Colouration in Animals and Plants

by Alfred Tylor

The most remarkable point is, however, the pattern on the abdomen, which, though varied in all possible ways, always preserves a general character, so that we might speak with propriety of a spider-back pattern. This pattern is fairly well illustrated in the genus Lycosa, but is seen to perfection, and in its simplest form in Segestria senoculata, [Plate VIII., Fig. 1], and in Sparassus smaragdulus, [Plate VIII., Fig. 2].

This peculiar pattern is so like the dorsal-vessel that lies just beneath, that it is difficult to avoid the conclusion that we have here an actual case of the influence of internal organs on the integument, and this we believe to be the case. No matter how curious the abdominal markings may seem to be, they never so far depart from this fundamental pattern as to appear independent of it.

Thus, in the genus Lycosa, which is by no means the best for the purpose, but is chosen as illustrating Gegenbaur's diagram, [Plate VIII.], we have the dorsal-vessel well marked in L. piscatoria, [Plate VIII., Fig. 3], from which may be developed the other forms. In L. andrenivora, [Plate VIII., Fig. 4], the male shows the vessel-mark attenuated posteriorly; and in the female, [Fig. 5], the hinder part has become broken up into detached marks, still preserving the original shape, while the upper part remains practically unchanged. In L. allodroma the disintegration of the mark has further advanced, for in the male, [Fig. 6], the upper portion has lost something of its shape, and the lower part is a series of isolated segments. This process is carried still further in the female, [Fig. 8], where the upper portion is simplified, and the lower almost gone. In L. campestris, [Fig. 10], the mark is reduced to a stripe, corresponding with the upper part of the vessel-mark only: and, lastly, in the male L. agretyca, [Fig. 7], this upper part is represented by two spots, though even here traces of the original form can be seen.

A simplification of marking of another sort is seen in L. rapax, [Fig. 13], where the chamber-markings are almost obliterated, and merely an irregular stripe left. The stages by which this modification is arrived at are too obvious to need illustration.

In some species the lower portion of the vessel-mark is reduced to small dots, as in L. cambrica, fluviatilis, piratica, and others; and the stages are very clear. Starting with the isolated chamber-marks, as in L. allodroma, [Fig. 5], we get, firstly, a set of spots, as in L. picta, which, in the female, [Fig. 16], are still connected with the chamber-marks, but in the male, [Fig. 17], are isolated. This leads us, by easy steps, to such forms as L. latitans, [Fig. 14], which consists of a double row of spots upon dark stripes.

The intimate connection thus shown to subsist between the characteristic decoration of the abdomen of spiders, and the shape of the important dorsal organ beneath, seems to be strong evidence of effect that internal structure may have upon external decoration. [37]

The cephalothorax of spiders, being covered with a hardened membrane, does not show such evidence clearly, for it appears to be a law that the harder the covering tissue, the less does it reflect, as it were, the internal organs. The hard plates of the armadillo are thus in strong contrast to the softer skins of other animals.

Nevertheless, there does appear, occasionally, to be some trace of this kind of decoration in the cephalothorax of certain spiders, though it would be hard to prove. The blood vessels of this part (see [Fig. 9]), though large, are not nearly so prominent as the great dorsal vessel. The chief artery enters the cephalothorax as a straight tube, forks, and sends branches to the limbs, palpi, and eyes. In many species, notably in the genus Thomisus, a furcate mark seems to shadow the forked aorta. This is best shown in T. luctuosus, [Plate VIII., Fig. 11]. Moreover, in this and other genera, lines frequently run to the outer pair of eyes, which alone are supplied with large arteries, see [Fig. 9].

However this may be, it is certain that the entire decoration of spiders follows structural lines, and that the great dorsal vessel has been emphasized by the peculiar pattern of the abdomen.