The Queen

Emery thought there was no way an acolyte could control all of these little creatures with their own prowess alone. 

Hence, he guessed there must be something that helped the acolyte control these little creatures. He assumed there must be a leader or commander amongst them. Emery also remembered from the observation he did during his childhood days ants were basically slaves to their superior. 

Therefore, based on all these deductions and information, Emery knew it was likely there was the leader type of these creatures somewhere in the arena.

As he was being cornered by the creatures earlier, Emery tried his best to locate the so-called queen. Looking at the open arena, he couldn't see anything that could be of use as a hideout for the queen. And then, it hit him.