The name yellow-faced bee refers to the yellow markings on the face of the males; females have a black face. Historically, Hylaeus hilaris was known from the dry coastal habitat on Maui, Lanai, and Molokai. 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. Hilaris yellow-faced bee is considered one of the most colorful of the Hylaeus species. It is a medium sized bee, relative to other yellowfaced bees. The face of a male is almost entirely yellow with a yellow stripe continuing above the antennal sockets. Males have slightly smoky to smoky wings and a red abdomen with white bands of hair on the abdominal segments. The adult female has a reddish to brownish face and antennae; the abdomen is dark with bands of hair on the segments. The frons (central forehead) and vertex (tip of the head) of the female are slightly swollen (Daly and Magnacca 2003, p. 106). A more detailed description of the species is provided by Daly and Magnacca (2003, pp. 103-106). Characteristics of the egg, larva, and pupa stages of the hilaris yellow-faced bee are unknown (Magnacca 2005, entire). Endemic to the Hawaiian archipelago, this medium-sized, soliatary bee is a cleptoparasite and is completely dependent (obligate) on several other ground or crevice nesting Hylaeus species for reproduction. The female does not build or provision her own nest, rather, she enters the nests of other Hylaeus species and lays her own eggs (Perkins 1899, p. 106; Zimmerman 1970, p. 36-37; Daly and Magnacca 2003 in litt., p.8; Magnacca 2005, entire). When the eggs hatch, the larvae consume the pollen and nectar provisions in the host nest. After passing through three successive molts, the larvae form pupae (a resting form) in which, they metamorphose (undergo change), and emerge as adults. The brood cycle from egg to adult takes about 30 to 60 days (Graham 2015 in litt., entire), during which time the solitary females do not provide parental care or defend their brood. Cleptoparasitic females, including hilaris yellow-faced bee females, have short, straight hairs on the foreleg(Daly and Magnacca 2003, p. 105). Perkins (1899, p. 106) hypothesized that these straight hairs are associated with the cleptoparasitic habits of hilaris yellow-faced bees; as this species lacks the need to provision her nests with pollen. Yellow-faced bees feed on pollen and nectar almost exclusively from native plants found in the dry coastal habitat.