The robber fly, which is sting-less and tasty, closely resembles the unpalatable bumblebee. A southern toad, which had previously seen neither fly nor bee, was presented with a robber fly, which was promptly eaten. A bumblebee was then offered – the toad grabbed it, was stung, and spit out the bee. A subsequent bumblebee was refused. Then another robber fly was offered – and, its lesson learned, the toad backed away. To prove that the toad was still hungry, the researchers then provided a dragonfly, which was immediately eaten.
I am not aware of research concerning how long such lessons are retained – but my own experience offers some clues. I have long kept green frogs, Rana (Lithobates) clamitans in an outdoor pen, where I used ripe fruit to attract insects for them to eat. Year after year, I observed the same frogs to studiously avoid yellow-jackets and other wasps, while snapping up flies and beetles located close to the wasps. It would appear that they were stung at one point, and that the lesson lasted, as far as I can tell, for at least 6 years.
The book to which I referred above is one in the wonderful Life Nature Library series published by Time, Inc. Don’t let the publication dates fool you – they are packed with original observations and unique photos.