Historically, Hylaeus assimulans was known from numerous coastal and lowland dry forest habitats up to 2,000 ft (610 m) in elevation on the islands of Maui (coastal and lowland dry ecosystems), Lanai (lowland dry ecosystem), and Oahu (coastal and lowland dry ecosystem). All Hylaeus bees roughly resemble small wasps in appearance; however, Hylaeus bees have plumose (branched) hairs on the body that are longest on the sides of the thorax, which readily distinguish them from wasps (Michener 2000, in litt.). In general, Hylaeus species are small to medium sized bees with forewing lengths of about 0.12 to 0.31 inches (in) (3 to 8 millimeters [mm]), slender bodies that are usually black, short-bilobed tongues, and two submarginal cells in the forewing (Daly and Magnacca 2003, p. 12). Males of most of the Hylaeus species and females of several species have yellow marks on their face, hence the common name “yellow-faced bees.” Hylaeus bees, in general, lack the elongated hairs on the hind legs that other bee genera use to carry pollen externally. The lack of these hairs gives them a wasp-like appearance. But, yellow-faced bees can be distinguished from wasps by the presence of branched hairs on the body that are longest on the sides of the thorax. Hylaeus assimulans is distinguished by its large size relative to other coastal Hylaeus species in the family Colletidae and subfamily Hylaeinae. It has slightly smoky to smoke-colored wings. The male is black with yellow face marks, with an almost entirely yellow lower face region and additional marks on the sides that narrow towards the top. The male also has brown appressed hairs on the tip of the abdomen. The female is entirely black, large-bodied, and has no distinct punctation on the abdomen (Daly and Magnacca 2003, p. 56). A more detailed description of the species can be found in Daly and Magnacca (2003, pp. 55-58). Transport of the pollen in the internal crop is unique to bees in the subfamily Hylaeinae (Michner 2000, p. 15).