Tuesday 25 May 2010

The leaf-cutters are back

ResearchBlogging.orgA male leaf-cutter has been patrolling in the garden the last few days, when the weather has been quite hot. Leaf-cutters are summer bees, emerging from their nests at the end of May-beginning of June and dissapearing for the year at the end of August. The male has been circling around the broom and other flowers, in a regular path, landing to bask for no more than two shots in a sunny leaf or flower. Occasionally, he stopped to feed on some bluebells.
They are Megachile willughbiella (thank you to eucera from WAB for confirming the ID). The males bear white and golden 'boxing gloves' on their forelegs, which are enlarged concave-shaped furry tarsi, contain glands producing odor chemicals and are involved in mating.
Males use the 'gloves' as blind-folds: to cover the female's eyes during mating. They also have various morphological modifications under their heads, mandibles and legs to hold and fold down the female's antenna, head, wings and abdomen in position during mating, so that exposure to the odor glands is maximised. It is not entirely clear why the female should be blinded in such a way, and it has been suggested that the males effectively immobilise the female during mating and that the odors contribute to 'calm' her.
This drawing, taken from a figure in Wittmann and Blochtein's work below) shows the position of the mating pair. The hatched parts are modified to aid during mating.
Females, however, can and do repel advancing males. Sexually dimorphic forelegs of males appear in various bees (some leaf-cutter and carpenter bees) and many sphecid wasps, and are all used during mating to cover female eyes, suggesting a common function.

More information
Wittmann, D. and Blochtein, B. (1995). Why males of leafcutter bees hold the females' antennae with their front legs during mating Apidologie, 26 (3), 181-196 DOI: 10.1051/apido:19950302


Threadspider said...

What a great post. I am learning to identify some solitary bees but it is very difficult, so observations such as yours about "boxing gloves" really helps.
I shall be on the look out for leaf cutters from now on. I think I have some leaf damage already

Africa Gomez said...

Thank you Threadspider. The 'gloves' are helpful, but beware that although willughbiella is one of the commonest species, often found in gardens, there are two other British species that have the 'gloves' M. maritima and M. circumcincta, which are more localised.