Fig. 8 (repeated). Caterpillars of the Buckthorn Hawk-moth, Deilephila hippophaës. A, Stage III. B, Stage V. r, annular spots.
Let us consider the ontogeny of these different forms of markings, beginning with the eye-spots. It appears that these develop from a sub-dorsal stripe, which appears in the young caterpillar in the second stage of its life, and from it, in the course of the further development, two pairs of large eye-spots are formed. Even in the young caterpillar, scarcely one centimetre in length (Fig. 116), it can be observed that the fine, white sub-dorsal line takes a slight curve upwards on the fourth and fifth segments (C), and on the lower edge of these curves a black line is laid down (D). This is then continued to the upper side (E), and encloses the piece of the sub-dorsal stripe (F and G), and thus there arises a white-centred, black-framed spot which only requires to grow and to differentiate a blackish shadow-centre, the pupil (G), to give the impression of a large eye. This occurs as the caterpillar goes on growing, and after the fourth moult or ecdysis the eyes have already some effect, as the animal is six centimetres in length, but they become even more perfect in the fifth and last stage. During this development of the eye-spot the sub-dorsal stripe disappears completely from the greater part of the caterpillar, persisting only on the first three segments (Fig. 116, B-F).
Fig. 116. Development of the eye-spots in the caterpillar of the Elephant Hawk-moth (Chærocampa elpenor). A, Stage I, still without marking, simply green. B, Stage II, with sub-dorsal stripe (sbd). C, sub-dorsal line somewhat later, with the first hint of the eye-spot (Au) on segments 4 and 5. D, eye-spots in Stage III of the caterpillar, somewhat further developed than in E, the third stage. F, Stage IV. G, the anterior eye-spot at the same stage.
When we consider that this stripe in the little caterpillar a centimetre long, which lives on the large leaves of the vine, or on the obliquely ribbed willow-herb (Epilobium hirsutum), is quite without protective value, its occurrence at that stage can only be regarded as a phyletic reminiscence due to the fact that the ancestors of these species of Chærocampa possessed longitudinal stripes in the adult state, probably because at that time they lived on plants among the grass, and that, later, when the species changed their habitat to plants with broad leaves which had arisen in the meantime, eye-spots were developed in addition to the green or brown protective colouring which they retained. Thus the modern development of these spots mirrors their phyletic evolution very faithfully; on the two segments there were formed, from pieces of the sub-dorsal line, first white spots ringed round with black, then unmistakeable eyes with pupils (C, D, G). This transformation can only have begun in the fairly well-grown caterpillar, because it was only of any use to it; but later on it was shunted further back in the ontogeny, from the sixth and fifth to the fourth and third caterpillar stage, not in its complete development, but in more and more incipient form; and nowadays the first traces of eyes, as we have already seen, are visible in the course of the second stage. The marking of the more remote ancestors, the longitudinal striping, is now lost in proportion as the eye-spots develop, perhaps because the former would take away from the full effect of the latter. The longitudinal stripes are still quite plainly visible on the first three segments, but these segments are drawn in and are scarcely noticeable when the caterpillar assumes a defiant attitude (Fig. 4).
In the case of marking with ring-spots, which is found especially in species of the genus Deilephila, the ontogeny discloses that it has developed phyletically from the sub-dorsal stripe; in the young stage of this caterpillar also, the sole marking is longitudinal striping; in Deilephila zygophylli, from the steppes of Southern Russia, this persists apparently through all the stages, but in the others it disappears almost completely in the later stages, but only on the segments on which the spot-marking has developed from it. This happens in a manner similar to that in which the eye-spot in Chærocampa arises, a piece of the white sub-dorsal stripe is enclosed above and below by a semicircle of black, and later these semicircles unite, and cut off the portion of the sub-dorsal line, and form a black spot with a light centre within which a red spot frequently appears (Fig. 117, A).
Fig. 117. Caterpillar of the Bed-straw Hawk-moth
(Deilephila galii). A, Stage IV, sub-dorsal
stripe still distinct, the annular spots are still
incompletely enclosed in it. B, fully-formed
caterpillar without trace of a sub-dorsal stripe,
but with ten annular spots.
In most species these ring-spots occur on many segments (10-12) (Fig. 117, B), and in cases where they are of importance in making the caterpillar conspicuous and easily seen they sometimes form a double row. But we know one species, Deilephila hippophaës, in which only a single ring-spot exists, and it is a large brick-red spot on the second last segment, mimicking the red berry of the buckthorn (Fig. 8, A and B, r). But individuals also occur in which there are, on the five or six segments in front, smaller ring-spots which become less distinct the further forward they are, and in most caterpillars it is possible, on careful examination, to recognize little red dots on the faded sub-dorsal stripes of these segments (Fig. 8, B). We might be disposed to think, on this account, that the ancestors of D. hippophaës bore rings on all the segments, and that these had gradually become vestigial on the majority of them, because they had lost their earlier biological importance, and now, by adaptation to the buckthorn, could only be of use on the second last. But when we take the ontogeny also into account we find in the young caterpillar only a simple sub-dorsal line, upon which, in the third stage, the red spot of the tail-horn segment appears (Fig. 8, A).