This is Diskman's Enderbeetles mod. It adds some giant bugs that you can grow from endermites by feeding them shulker shells. The behemoths can then be bred with the same shells, and can be killed for carapace and chitin. These materials can be used to craft many helpful things.
Carapace and Phantom membrane can be used to craft an elytron, which can then be used, in large numbers, to craft the elytra.
Chitin can be crafted with sand and gravel similarly to concrete to make biolith, which is a decorative block similar to stone.
Recommended to be played with a mod that allows the respawning or farming of shulkers (before this was added to vanilla), and with JEI to view the crafting recipes for the different items.
