Eristalis tenax is a European hoverfly, also known as the drone fly. It has been introduced into North America and is widely established.
The larva of E. tenax is a rat-tailed maggot. It lives in drainage ditches, pools around manure piles, sewage, and similar places containing water badly polluted with organic matter. The larva likely feeds on the abundant bacteria living in these places.
When fully grown, the larva creeps out into drier habitats and seeks a suitable place to pupate. In doing so it sometimes enters buildings, especially barns and basements on farms. The pupa is 10–12 mm long, grey-brown, oval, and retains the long tail; it looks like a tiny mouse.
The adult fly that emerges from the pupa is harmless. It looks somewhat like a drone honey bee, and likely gains some degree of protection from this resemblance to a stinging insect. The adults are called drone flies because of this resemblance. Like other hover flies, they are common visitors to flowers, especially in late summer and autumn, and can be significant pollinators.
In its natural habitat, E. tenax is more of a curiosity than a problem, and the adults are beneficial pollinators.
Source: en.wikipedia.org/wiki/Eristalis_tenax - 09.11.2011