The colours, odours, and sounds of animals, like the colours and odours of plants—so far as they may be assumed to be in some way connected with the reproductive functions—are, as a rule, complementary to each other. Stridulating insects are generally not conspicuously coloured. Among the Homoptera, there do not seem to be any well-marked cases of ornamental differences between the sexes. Among crickets, the Locustidæ, and grasshoppers, some species are beautifully coloured; but Mr. Darwin says, “It is not probable that they owe their bright tints to sexual selection. Conspicuous colours may be of use to these insects by giving notice that they are unpalatable.” Other species have directly protective colours. The bright hues of stridulating beetles seem to be of use chiefly for protective and warning purposes; whereas species belonging to the orders Neuroptera and Lepidoptera, often extremely conspicuously coloured, are not remarkable for any stridulous sounds. Frogs and toads, which have an interesting sexual character in the musical powers possessed by the males, are evidently coloured according to the principle of protection, or sometimes tinted with conspicuous hues in order to be more easily recognized by their enemies as a nauseous food. Of Reptiles, the Lacertilia excel mainly in bright tints; the Chelonia, Crocodilia, and Ophidia, in sounds and odours. Among Birds, in one instance at least, the male is remarkable for his scent. “During the pairing and breeding season,” says Mr. Gould, with reference to the Australian musk-duck, “ ... this bird emits a strong musky odour;” it is not ornamented with any conspicuous hues.[1502] Sexual colours and the power of song are generally complementary to each other among Birds. “As a general rule,” Mr. Wood remarks, “it is found that the most brilliant songsters among the birds are attired in the plainest garb; and it may safely be predicted of any peculiarly gorgeous bird, that power, quality and sweetness of voice are in inverse ratio to its beauty of plumage.”[1503] Thus, of the British birds, with the exception of the bullfinch and goldfinch, the best songsters are plain-coloured, and the brilliant birds of the tropics are hardly ever songsters. The wild camel in the desert of Kum-tagh has a reddish, sandy hue, and the males, “even during the rutting season, utter no sound, but find their consorts by scent.”[1504] The musk-deer, well known for the intolerable perfume which the males emit at the pairing time, is also entirely silent.[1505]