one time i had a use for flies and we had heaps around outside that year which was lucky.They all used to sleep on the tar paper on the roof of the carport and garage.So i would wait till they got a bit dopey whenit cooled off and made a little mesh bag to fit inside the old vacum cleaner i use for the shed.Would just go around and suck them up as they slept and then take the bag full of flies and chuck them in the tank.Another method was to get a net and run through long grass catching whatever,also used a quad bike like this and got heaps of baby crickets and grasshoppers,just tipped them into a bucket and whacked the lid on quick.