A bumble bee, Bombus sp (Hymenoptera: Apidae) infested with mites. I was rather surprised to come across parasites so early in the spring as I had assumed that they don't really get going till later in the year. This bee was rather torpid: it didn't try to fly away when I approached it, but just waved a leg at me.
Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-03-28.
This afternoon I overheard a discussion of bee mites on BBC Radio 4. Apparently the mites that are commonly seen on bumble bees in the spring are not thought to be parasitic, but are just hitch-hikers. They spend most of the year in the bee's nest scavenging on scraps of food but in the spring, when the queen bee is about to leave in search of a new nest, the mites climb onto her, so as not to be left behind. When the queen founds a new nest, the mites dismount and make it their new home.
The above bee is therefore probably a queen.