The power of predicting what will happen in a given case is always considered to be a crucial test of a true theory, and if so, the theory of warning colours, and with it that of mimicry, must be held to be well established. Among the creatures which probably have warning colours as a sign of inedibility are, the brilliantly coloured nudibranchiate molluscs, those curious annelids the Nereis and the Aphrodite or sea-mouse, and many other marine animals. The brilliant colours of the scallops (Pecten) and some other bivalve shells are perhaps an indication of their hardness and consequent inedibility, as in the case of the hard beetles; and it is not improbable that some of the phosphorescent fishes and other marine organisms may, like the glow-worm, hold out their lamp as a warning to enemies.[115] In Queensland there is an exceedingly poisonous spider, whose bite will kill a dog, and cause severe illness with excruciating pain in man. It is black, with a bright vermilion patch on the middle of the body; and it is so well recognised by this conspicuous coloration that even the spider-hunting wasps avoid it.[116] Locusts and grasshoppers are generally of green protective tints, but there are many tropical species most gaudily decorated with red, blue, and black colours. On the same general grounds as those by which Mr. Belt predicted the inedibility of his conspicuous frog, we might safely predict the same for these insects; but we have fortunately a proof that they are so protected, since Mr. Charles Home states that one of the bright coloured Indian locusts was invariably rejected when offered to birds and lizards.[117] * * * * * The examples now given lead us to the conclusion that colours acquired for the purpose of serving as a danger-signal to enemies are very widespread in nature, and, with the corresponding colours of the species which mimic them, furnish us with a rational explanation of a considerable portion of the coloration of animals which is outside the limits of those colours that have been acquired for either protection or recognition. There remains, however, another set of colours, chiefly among the higher animals, which, being connected with some of the most interesting and most disputed questions in natural history, must be discussed in a separate chapter. FOOTNOTES: [Footnote 92: _Nature_, vol. iii. p. 165. Professor Meldola observed that specimens of Danais and Euplaea in collections were less subject to the attacks of mites _(Proc. Ent. Soc._, 1877, p. xii.); and this was corroborated by Mr. Jenner Weir. _Entomologist_, 1882, vol. xv. p. 160.] [Footnote 93: See Darwin's _Descent of Man_, p. 325.] [Footnote 94: _Transactions of the Entomological Society of London_, 1869, p. 21.] [Footnote 95: _Ibid._, p. 27.] [Footnote 96: _Nature_, vol. iii. p. 147.]