Curly wings are only available from April to October.
These are house flies that have been selectively bred for curly wings. Curly wings cannot fly and instead hop around. The gene for normal wings still exists in the breeding pool so every generation a few normal type flies will be produced. The easiest solution to getting rid of these and keeping the curly wing lines as curly as possible is to simple open up the container they are held in outside and let the normal types fly away! I have been breeding and raised them for 5 years now.