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Herein we report on the nesting biology of ground-nesting Trachusa (Heteranthidium) larreae (Cockerell) from New Mexico and Arizona, an oligolege of creosote bush, Larrea tridentata (DC.) Coville (Zygophyllaceae). Nests are single, slanting, open burrows at the lower end of which are horizontal cells lined with resin collected from creosote bush, also the source of the orange, mealy-moist provisions. Eggs are placed on the surface of the provisions, and the first three instars remain in the same position as the eggs from which they hatched. The fourth instar separates its body from the provisions, and only the fifth (final larval) instar moves around the brood chamber while consuming remaining provisions and defecating prior to cocoon spinning. It is suggested that middorsal body tubercles and an integumental body vestiture of short setae and setiform spicules restricted to this instar are adaptations enabling the movement of the fifth instar not only of this species but possibly those of other Megachilinae, all of which have a body vestiture and presumably middorsal body tubercles.
The cocoon is spun after most of the feces are voided. Like cocoons of many other Megachilidae, it bears a pronounced nipple at its anterior end. From its construction as well as by comparison with cocoons of other bee taxa, the nipple seems to serve a number of functions: it enables exchange between the interior air of the cocoon and the external ambient air; it screens out parasites and predators from attacking the cocoon inhabitant; and it probably regulates cell humidity.
Eggs are briefly characterized, and three females each were found to have three ovarioles per ovary and to carry a single mature oocyte, which is classified as medium in Iwata and Sakagami's classification of egg/mature oocyte size relative to body size of female. The fifth instars (both preand postdefecating forms) are described and found similar to those of other Anthidiini.