it basically depends on the size of your archer.
if you are keeping small size 1-2 inch, meal worm or blood worms would be ok.
if you are keeping them in an open area like a pond and stuff,
look for empty spider webs.
chances are that your archers have shot down the spiders and consumed them.
mine feed on small spiders and meal worm and at times frozen blood worm.
when your archers have stabled down, try to catch a small spider and dangle it above the archer.
you would see it try to shoot down the spider, if it keep missing, try to lower it or drop it in the water.
your archer would usually consume it directly.