Hoverfly

A small black hoverfly with large bright orange antennae: probably a female Cheilosia pagana (Diptera: Syrphidae).  According to  Stubbs and Falk (British Hoverflies, 2nd edition, 2002), the female of this species is "conspicuous in the field because it has a huge orange third antennal segment, clearly wider than the front femur, but the males are less obvious with their small antennae". 

The flowers are those of hedge mustard (Sisymbrium officinale), a strange looking plant in which the flower heads are at the tips of long, straggly, often up-curved branches which have narrow pods aligned along their length, giving the impression of a green multi-strand cable.

Photo taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-09-04.