Flies are usually just a minor annoyance to people. They're always looking for food, they're always hungry, always trying to steal your dinner. Imagine what a really large fly would be like -- one the size of a mouse, a cat or dog, or even the size of a pony.
Herbie Butterworth is telling a friend how he'd like to have a pet fly. He'd like to give it a collar and put it on a leash and treat it like any other pet. This wouldn't work with a fly of a normal size. He wants to breed larger flies.
When you have flies that are many times larger than normal, what do they eat? How can they get enough of it, with their appetites increasing with their size?
This here fly is 8 inches long.