come to differ in colour from the males for the sake of protection. _Mutilla Europæa_ emits a stridulating noise; and according to Goureau[483] both sexes have this power. He attributes the sound to the friction of the third and preceding abdominal segments; and I find that these surfaces are marked with very fine concentric ridges, but so is the projecting thoracic collar, on which the head articulates; and this collar, when scratched with the point of a needle, emits the proper sound. It is rather surprising that both sexes should have the power of stridulating, as the male is winged and the female wingless. It is notorious that Bees express certain emotions, as of anger, by the tone of their humming, as do some dipterous insects; but I have not referred to these sounds, as they are not known to be in any way connected with the act of courtship. Order, _Coleoptera_ (Beetles).—Many beetles are coloured so as to resemble the surfaces which they habitually frequent. Other species are ornamented with gorgeous metallic tints,—for instance, many Carabidæ, which live on the ground and have the power of defending themselves by an intensely acrid secretion,—the splendid diamond-beetles which are protected by an extremely hard covering,—many species of Chrysomela, such as _C. cerealis_, a large species beautifully striped with various colours, and in Britain confined to the bare summit of Snowdon,—and a host of other species. These splendid colours, which are often arranged in stripes, spots, crosses and other elegant patterns, can hardly be beneficial, as a protection, except in the case of some flower-feeding species; and we cannot believe that they are purposeless. Hence the suspicion arises, that they serve as a sexual attraction; but we have no evidence on this head, for the sexes rarely differ in colour. Blind beetles, which cannot of course behold each other’s beauty, never exhibit, as I hear from Mr. Waterhouse, jun., bright colours, though they often have polished coats: but the explanation of their obscurity may be that blind insects inhabit caves and other obscure stations.