I next made the experiment of placing before a lizard a caterpillar as much as possible like that of _E. Jacobææ_. Half grown larvæ of _Bombyx Rubi_ likewise possess golden yellow (but narrower) transverse rings on a dark ground, and they are much more hairy than those of _E. Jacobææ_. The lizard first applied its tongue to this caterpillar and then withdrew it, so that I believed it would also be avoided; nevertheless it was subsequently eaten. The caterpillars of _Saturnia Carpini_ were similarly devoured in spite of their bristly hairs, and likewise cuspidate larvæ (_Dicranura Vinula_), notwithstanding their extraordinary appearance and their forked caudal horn.[152] These lizards were by no means epicures, but consumed large numbers of earth-worms, slugs, and great caterpillars, and once a specimen of the large and powerfully biting Orthopteron, _Decticus Verucivorus_. Creatures which possessed a strongly repugnant odour were, however, always rejected, this being the case with the strongly smelling beetle, _Chrysomela Populi_, as also with the stinking centipede, _Iulus Terrestris_, whilst the inodorous _Lithobius Forficatus_ was greedily eaten. I will call particular attention to these last facts, because they favour the supposition that with rejected caterpillars a disgusting odour--although perhaps not always perceptible by us--is the cause of their being unpalatable.
Striking colours are of course only signals of distastefulness, and the experiment with _Bombyx Rubi_ shows that the lizards were from the first prejudiced against such larvæ, the prejudice only being overcome on actually trying the specimen offered. A subsequent observation which I made after arriving at this conclusion, is most noteworthy. After the lizard had learnt by experience that there might be not only distasteful caterpillars (_E. Jacobææ_), but also palatable ones banded with black and yellow (_B. Rubi_), it sometimes tasted the _Jacobææ_ larvæ, as if to convince itself that the insect was actually as it appeared to be, viz., unpalatable!
A striking appearance combined with a very perceptible and penetrating odour is occasionally to be met with, as in the caterpillar of the common Swallow-tail, _Papilio Machaon_. I have never seen a lizard make the slightest attempt to attack this species. I once placed two large specimens of this caterpillar in the lizard vivarium, where they remained for five days, and finally pupated unharmed on the side of the case.