Answer
Mealworms can be used as tarantula prey, especially for smaller spiders, but they burrow quickly and need monitoring.
Stop them burrowing
Offer them in a smooth-sided dish or crush the head immediately before offering if you need to prevent escape into the substrate.
Prey size
Use a worm the spider can subdue easily. Cut fresh sections can be used for tiny slings that scavenge.
Remove leftovers
Do not leave mealworms or beetles hidden in the substrate around a premoult or moulting spider.
Variety
Mealworms do not need to be the only feeder. Crickets, locusts and roaches can be rotated according to availability and spider size.