According to Walton's original 1910 description (at BHL,
here, figures of plate IX
here),
champlainii has dense yellow pile on the first through the fourth abdominal tergites (fifth on male), with 'anvil-shaped' black patches on the dorsum of first three, while
grossa has first tergite black, separated from subsequent tergites by a deep groove, and lacking the dorsal black patches. The proboscis of
champlainii is thinner and more tapered than
grossa, which has a proboscis that is thicker towards the distal end. These two male specimens from the Carnegie Museum of Natural History collection fit the original description: