14 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 6G 



joint two and one-half times the second ; arista with long but sparse 

 piumosity; facial ridges flat, only a few hairs above vibrissae; bucca 

 yellow, about one-ninth the eye height. 



Mesonotum blue-black, with white pollen visible more from behind, 

 not distinctly striped. Chaetotaxy: dorsocentral 2, 3; acrostichal 

 2. 1; humeral 3; posthumeral 2; presutural 1; notopleural 2; supra- 

 alar 3; intraalar 2; postalar 3; scutellum with 1 lateral; 1 apical, 1 

 rather small discal; sternopleural 2, 1. Calypters rather dark, the 

 anterior with black rim. Spiracles large, yellow. 



Abdomen blue-black, wdth a little Avhite pollen visible in certain 

 directions, not tessellated ; first segment with one large lateral margi- 

 nal ; second with two laterals and sometimes a median marginal pair 

 (21^ times in five males) ; third segment with strong marginal row of 

 10; fourth segment with distinct discal row^ of 4 to G (usually not 

 continuing down the sides), and an apical row of 6 to 8. Genital 

 segments rather large, shining black, with hair but no bristles. Inner 

 forceps black, flat and broad at base, but tapering to a slender, sharp 

 tip ; outer forceps black, narrowed at base, broader in middle, with 

 sharp tip. Fifth sternite with ordinary deep incision in middle and 

 tv»^o large, black, subshining, flat lobes. 



Legs black, middle tibia with flexor bristle, hind tibia with large 

 calcar below middle. 



Wings lightly infuscated, hind crossvein deeply so; fourth vein 

 beyond hind crossvein bowed a little backward so as to widen the 

 apical cell, which includes at its tip a costal segment about one- 

 seventh of the preceding. 



Female. — Front of equal width almost to antennae, 0.25 the head 

 width (average of three, 0.24, 0.25, 0.27), the middle stripe red over 

 halfway up. 



Length 8 to 8.5 mm. in both sexes. 



Described from 6 males and 4 females, I>a Suiza de Turrialba, 

 Costa Rica, February 22 to March 29, 1923, and September 5 and 

 October 1, 1921. In Prof. L. Melander's collection. 



Paratypes.—M^ilQ and female. Cat. No. 26797, U.S.N.M. 



MESEMBRINELLA SEMIFLAVA, new species 



Male. — Front almost as wide as the narrow ocellar triangle, the 

 narrow parafrontals touching for some distance; frontal bristles 

 beginning about the middle; ocellars long, proclinate, the post- 

 ocellar pair about half as long ; one pair of smallish verticals. The 

 head is yellow except upper two-thirds of back and upper third of 

 front; antennae and palpi yellow; third antennal joint not much 

 more than twice the second which is a litle longer than in some 

 species; arista long, with long but sparse piumosity; facial ridges 



