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The Flus' air sacs seperated into two lobes, each increasing in size on the inside of the body. This allowed for significant longer sustained floating, as well as a decreased need to return to water.
The increased space in between the air sacs allowed the Flus' mandibles to grow in size, and is now used more aggressively.
The poison glands of old adapted into a thing coating of the poison along the Flus' body. This poison is alkaline in nature, and neutralizes the Blade Worem's acid. Though it is now immune to the Blade Worem's acid, the Flus' soft yet rubbery hide is not a very good defense against the Blade Worem's blades and teeth. This, combined with their slow floating speed, makes the Flus a major prey for the Blade Worem.
These recent adaptations allowed the Flus to attack prey by dropping on top of it.