84 Colouration in Animals and Plants. 



Fig. 13, where the chamber-markings are almost obliterated, and 

 merely an irregular stripe left. The stages by which this modifi- 

 cation 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, Jluviatilis, 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.* 



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. 



* The decoration of many of the Hoverer flies and wasps is of a similar character. 



