Live insects for sure. When I was in the high school, we had a live tarantula in our biology lab. The teacher was feeding it with live crickets. You can buy live crickets from many pet stores either online or locally. You can also buy mealworm and other types of feeder insects from pet stores.
If you are unable to buy the feeder insects, you can always catch some yourself.  Although I have never owned a tarantula myself, I have a little experience with feeding a spider in captivity with live house flies I caught.
How big is this tarantula?
The one in our biology lab was as big as my hand. The little spider I had for a while was smaller than my pinky finger nail.
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What Do You Feed Spiders?
