For taxonomic work on Hawaiian endemic *Hylaeus* species, what subtle features are often relied upon?

The relative length of antenna segments or minute sculpturing on the thorax surface.

In regions like Hawaii, where several *Hylaeus* species face critical endangerment or extinction, distinguishing between them requires highly specific morphological keys used by entomologists, as visual cues alone may be insufficient or misleading. The physical examination often moves beyond easily visible traits like general size or facial patterns. Researchers focus on features that require close inspection, such as the precise relative lengths of the various segments that make up the antennae, or the minute structural details, referred to as sculpturing, present on the surface of the thorax. These minute details provide the critical data needed for accurate taxonomic classification and tracking of these vulnerable island populations.