HYDROPHORUS. 181 



above at each side, and ending below in an obtuse angle. Lower 

 orbit sometimes enlarged into cheeks (sp. 7, 8). Supercilia black 

 spines ; lower occiput bearded with pale hairs. Proboscis, with the 

 palpi, moderately prominent. Antenna; generally shorter than the head, 

 with the first joint naked ; the second shortest, verticiUate ; the third 

 joint scarcely longer than broad, the thickened base separated from the 

 compressed rounded tip by a line inside descending from the insertion 

 of the arista and ending with a tooth below ; arista dorsal, nearly 

 naked, the last joint deflected, making an angle with the penultimate, 

 which is about as long as the third joint (except sp. 1, 2). Scutelhuu 

 usually with two bristles at each side; (except sp. 1, 10.) Prosteruum 

 with a spine at each side, or a bristle stronger than in Dolicliopus, 

 sometimes with more than one (sp. 3, 10). Metathoracic epimera much 

 dilated, extending to the base of the second segment of the ab- 

 domen. TVimjs longer than the body, oblong ; xcitk the discal trans- 

 verse vein very close to the hind margin, seldom distant by near its own 

 length. Alula> fringed with fine pale hairs. Halteres usually pale, 

 sometimes with a dusky spot at the end. Abdomen in general not 

 longer than the thorax, depressed, finely pubescent, with the incisures 

 not ciliated ; five segments, the first not shorter than the following ; 

 the foveolai of the inflected lateral margins very conspicuous. In the 

 male the abdomen is a little compressed behind, with the fourth ventral 

 segment prominent ; the hi/popi/gium for the most part imbedded, tcith a 

 single inflected appendage beneath (not always visible), composed as if 

 of two lamcllce muted by the hind edge to form a keeled vessel (except 

 sp. 1, 3). Fore legs short ; the coxcb but little compressed, very freely 

 articulated, little shorter than the femora, which are swelled towards 

 the base, and form with the cm-ved tibia a strong pincer, the spines of 

 the contiguous edges interlacing ; the posterior legs long and slender, 

 the coxre usually unarmed, the femora and tibia armed tvith short scat- 

 tered spines, the hind metatarsus unarmed, little longer than the second 

 joint ; (except sp. 1, 2.) 



The species of this genus^ as the name imports, live mostly on 

 the surface of the water, of ponds and lakes, or even of the sea. 

 Some of them have been found on pools frozen over in winter. 



The British species are not numerous, and may be arranged 

 thus : — 



a. Posterior femora with a single spine in front near the tip. 



b. Abdomen banded. Species 1. 



b b. Abdomen not banded. Species 2. 

 a a. Posterior femora with scattered spines. 



c. Fore tibia obliquely produced at the tip inside. Species 3. 

 c c. Fore tibia with a straight tip. 



d. Arista deflected at a rectilineal angle. 



e. Wings with numerous dusky spots. Species 4. 

 e e. Winffs with two duskv dots. 



