The male of Lygistorrhina sainctaecatharinae is readily distinguished from all known Lygistorrhina by its reduced number of apical abdominal bands. The female of sanctaecatharinae with its completely black abdomen is not likely to be confused with any other known New World species.
Male: Head: Dark brownish black, moutharts pale brownish yellow except darker near tip, about as long as hind tibiae; antennae light brown, with medial flagellomeres slightly yellowish, about twice as long as head, as long as hind basitarsi, with verticals distinct and as long or longer than flagellomere. Thorax: Dark brownish black, dull; pleuron sparsely grayish pollinose except for large subshiny basoventral area on sternopleuron; mesonolum with 2 submedial grayish pollinose vittae, with vittae broadly joined in front of scutellum, from posterior view with these vittae appearing dark; rest of mesonoturn very sparsely grayish pollinose except densely pollinose in front of scutellum and postalar callus and behind humerus; halter yellow; scutellum silvery pollinose, with a single row of 6-8 marginal bristles. Legs: Anterior 4 legs yellow except dark apical 4 tarsal segments, basal 3/4 in front coxa and all of middle coxa; hind leg dark brownish black except yellow trochanter, basal 2/3 of femur and basal 2/3 to 1/2 of tibia; tibial spurs single on anterior legs, double on hind leg; inner spur of hind leg about 1/2 as long as outer. Wing hyaline, microtrichose. Abdomen: Black, with distinct apical yellow bands on only 2nd, 3rd and usually 4th segments (both sterna and terga), with indistinct apical band on 5th sterna and rarely with lateral apical comers of 5th tergum slightly yellowish. Male genitalia black; basistyle about 1/3 longer than wide; distyle simple, about 2/3 as long as basistyle, thickened apically and ending in a short capitate seta on upper internal angle, short pilose with 2 long setae on inner margin; 9th tergum very large, as long as basistyle, elliptical.
Female: Quite similar to male, but differs as follows: eyes much smaller and thus front much broader; hind femur much more extensively dark brownish, in some specimens all brownish black; abdomen much shorter and stouter, all black except yellow cerus, without a trace of yellow apical bands.
Measurements: [Average (range; number of specimens measured)]; overall length, 4.86 mm (4.08-5.36; # 16); 3.16 mm (2.96-3.36; #4); mouthparts, 1.70 mm (1.42-2.74; #10); wing, 2.41 mm (23.2-26.0; #11); fore femora, .75 mm (.70-.88; #11); middle femora, .86 mm (.82-.96; #11); hind femora, 1.18 mm (1.12-1.28; #11); middle femora, .86 mm (.82-.96, #11); hind femora, 1.18 mm ( 1.12-1.28; # 11). Leg ratios [femora:tibiae:basitarsi: tarsi]: front leg, 1:1.07:0.96:1.04 ( #11); middle leg, 1:1.25:0.92:0.99 ( #11); hind leg, 1:1.42:0.73:0.95 (#11).