>> reporter: kosaka's biggest rival is this man.e's two-time champion and veteran trainer sakuma who works at a local iron works. >> caught a whole bunch of them. >> reporter: to make his spider the strongest, he feeds them flies. >> i treasure them for now. they're like family. >> reporter: he feels only soft just born female flies with the wings clipped off to make them easier to eat. treat it like royalty. maybe not. he makes his spiders fight and trains them like crazy. okay. let's go. >> reporter: he makes the good ones spar three times a day. no! get him. >> reporter: he ranks his spiders by the results and then decides which to enter. moving their behinds up and down is called behind bowing. it shows they are fired up. >> translator: yeah, that's good! look, you quit. he's my number one. my god, don't broadcast this bit, okay? this one won't do. what got into him? >> reporter: he has no use for losers. this is how he trains the strongest and fiercest jumping spider. >> translator: there are other important things in life, but