Stocky

Of the seven species of leafcutter bee found in the UK, Willughby's Leafcutter (Megachile willughbiella) is the most widespread, and probably the easiest to identify. 

Males are about 12mm long, with pale faces and the short, square abdomen that's common in male leafcutters.  However, unlike the male Patchwork Leafcutter (M. centuncularis) I posted about ten days ago, they have pale, expanded tarsi on their forelegs, with extravagant fringes of white hair. You can see this very clearly among the males on Steven Falk's species page, but in my experience they often settle with their forelegs tucked underneath them, giving only a teasing glimpse of this useful identification feature.. 

Females are larger, at around 15mm long, and stockier. For comparison, female Patchwork Leafcutters are around 12mm, with a slimmer build and a more pointed abdomen, and Wood-carving Leafcutter females (M. ligniseca) are about 16mm in length. Crucially, the pollen brush beneath the abdomen is different in these three bees: in Patchwork Leafcutters it's bright orange (sometimes showing as an orange halo around the abdomen, if you're viewing the bee from above), in Wood-carving Leafcutters it's buff, with a few black hairs at the tip, and in Willughby's Leafcutters it's orange at the base and black below the last two abdominal segments. 

In a fresh bee like this one it's easy to see the colour of the pollen brush - especially as all leafcutters tend to feed with their heads down and their bums in the air - but when female leafcutters are foraging, their abdomens rapidly become so heavily coated with pollen that the scopal hairs are virtually invisible. Even then though, Willughby's Leafcutters tend to give the impression that the tip of their tail is black, and taken together with their size and shape this is usually enough to seal the identification.

In case you haven't seen it, I've now updated and reposted my blip from last Wednesday.