These horrible and fat creatures are a scourge to the worm farm. They invaded the farm during the Christmas period due to a piece of contaminated meat entering the system. I believe they are the maggot of the green bottle fly, something that is in high numbers in my area due in no small part to the next door neighbours cats. These critters are eatting their way through perfectly good worm food and producing a pretty useless compost. The sooner these things are eliminated the better.
