In the known species of the genus now existing these spots have different meanings.
In some species (certainly in Galii, and probably in Euphorbiæ and Mauritanica) the conspicuous ring-spots serve as signals of distastefulness for certain enemies (not for all).
In a second group of species they serve as a means of alarm, like the eye-spots of the Chærocampa larvæ (Nicæa? light form of Galii?).
Finally, in a third group, of which I can at present only cite Hippophaës, they act as an adaptive resemblance to a portion of a plant, and enhance the efficacy of the protective colouring.
5. Subordinate Markings.—If, from the foregoing considerations, it appears that the three chief elements of the Sphinx-markings—longitudinal and oblique stripes, and spot formations—are not purely morphological characters, but have a very decided significance with respect to their possessors, there should be no difficulty in referring the whole of the markings of the Sphingidæ to the action of natural selection, supposing that these three kinds of marking were the only ones which actually occurred.
In various species, however, there appear other patterns, which I have comprised under the term “subordinate markings,” some of which I will select, for the purpose of showing the reasons which permit of their being thus designated.
I ascribe to this category, for example, that fine network of dark longitudinal streaks which often extends over the whole upper side of the caterpillar, and which is termed the “reticulation.” This character is found chiefly in the adult larvæ of Chærocampa, being most strongly pronounced in the brown varieties: it occurs also in Deilephila Vespertilio, Pterogon Œnotheræ, and Sphinx Convolvuli. As far as I know, it is only associated with adaptive colours, and indeed occurs only in those caterpillars which rest periodically at the base of their food-plants among the dead leaves and branches. I do not consider this reticulation to be a distinct imitation, but only as one of the various means of breaking up the large uniform surface of the caterpillar so as to make it present inequalities, and thus render it less conspicuous. There can be no doubt as to the dependence of this character upon natural selection.
There is, however, a second group of markings, which must be referred to another origin. To this group, for instance, belong those light dots in Chærocampa Porcellus and Elpenor which have been termed “dorsal spots.” I know of no other explanation for these than that they are the necessary results of other new formations, and depend on correlation (Darwin), or, as I may express it, they are the result of the action of the law governing the organization of these species.
As long as we are confined to the mere supposition that the character in question may be the outward expression of an innate law of growth, it is permissible to attempt to show that a quite similar formation in another species depends upon such a law.
Many of the dark specimens of Sphinx Convolvuli show whitish dots on segments six to eleven, one being situated on the front edge of each of these segments, at the height of the completely vanished subdorsal line ([Fig. 52]). These spots vary much in size, lightness, and sharpness of definition. Now it might be difficult to attribute any biological significance to this character, but its origin becomes clear on examining light specimens in which the oblique white stripes are distinct on the sides and the subdorsal line is retained at least on the five or six anterior segments. It can then be seen that the spots are located at the points of intersection of the subdorsal and the oblique stripes (Fig. 16, [Pl. III].), and they can accordingly be explained by the tendency to the deposition of light pigment being twice as great in these positions as in other portions of the two systems of light lines. Light spots are thus formed when the lines which cross at these points are partially or completely extinct throughout their remaining course.