EPOW - Ecology Picture of the Week

Explanation:   It is 103^oF (39^oC) in the parching sun.  We are kneeling over a Coville's barrel cactus in bloom, in a remote mountain park outside the tiny hamlet of Oracle, Arizona, about an hour north of Tucson's northwest boundary.  The bloom has a visitor carrying yellow luggage ... pollen sacs ... performing the key ecological function of pollination for this plant.

This is a halictid bee (of family Halictidae), probably an undescribed species of genus Dialictus.  This genus of hundreds of species (500 in North America) has not been "monographed," which is entomologist-speak for describing and naming of all the species.  In fact, this genus has recently been combined with three others into the  more comprehensive genus of "sweat bees," Lasioglossum.

Coville's barrel cactus in partial (sequential) bloom,
southern Arizona USA

Most halictid bees are tiny, ranging 1/5 to 3/5 of an inch (5-15 mm) long.  Some halictid bees are called "metallic bees" because of the sheen on their exoskeleton, and most or all seem to be pollinators of flowering plants.   

Coville's barrel cactus blooms pollinated by halictid bees
in the American Southwest deserts

Halictid bees sometimes nest in large numbers in ground burrows, but this one was flying solo, literally.  Their ground burrows usually consist of a main vertical tunnel with lateral tunnels branching from it, each ending in a small cell.  Like many desert holes, when abandoned, the burrows of halictid bees might be secondarily used by other insects and organisms, such as ants.  This may be another key ecological function of halictid bees -- provision of tunnels for other secondary burrow-using species. 

Acknowledgments:  My thanks to Andy Moldenke, Entomologist, Oregon State University, for the identification.