THE WATER TREADERS. 613 



with Hydrometra, Microvelia and Rheumatobates, they are not dependent 

 upon the chance and uncertain fare of terrestrial insects caught upon the 

 surface film but find another, and indeed a more constant source, in the 

 organisms that dwell below but come up to the surface film. Among 

 these, Ostracods and like forms are available as more or less staple 

 food and Mesovelia have been observed exploring the sides of floating 

 Typha and the tangled mats of alga? for such Crustacea which they spear 

 from the surface of the water." 



The principal literature treating of our species of the family 

 is by White, 1879 ; Uhler, 1884, 1894a ; Champion, 1898, and 

 Hungerford, 1917, 1920, 1924, 1925. 



I. Mesovelia Mulsant & Rey, 1852, 138. 



The principal characters of this genus are set forth under 

 the family heading. In addition, its members have the head 

 much prolonged and declivent before the eyes ; hind margin of 

 pronotum subtruncate ; clavus very large, wholly membranous ; 

 membrane without veins ; hind coxae rotatory. 



KEY TO EASTERN SPECIES OF MESOVELIA. 



a. Larger, length 3.8 — 4 mm. ; hind margin of front and middle femora 

 with a row of spines; beak reaching base of hind coxa?; first 

 genital of male with tufts of black seta? on lower side. 



589. MULSANTI. 

 aa. Smaller, not over 2.8 mm. ; hind margin of front and middle femora 

 without spines ; first genital of male without seta?. 

 b. Beak reaching first ventral. 

 c. Head brown with a narrow median pale line; joint 1 of antennae 

 not passing hind margin of eye; general color mottled brown, 

 shining. 590. douglasensis. 



cc. Head grayish with two narrow parallel dark lines; joint 1 of 

 antenna? surpassing hind margin of eye; general color olive- 

 green, thickly covered with a fine grayish bloom. 



591. CRYPTOPHILA. 



bb. Beak reaching between middle coxa?; head yellow with two brown 

 stripes. 592. amcena. 



589 (814). Mesovelia mulsanti White, 1879, 268. 



Elongate, rather slender, somewhat spindle-shaped. Color greenish 

 with a yellowish tinge, fading to brownish-yellow; hind lobe of pronotum 

 fuscous with a yellowish median stripe; antenna?, collar, sides and tip 

 of scutellum, veins and cuneus of elytra and tarsi, brownish or tinged 

 with fuscous-brown ; under surface pale yellow. Joints 1 and 3 of anten- 

 na? subequal, each about as long as head; 2 shortest, 4 one-third shorter 

 than 3. Front and middle femora armed beneath with numerous rather 

 stout seta?. Length, 3.8 — 4 mm. (Fig. 153). 



