This is his adjuchas form.
Looks like an insect with a large beetle shell (formed from a gillian mask split down the middle), 9 locust heads with hollow masks and humanoid spines running along the necks.
From under the shell stick out a whole bunch of segmented limbs, with hands, claws and pincers.

He'll go arrancar soon though, then he'll be humanoid with the leftover mask forming a ridge on the top of his head like a headband, the spines hanging behind it between the dreadded hair.