From Halopedia, the Halo wiki
Scrub grubs are a type of arthropod, like the Yanme'e, though far smaller and non-sentient. They are anxious creatures with hairs that cover their turgid bodies. These hairs are always in motion, sensing for danger. On long space flights, their numbers have to be contained in order to prevent them from damaging any vital systems. When they are smashed they turn into a mealy liquid. The Unggoy often hunt these creatures for sporting games during space travel.
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