HUNGERFORD: AQUATIC HEMIPTERA. 165 



those with well-developed swimming fringes. Pronotum is transverse, 

 moderately convex, and narrower in front than behind. The venter of 

 the abdomen is equipped with a median longitudinal keel or carina and 

 provided along the lateral margins with guard hairs for closing over the 

 two longitudinal troughs, thus forming air channels. — From Uhler and 

 Parshley. 



The family Notonectidae embraces, according to Kirkaldy, two sub- 

 families: Pleinae and Notonectinae. In his "Revision of the Notonectidae, 

 Part I," in Trans. Ent. Soc. London (3), vol. 35, p. 393-426, 1897, this 

 author begins his systematic revision of the family. This is continued in 

 a second paper which appeared in the "Wiener Entomologischen Zeitung" 

 for 1904, and entitled "Uber Notonectiden." 



In the following year Bueno published "The genus Notonecta in Amer- 

 ica North of Mexico." 



Kirkaldy lists six genera in the subfamily Notonectinae: Notonecta L. 

 Anisops Spin., Enithares Spin., Martarega B. White, and Nychia Stal. 

 It is in his second paper that he erects the genus Buenoa which is allied 

 to Anisops. In this country we have the three genera: Notonecta, 

 Buenoa and Plea. These may be separated by the following: 



KEY TO GENERA. 



A. Legs are quite similar. Plea Leach. 



AA. Legs dissimilar, hind legs flattened and fringed for swimming. 



B. Last segment of antennas much shorter than the penultimate. 



Notonecta Linn. 

 BB. Last segment of antennas longer than the penultimate. 



Buenoa Kirk. 



Of these three genera Notonecta is the commonest, being represented 

 in this country by 12 species, the genus Buenoa comes next with 5, while 

 the little Plea so far is credited in the literature with but one. 



Genus PLEA Leach. 



DESCRIPTION OF GENUS. 



"The thorax is obscurely hexagonal, with the hinder margin pi'ominent 

 and rounded; the head as broad as the broadest part of the thorax; the 

 eyes are rather oblong, without the least tendency to converge behind; 

 the hinder pair of legs not more ciliated* than the others, but are termi- 

 nated by very strong and distinct claws; the tips of the elytra accumi- 

 nated and entire." — Leach. 



One species so far listed for the United States, which is Plea striola 

 Fieber. Kirkaldy, 1904, remarks: "Wahrscheinlich haben Uhler und 

 champion zwei oder drei arten hier vermischt." An examination of the 

 material in various collections would suggest that we may have more 

 than one species. Kirkaldy lists two genera in the subfamily Pleinse — 

 Plea and Helotrephes. To the first he ascribed 12 species, and to the lat- 

 ter, 4 species. It is interesting to note that he considers the Plea minutis- 

 sima of Leach and of Fieber under P leachi McGregor and Kirkaldy. 



* They are more ciliated in our own species. 



