The purpose of this study is to describe the mature larvae of all taxa of the tribe Osmiini (Megachilidae) whose postdefecating larvae have been collected and are available. The hope is that data uncovered will eventually lead to a better understanding of the interrelationships of included taxa and of their anatomies, behaviors, and ecologies (i.e., their natural histories). The species included are representatives of the following taxa: Ashmeadiella, Atoposmia, Chelostoma, Heriades, Hoplitis, and Osmia (Hoplosmia). We also include recently described larval representatives of Haetosmia and Ochreriades. Because of its large size and diversity, the genus Osmia alone (with the exception of the subgenus Hoplosmia) is allocated to a separate future investigation. Larvae of the tribe share many features with one another and also with other members of the Megachilidae. Anatomical structures of larval Osmiini whereby they can be differentiated from other megachilids were not identified. However, most (though not all) members of the tribe fall into one of three main subgroups on the basis of: (1) body form, (2) presence or absence of middorsal, intersegmental tubercles on basal abdominal segments 1–5, and (3) the depth of intersegmental constrictions.

In the process of gathering specimens the authors uncovered new information concerning nesting biology with respect to nest location and structure and to cocoon structure and function. These data were derived from the authors' own field studies as well as from specimens and field data collected through the years by others.