What colour are bluebottles?

[Colin Cracknell]

As a child I used to wonder why big fat flies were known as bluebottles, 
when they were black. As time went on, I saw occasional blue ones, but 
almost all seemed to be black.

A few years ago I started photographing insects, thus looking at them 
more closely and more often. In high summer I have plenty of interesting 
subjects, but in spring I've had several months of nothing and there's 
still not much around. Thus I'm likely to look at less promising 
subjects, just for practice. Subjects like bluebottles - and they all 
have blue abdomens!

I can think of several possible explanations for this. The obvious one is 
that I've made a mistake in identification; that the blue specimens are 
Calliphora vomitoria and the black ones are something similar. Possible, 
but I doubt it. The plate in Colyer & Hammond's "Flies of the British 
Isles" has only the faintest touch of blue.

My current working hypothesis is that the colour is in some way 
temperature-dependent. At the start of this season all the flies I saw 
were brilliant blue. In a spell of warmish spring weather they seemed 
duller, then back to blue as the weather cooled again. The British summer 
being what it is, there hasn't yet been a long spell of hot weather to 
bring out the fully black form.

Am I talking rubbish? Has any serious work been done on this? What is the 
metallic pigment in Calliphoridae anyway?