A spider on shaft can be made on Karamja during the Tai Bwo Wannai Cleanup minigame. Kill a Jungle Spider to get spider carcass, then use an arrow shaft with the spider carcass.
When the spider is on the shaft, it can be cooked. A Cooking level of 16 is needed. When eaten, it heals a random amount of health between 70 and 100, and the shaft is destroyed.
Spiders on shafts are sold by Hudo, the gnome grocer in the Grand Tree. He also sells spiders on sticks, but they cost 10 coins more than spiders on shafts.