[CHAPTER XII.]
Arachnida.
T
THE Arachnida include the scorpions and spiders, and as the former are tolerably uniform in colour, our remarks will be confined to the latter.
The thorax is covered with a horny plate, while the abdomen only possesses a soft skin, and neither show any traces of segmentation. From the thorax spring four pairs of legs, and a pair of palpi, or feelers. Immediately beneath the skin of the abdomen lies the great dorsal vessel, which serves as a heart. This vessel is divided into three chambers, the general aspect of which is shown in [Fig. 9, Plate VIII.], taken from Gegenbaur's Comparative Anatomy.[35]
From this heart the blood passes by vessels to each of the limbs, the palpi, etc., as offsets from the double-branched aorta. The shape of this dorsal vessel is peculiar, and its importance in respect to colouration will be immediately apparent.
The primary scheme of colouration in the Arachnida seems to be the distinguishing of the cephalothorax from the abdomen by a different colour. Thus, of the 272 species of British spiders represented in Blackwell's work,[36] no less than 203 have these parts differently coloured, and only 69 are of the same hue, and even in these there is often a difference of tint. So marked is this in certain cases that the two parts form vivid contrasts. Of this cases are given in the following list.
| Cephalothorax. | Abdomen. | |
| Eresus cinnabarinus, | Black, | Bright Red. |
| Thomisus floricolens, | Green, | Brown. |
| —— cinereus, | Brown, | Blue. |
| —— trux, | Red, | Brown. |
| Sparassus smaragdulus, | Green, | Red and yellow. |
As a rule the abdomen is darker than the cephalothorax, and many species have the former red-brown and the latter black.
The legs, usually, take the colour of the cephalothorax, and are, hence, generally lighter than the abdomen, but to this there are exceptions. Where the individual legs differ in colour, the two first pairs are the darkest, and the dark hue corresponds in tint with the dark markings on the cephalothorax. The joints of the legs are in many species emphasized with dark colour, which is often repeated in bands along the limb.