Volucella inanis, Hoverfly larvae
These are larvae of Volucella inanis, a large species of hover-fly (Syrphidae). They develop inside wasps' nests where they act as scavengers on debris that accumulates in the bottom of the nest and also feed on the wasp larvae. When fully grown, the fly larvae leave the nest and then wander around until they can find a suitably sheltered nook or cranny where they can pupate. This is when they are sometimes found crawling about inside houses, and although they are completely harmless to people the sight of these large fly larvae inevitably causes concern when people don't know what they are. In the UK, V. inanis is largely confined to southern England, being commonest in the suburbs around London. It can be distinguished from the four other British Volucella species by its pale yellow abdomen with narrow black bands.