Although the tubes in your fly zapper may appear to be working and giving out plenty of light after use of around 10000 hours run time the amount of useful insect attracting UV light coming from your old lamps drops to a level where it will no longer be attracting insects to your bug killer These Replacement UV blacklight bulbs for bug killer machines fit most 20w/40w fly zappers that use 60cm T8 bulbs such as Vermatik Futura Z40, ACV40WP, ACV40