Just an introductory comment on the endemic Florida species Stelis ater:  There is some difference of opinion among local entomologists about whether the only other black Stelis species of the eastern United States --  S. diversicolor -- is found in Florida.    It is notable that the Discover Life database records that S. diversicolor has been found at least once, somewhere in Florida. Gainesville is the stomping ground of Stelis ater's discovery.

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The Discover Life Stelis key, citing Mitchell, notes that Stelis ater and S. diversicolor might be separated by two traits.   (1) S. diversicolor tends to run smaller (c. 8 mm) than S. ater (c. 10 mm).   (2)  On S. diversicolor, fewer than 50 % of the pits on the scutum touch each other.   By contrast, on S. ater, more than 50% of the pits on the scutum touch each other.  The male S. ater has never been formally described.  The University of Florida bee key has some excellent photos, however, of dead specimens of both genders.  (https://entnemdept.ufl.edu/hallg/melitto/floridabees/stelis.htm).  That key does not include Stelis diversicolor as a native Florida species.

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Below are four black Stelis --  three females and a male.   None of the females precisely fits Mitchell's description of the species.  The first female measures 9.5 mm, but the other two are larger -- 12 and 13 mm -- and seem far out of the size range of a typical Stelis diversicolor.  Nonetheless, all three female bees seem to lack the character that Mitchell used to distinguish Stelis ater from Stelis diversicolor:  the scutum of each is not particularly densely pitted.  The male, by contrast, has a densely-pitted scutum.  All four bees are from Gainesville, except for the largest female, which is from much farther to the southeast (Brevard County).  All were feeding on yellow thistle (Cirsium horridulum), except for the large Brevard Co. female, which was found on Nuttall's thistle (Cirsium nuttalli).  In all four cases Osmia chalybea was found within a few yards of the Stelis. 

All four bees have these characteristics:  they are all black bees,  with dark, black-veined wings.    Each has a long face, with three-toothed mandibles.  Much of each bee's body is coarsely pitted (vertex, thorax, tergum, sternum). White hairs cover parts of the face, vertex and thorax. The hairs on the vertex are all light and very sparse. The females' abdomens are banded with sparse white hairs on T1, T2 and T3.   On the females, T4 has white hairs laterally.  Sparse white hairs also band the sternum.  On the male,  T1-T4 are banded with spare white hairs, and T5 has white hairs laterally.

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Osmia chalybea, the documented host of Stelis ater  (Rozen 2014) was found within a few yards of all four Stelis.   Available literature on Stelis ater associates it exclusively with thistles. 

In Texas, as in Florida, Osmia chalybea is common wherever thistles abound.  telis diversicolor visits thistles and a range of other flora.   The host of Stelis diversicolor has never been established.  But, on the other hand, Osmia chalybea has never been ruled out as the host of Stelis diversicolor.  Other promineent Texas Osmia include O. texana and several O. (Dicertosmia). The known hosts of Osmia (Diceratosmia)  do not include Stelis diversicolor.  

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About size:  Although Mitchell estimated the size range of Stelis diversicolor to be around 8 mm, there are specimens as large as 10 mm in Texas collections.  They may well run larger - - a concerted effort to determine their size range might produce larger specimens.