Russel Wallace : Alfred Russell Wallace (sic)
"1. The development commences with a state of simplicity, and advances gradually to one of complexity.
"2. New characters first make their appearance in the last stage of the ontogeny.
"3. Such characters then become gradually carried back to the earlier ontogenetic stages, thus displacing the older characters until the latter disappear completely."
These laws are liable to be modified in various ways by the influence of natural selection, and especially by the need for protection, whence arise the various markings of the different groups, and the peculiar divergences often noted in their development at corresponding ages.
Having thus established the general developmental history of the markings of caterpillars, and explained by a few simple principles the chief anomalies they present, Dr. Weismann passes on to the still more interesting inquiry as to the biological value or actual meaning and use of the markings in each individual case. He first shows that colour itself, irrespective of marking, has a distinct biological value, being always either protective or a signal of uneatableness. The Sphinx larvæ when young are almost always green, resembling the leaves of the plants on which they feed and rest. When they get larger, however, they frequently change to brown, and this change is always accompanied by a change in habits, the insect feeding at night, but during the day concealing itself on the ground or amidst dead leaves and branches. This occurs chiefly among the species which feed on low shrubs or herbs and can thus easily descend to the ground to conceal themselves during the day; while those which feed on large trees acquire markings which assimilate them more completely to the foliage or flowers which surround them. The simplest form of marking--longitudinal stripes--is common on all caterpillars which feed on grasses or other plants in which straight lines are a prevalent feature, and this style of marking is that which usually appears in the young sphinx larvæ. But as they grow larger diagonal stripes or bands variously tinted or shaded appear, and this style of marking is found to assimilate so well with the oblique veining of the leaves that the caterpillars are very difficult to see when resting among them. This is the case even when the oblique lines are margined with violet or other bright colours, since, however conspicuous these markings may be when the insect is examined in captivity, they are found to blend perfectly with the lights and shadows of the foliage which surround it in its natural habitat. As an example we have the following account of the brilliantly coloured larva of the Death's-Head Moth on one of its natural food-plants:--
Many of the adult sphinx larvæ however are adorned with ring-spots or eye-spots, and these have been found to serve two distinct purposes. Sometimes they occur on several of the segments, but of slightly different sizes, as in a North American species (Chærocampa tersa), the red spots on which imitate the small red flowers of the plant on which it feeds; while in the European Deilephila hippophaes, the grey-green larva with orange-red spots so exactly assimilates to the foliage and fruit of the sea-buckthorn on which it feeds, that Dr. Weismann has often shown to people as many as six or eight of the large caterpillars on one buckthorn branch without their being able at once to detect them. In other cases we find very large eye-spots on the fourth or fifth segments only, coupled with the habit of retracting the head and first three segments, so that an appearance is produced of a broad head with two very conspicuous eyes. Whenever the insect is disturbed it thus retracts itself, and it has often been conjectured that the effect is to frighten away its enemies. This Dr. Weismann has proved to be actually the case. On placing the larva of the Elephant Hawk-Moth (Chærocampa Elpenor) in a trough used for feeding fowls, a number of sparrows and chaffinches flew down from the neighbouring trees to pick up some stray food.
On removing the caterpillar the birds again assembled, and at once entered the trough to feed. Fowls were also frightened at first, and would draw back just as they were going to peck at the caterpillar. At last, after several had tried, and even made ineffectual attempts to peck, one more courageous than the rest would actually touch it, and after a time, finding nothing disagreeable, the insect would be swallowed. In the genus Deilephila, however, there are uneatable caterpillars, and these have strongly contrasted black and white or yellow spots combined with the habit of fully exposing themselves upon their food plants. Dr. Weismann experimented with two species (D. galii and D. euphorbiæ) and found that they were refused by birds, though the latter was eaten by lizards. It is to be noted however that the experiments were made with a South European species of lizard, not that of Germany, so that the result has not a direct bearing on the point.
The general conclusion at which Dr. Weismann arrives is, that all the varieties of colour and marking occurring in the Sphingidæ have a distinct biological value, and can in every case be traced to the action of natural selection and correlation of growth.
The next essay is not quite so interesting or important. It is an endeavour to prove, by a distinct line of inquiry, that the markings of the larvæ are not due to a "phyletic vital force" or to general laws of growth and development. The different groups of Sphingidæ are minutely examined and compared in their three stages of larva, pupa, and imago, and it is shown that the changes that occur from species to species in each stage are to a great extent independent of the changes in the other stages. Numerous examples of this want of phyletic parallelism are given, and it is hence argued that the modifications which occur must be due to an adaptation to the special conditions to which the insect is exposed in one or other of its states, not to any innate law of variation and development, which, it is argued, would affect all the stages pari passu and produce a "phyletic parallelism" which does not actually exist. The same general facts are shown to prevail, not only among Lepidoptera generally, but among all insects and crustacea--or generally among all organisms which undergo a metamorphosis.
This instalment of the work has been admirably translated and edited by Mr. Meldola, who, in a series of valuable notes and an Appendix, has brought up the information on every branch of the inquiry to the latest date. The six coloured plates of larvæ in their several stages are very well executed, and serve to illustrate the somewhat complex discussion in a clear and effective manner.