I came across an interesting set of photos which depict a toad's reaction when presented with a late-instar Deilephila elpenor (Sphingidae) caterpillar:
According to the photographer the toad has adopted a
"snake-defensive" posture, however I'm unsure of the validity of this
statement. No doubt that snakes are important predators of frogs and toads, but
I know that large frogs and toads also attack and eat small snakes. This is
clearly an artificial setting, but it is plausible that frogs and toads might
come across a late-instar Deilephila elpenor caterpillar as they
pupate amongst leaf litter and other debris.
More information about the natural history of Deilephila elpenor can be found here.