A mixture of cornstarch and water becomes solid if you hit it. Ketchup becomes runnier if you shake the bottle. Saliva is like ketchup: Forces makes it less viscous. But while human saliva becomes around ten times less viscous if you apply force to it, frog saliva becomes a hundred times less viscous.
So when a frog tongue strikes an insect, its saliva flows freely and readily seeps into every crack and gap. When the tongue slows down and starts retracting, the saliva thickens again into a paste, the equivalent of a closed fist grasping the insect for the journey back.
“The analysis helps to explain many bizarre observations, like why frogs use the backs of their eyeballs to push prey down their throats,” says Kiisa Nishikawa from Northern Arizona University. When the insect’s in the frog’s mouth, the frog has to get it off its tongue. Fortunately, all of its adhesive tricks work best in the perpendicular direction—it may be really hard to pull the insect off, but it’s comparably easy to slide it off. The frog just needs something to push against the insect—so it uses its eyeballs. Twelve years ago, Robert Levine used X-ray videos to show that a frog swallows, it retracts its eyeballs inwards, and uses these to push victims off its tongue.Cat tongues are another kettle of fish.