The prey should never be more that half the size of the spider and preferable about one quarter the size. Crickets and mealworms are the most readily available in pet shops or bait shops and are the safest source of disease free food. Wild caught crickets, grasshoppers, grubs, or caterpillars are acceptable but do pose some risk of introducing parasites or diseases. Beetles and moths are less likely to be accepted as prey but this will depend on the spider. Avoid bees, wasps, and ants which could sting the spider.
Some spiders are voracious and eat nearly anything of the right size.
Others will prove to be finicky eaters. Try a variety of foods until one is taken. It is generally not necessary to vary the diet much. Occasionally a spider can be induced to eating a piece of meat. Select lean raw beef if you try this and dangle it on string to elicit an attack response.