sexes 

 is stiffened and made rigid by a concentration of the segments along the 

 venter, and by two keel-like lateral expansions of the abdominal segments. 

 Along these keels the segments have become so firmly cemented that the 

 joints between the segments do not show, thus giving to the keels the ap- 

 pearance of continuous structure. 



The life-history of H. lineata is similar to that of other Hemiptera 

 in that there are several broods during the summer. The insect hiber- 

 nates in the adult stage, and during the first warm days of spring crawls 

 stiffly out from under the rubbish along the banks, where it has passed the 

 winter. When the weather becomes warm enough (the first to tenth of 

 May at Ithaca), egg-laying begins ; the female becomes restless and stalks 

 about in search of a place to deposit an egg. The laying of an egg by 

 this stiff-abdomened, clumsy creature is accomplished in a very peculiar 

 manner : Backing up to a grass stem or almost any firm object which 



