From these considerations it still appears most probable to me that _D. Euphorbiæ_, and the nearly allied _D. Dahlii_ and _Mauritanica_, bear conspicuous ring-spots as signs of their being unpalatable to the majority of their foes. The fact that these species feed on poisonous _Euphorbiaceæ_, combined with their habit of exposing themselves openly by day, so as to be easily seen at a distance, may perhaps give support to this view. As these insects are not protectively coloured, this habit would long ago have led to their extermination; instead of this, however, we find that in all situations favourable to their conditions of life they are among the commonest of the _Sphingidæ_.
Thus, _D. Euphorbiæ_ occurs in large numbers both in South and North Germany (Berlin); and Dr. Staudinger informs me that in Sardinia the larvæ of _D. Dahlii_ were brought to him by baskets full.
But if the conspicuous ring-spots (combined of course with the other bright colours) may be regarded as signals of distastefulness in many species of _Deilephila_, this by no means excludes the possibility that in some species these markings play another part, and are effective as a means of alarm. It even appears conceivable to me that in one and the same caterpillar they may play both parts against different foes, and it would certainly be of interest to confirm or refute this supposition by experiment.
In the light yellow variety of the caterpillar of _D. Galii_ the ring-spots may serve as means of alarm, and still more so in that of _D. Nicæa_, the resemblance of which to a snake has struck earlier observers.[154]
In those species of _Deilephila_ which conceal themselves by day, the ring-spots cannot be considered as signals of distastefulness, and they must therefore have some other meaning. As examples of this class may be mentioned _D. Vespertilio_, which is protectively coloured both in the young and in the adult stages; and likewise _D. Hippophaës_, in which this habit of concealment is associated with adaptive colouring. In the case of the first-named species, it appears possible that the numerous large ring-spots may serve to alarm small foes, but the truth of this supposition could only be decided by experiment. In _D. Hippophaës_, on the other hand, such an interpretation must be at once rejected, since most individuals possess but a single ring-spot, which shows no resemblance whatever to an eye.