Tuesday, 8 June 2010

Pollen gathering face brushes

ResearchBlogging.orgMost non-parasitic female bees collect pollen as well as nectar to provision their brood cells. Pollen is brushed from the plant anthers using their front legs and stored on specialised structures on the bee body or legs. Some bees, however, specialize on collecting pollen from flowers with raised anthers, which touch over the bee's head or thorax when bees land on them. These are called nototribic flowers and include species from the Lamiaceae (the mint family) and Scrophulariaceae (the figwort family). Although bee-pollinated plants benefit from bees taking nectar - exchanging nectar for inadvertent pollination - they do not benefit when potential pollinators efficiently gather the pollen for their offspring consumptions instead. Nototribic plants in response to specialised pollen gathering by bees, have flowers that make pollen hard to collect, even when they have plentiful nectar. Only bees equipped with either specialised behaviour or morphological modifications, or both can effectively make use of their pollen.
 Mueller found that only 13 species of Central European bees (including Anthophora furcataAnthidium manicatum and Osmia caerulescens) possess specialised structures to brush and collect pollen from nototribic flowers. In several of these species the specialisation is on the face of the bee and consists on a think covering of peculiarly shaped hairs, of wide bases and sharply bent ends. These specialised hairs seem to have evolved independently several times as the bees belong to different genera also containing species lacking such modified structures.
A photo of Anthophora furcata clypeus (top lip of the bee) and on the right a portion of it showing the specialised hairs (from Mueller 1996).

One of such bees, Anthophora furcata, the Fork-Tailed Flower Bee is a summer bee, active in June and July in the British Isles. These are long-tongued, relatively specialised species, which collects pollen mostly of Lamiaceae (and to a minor extent of Scrophulariaceae) flowers. They are often present in gardens where they feeds on Lavender, Tree Germander and Lamb's ears (Stachys byzantina) as well as foxgloves. A couple of males I spotted yesterday, the first of the year, were feeding on a patch of Stachys sylvatica, and the photo of one of them on top of the post shows the position of the top lip of the flower - which hides the stamens and pistil - touching the bee's head. 
Mueller also investigated the brood cell contents and pollen load of bees to ascertain the degree of specialisation of the bees and also looked into the actual bee's behaviours to gather the pollen to account for the position of the facial hairs. In the case of A. furcata:

[it] presses its head against the anthers and, by vibrating its indirect flight muscles, buzzes the pollen amongst the transformed hairs. No further head movements can be observed.

The 'buzzing' releases the pollen from the anthers, and the specialised hairs help retain the pollen grains on the face. The bee then removes the pollen grains adhering to the specialized pilosity by rapid stroking movements of the forelegs in flight after leaving the flower.
Female of A. furcata feeding on Lambs ears.
Although the bee behaviour could be thought as a form of 'pollen robbing', pollination can still take place while nectar sucking, as the pistil of the flower still touches the bee's thorax, making no contact with the specialised brush on the bee's face and the flower constancy of the bees makes it more likely that she will be carrying some of the same species pollen on her back.

More information
Muller, A. (1996). Convergent evolution of morphological specializations in Central European bee and honey wasp species as an adaptation to the uptake of pollen from nototribic flowers (Hymenoptera, Apoidea and Masaridae) Biological Journal of the Linnean Society, 57 (3), 235-252 DOI: 10.1111/j.1095-8312.1996.tb00311.x

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