Sat 2012-03-31 
A female Leucozona glaucia (Diptera: Syrphidae). And this is a male of the same species:
Photos taken above Hutton Village, near Guisborough, North Yorkshire, UK, on 2010-08-10 (the female), and in Bousdale, also near Guisborough, North Yorkshire, UK, on 2010-08-08 (the male).
Fri 2012-03-30 
From earlier this month: a pair of cluster flies, a Pollenia sp (Diptera: Calliphoridae), mating. In this species the females are significantly larger than the males.
Photos taken in Whiteknights Park, Reading, UK on 2012-03-10.
Thu 2012-03-29 
From a couple of years back: males and female green dock beetles, Gastrophysa viridula (Coleoptera: Chrysomelidae). The females are so swollen with eggs that their wing cases cover less than half of their abdomen.
Photos taken in Whiteknights Park, Reading University griounds, Reading, UK, on 2010-05-28.
Wed 2012-03-28 
Another Eristalis pertinax (Diptera: Syrphidae), this time a female as indicated by the large gap between the eyes on the top of the head.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2009-04-24.
Tue 2012-03-27 
A male Eristalis pertinax (Diptera: Syrphidae). This species is distinguished from the other similar Eristalis tenax and Eristalis similis by its orange front tarsi. A male because its eyes meet on top of its head.
For a top view of another male of this species see here.
Photos taken in Whiteknights Park, Reading University grounds, Reading, on 2010-07-30.
