Further investigation has attributed this scent to a substance generated by the mandibular glands of both males and females, composed of neral and geranial.

These have been categorized geographically as follows by Ikudome (1989):[6] A significant diversity of Hylaeus is found in Hawai'i — approximately sixty species are recorded as living there.

[9] The Hawaiian bee population and diversity is somewhat fluid — within the past ten to twenty years several Hylaeus species have been found and identified on the islands as new to science.

[15] Phylogenetic evidence suggests that the first Hylaeus species arrived on the island of Hawaii between 0.5 and 0.4 million years ago.

From here, it is believed that descendants of the initial (presumed ground-nesting) colonist separated in order to specialize in the different ecological niches available.

Using an equation (r=lnN/t) developed by other studies into the evolution of Hawaiian groups,[16] it can be calculated that the minimum speciation rate for Hylaeus on the island of Hawaii is 9.23 species per million years.

Hylaeus species lack features required for digging or excavation; instead, it is likely that the holes they inhabit were initiated by other insects (such as Plagithmysus beetles or Ectemnius wasps).

There are multiple reasons that extra species may not currently be recorded: colour markings and variations have been traditional identifiers leading to multiple junior synonyms, this genus is easily established outside of its native range, and heteroplasmy is known in some Hylaeus species which can lead to difficulties with specimen identification.

[19] Endemic species of Hylaeus are divided into two groups based on a distinctive feature: the form of the apex of the 8th metasomal sternum of males.

[19] H. agilis has been recorded nesting in a wide variety of pre-made holes in stems and wood, for example in Muhlenbeckia australis and Ripogonum scandens, as well as introduced blackberry.

[20] Phylogenetic data currently suggests that Hylaeus originates from Australia, arising around 30 million years ago.

[3] The Australian Faunal Directory lists 20 subgenera as being found in Australia:[21] 21 species have been formally identified and described in China.

Further surveying of bee fauna is likely to result in further groups being recorded, due to the close proximity of numerous other species and subgenera of Hylaeus.

The inner most cell is the one farthest from the nest entrance, and is characterised by a round distal end in order to fit the hole.

[24] Hylaeus lines their nest with a waterproof and contaminant proof clear cellophane substance that does not melt or dissolve with strong chemicals.

It is thought that the internal method of carrying pollen via the crop is an ancestral trait compared to scopal transport that other bees perform.

[30] They forage at a high rate of visitation on many plant species[31] and are considered to be an important contributor to pollination.

[34] Additionally, the Hylaeus forage within short distances of their burrows ultimately resulting in pollen limitations within their environment.

It is estimated that the species performs 90-95% of the pollination within the alpine area studied[32] with the Hylaeus also making up approximately 90% of foraging activities within their local environment.

Nearly half of the Hylaeus species found in Hawaii are under threat, primarily due to loss and alteration of habitats.

[17] In the past 30 years, only three specimens of Hylaeus facilis have been collected from the Hawaiian Islands - one from O‘ahu in 1975, one from Maui in 1993, and one from Moloka‘i in 2005.

The black household ant (Ochetellus glaber) has been observed depredating nests, removing pollen, eggs, larvae and pupae.

The African big-headed ant (Pheidole megacephala) can co-exist with H. anthracinus at low densities, however excludes them from ground-nesting opportunities when populations rise.

In contrast, the yellow crazy ant (Anoplolepis gracilipes) is known to expand rapidly and remove this species from its range.