The Colletinae: Going to Ground

In a recent post, I considered one of the families of short-tongued bees, the Halictidae. In this post, I'll turn my attention to members of one of the other short-tongued bee families, the Colletidae. Specifically, I'm looking at members of the subfamily Colletinae.

Mating ball of male ivy bees Colletes hederae, copyright Charles J. Sharp.

Members of the Colletidae differ from other bee families in that their glossa, the 'tongue' at the end of the proboscis, is apically bilobed or bifurcate. They are also distinctive in lining their nests with a plasticky, cellophane-like material. It has been thought that this material was made from dry saliva but the bulk of it is now known to come from a large gland in the abdomen that opens near the base of the sting (Almeida 2008). A nesting female will swallow droplets of the glandular secretion from her partially protruded sting then regurgitate it as she licks the wall of the nest cell. This waterproof lining both protects the cell from outside elements while preventing the loss of moisture from within. Many colletids, including colletines, leave the cell food provisions in a semi-liquid state; other bees whose nests are less watertight will dry and compact the provisions, presumably because the bulk of them would otherwise be lost before the larva hatched. In colletines, the egg is attached to the cell lining when laid, suspended above the provisions for the hatching larva to swandive into upon emergence. All colletids are solitary nesters with species nesting either in burrows in the ground or in hollows in vegetation; the majority of colletines are ground nesters*.

*One species, Colletes daviesianus, has apparently taken in Germany to boring its nests in the sandstone and mortar used in building construction.

Female Colletes daviesianus, copyright Donald Hobern.

In his 2007 edition of The Bees of the World, Charles Michener recognised five subfamilies within the Colletidae. The Colletinae were distinguished from three of these subfamilies by their retention of a covering of dense hair over the body (from the last subfamily, the Diphaglossinae, they differ in features of the glossa and wing venation). The hind leg of the female bears a well-developed scopa (dense arrangement of hairs for the carrying of pollen) on the femur and tibia with a corbicula (bare patch within the scopa where a ball of compacted pollen may be carried) on the underside of the femur. Hairiness is an ancestral characteristic for bees and phylogenetic studies have established that the Colletinae as recognised by Michener is a paraphyletic grouping (e.g. Almeida & Danforth 2009). As a result, it has been further subdivided with the name 'Colletinae' now restricted to what Michener had recognised as the tribe Colletini. As such, the Colletinae now includes just two genera of moderate-sized bees (seven to sixteen millimetres in length). The larger of these, Colletes, is found in temperate and tropical regions around the world except for the Indo-Australian region where it is notably absent. The other genus, Maurecolletes, is restricted to South America. One of the most distinctive features of Colletinae in the strict sense compared to other ex-colletines is the lack of the basitibial and pygidial plates, flattened and hardened plates possessed by other hairy colletids at the base of the hind tibia and at the end of the abdomen.

The absence of these plates is intriguing in light of the ground-nesting habits that seem to be the norm for Colletes (the nesting habits of Maurecolletes seem to be unknown). In other ground-nesting bees, the basitibial and pygidial plates are used to press the soil of the nest walls and opening into place. One would think this would mitigate against their loss. An explanation may be provided by the fact that some South American Colletes nest in the hollows of dead, pithy plant stems instead of in the ground, a characteristic shared with members of the less hairy colletid subfamilies belonging to the sister group of the colletines (Almeida & Danforth 2009). Ground-nesting Colletes species also bear noteworthy resemblances to stem-nesting colletids. Nest cells are closed with a layer of the cellophane-like wall membrane rather than the earthen plugs used by other ground-nesting bees. In many species, cells are lined up in a burrow divided by transverse partitions rather than placed in their own individual side branches. The possibility has been suggested that stem-nesting arose within the common ancestors of modern colletines and less hairy colletids. Ground-nesting in Colletes would then represent a secondary reversion by these species to the previous habit. When they did so, they retained the adaptations and habits that had originally been associated with their time in the twigs.

REFERENCES

Almeida, E. A. B. 2008. Colletidae nesting biology (Hymenoptera: Apoidea). Apidologie 39: 16–29.

Almeida, E. A. B., & B. N. Danforth. 2009. Phylogeny of colletid bees (Hymenoptera: Colletidae) inferred from four nuclear genes. Molecular Phylogenetics and Evolution 50: 290–309.

Michener, C. D. 2007. The Bees of the World 2^nd ed. John Hopkins University Press: Baltimore.