Do trap-jaw ant larvae need sand or soil to spin their cocoons?
Yes, Odontomachus spp. larvae need some kind of substrate to spin their cocoon, which is critical in their pupation process. This same characteristic is found in virtually all Ponerines, not just Odontomachus spp.
If you're keeping your colony in The Substratum, or another soil-based nest, no need to worry about providing additional substrate! The ants will take care of their larvae as they naturally would in the wild.
If you're keeping your colony in more of an artificial setup, you'll need to provide them with substrate directly in the nest. It is not sufficient to provide some in the foraging area. Sand, coco fiber, dirt, etc. all work; the ants are quite adaptable.