Did I here at once proceed to establish the biological value of larval markings--and especially of those of the _Sphingidæ_--so as to arrive in this manner at a conclusion as to their dependence upon natural selection, it would be impossible to avoid the consideration of the total coloration of the caterpillars, since the marking frequently consists only of a local strengthening of the colour, and cannot be comprehended without coming to this understanding. The action of the markings also often appears to be opposed to that of the colouring, making the caterpillar again conspicuous; so that the two factors must necessarily be considered together. I shall therefore commence the investigation with colour in general, and then proceed to treat of marking. IV. BIOLOGICAL VALUE OF COLOUR. The general prevalence of protective colouring among caterpillars has already been so frequently treated of that it is not here my intention to recall particular instances. In order to judge of the effect of marking, however, it will be well to bear in mind that these insects, being generally defenceless and thus requiring protection, have acquired the most diverse means of rendering themselves in some measure secure from their foes. The sharp spines which occur on the caterpillars of many butterflies (_Vanessa_, _Melitæa_, _Argynnis_), and the hairs on those of many moths, serve for protective purposes. Among other means of protection--although in a different sense--we have in all the species of the great family of the _Papilionidæ_ the strikingly coloured (yellowish red) odour-emitting tentacles concealed near the head, and suddenly protruded for terrifying foes; and likewise the forked horn at the tail of the caterpillars of the genus of moths _Harpyia_, the tentacles of which can be suddenly protruded in a similar manner. Adaptive colours and forms combined with certain habits[129] are, however, much more common than defensive weapons. Thus, the caterpillars of the _Noctuæ_ belonging to the genus _Catocala_ and its allies, feed only at night on the green leaves of various forest-trees; by day they rest in crevices of the bark on the tree trunk, which they resemble so perfectly in the colour of their peculiar glossy dull grey or brownish skin beset with small humps, that only sharp eyes can detect them, even when we are familiar with their habits.[130] The striking resemblance of many moths to splinters of wood is well known, and to this is added a habit which helps their disguise, viz., that of remaining stiff and motionless on the approach of danger, just like a splinter projecting from the branch.[131] Among the moths coming under this category may be mentioned _Cucullia Verbasci_, and particularly those of the genus _Xylina_, which, when at rest, closely resemble a broken splinter of wood in the colour and marking of their fore wings, and when touched, have a habit of drawing in their legs and falling without opening their wings as though dead.