Yes. They as big as bluebottles. But are green where the other is blue. Very beautiful but vile in equal measures. There's plenty of reading out there. Type in blowfly. Lucilia seracata (sp.)
Preventatives, we use clik or clik zen. Clik for lambs, click zen for ewes as its got a shorter action, ready for shearer, beginning of July. But saying that if we spray earlier the ewes will be clik ed too.