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Show 1887.] VALUE OF COLOUR AND MARKINGS IN INSECTS. 193 enemy, but palatable to another; and to this extent Wallace does point out a limit to the application of this principle of defence. But the counterbalancing limit which I suggested is of course entirely different, for I argued that a Vertebrate enemy may be forced by stress of hunger to eat an insect although unpalatable to it. Although the latter limit is thus quite distinct, it would certainly in time become identical with the former, as the distaste for the insect gradually disappeared after it had been repeatedly eaten. In fact it will be shown to be probable that many (if not all) of the instances in which an evidently distasteful insect is eaten by certain Vertebrates originally rose in this way. These suggested additions to Wallace's theory of protection by warning colours were capable of being put to the practical test. To achieve this object it was only necessary to ascertain whether an insect-eating Vertebrate could be induced by hunger to eat a gaily coloured and conspicuous larva which it was always known to refuse when other food was present, and which was evidently very much disliked on the few occasions of preliminary " tasting," which would always occur long before the time when the disgusting morsel would be reluctantly swallowed. I shall presently show that my suggestion was in every way confirmed by the test; but before giving an account of m y own experience I will allude to all the previous experiments which have been made in support of Wallace's theory. I. Brightly Coloured or Conspicuous Larva. At a meeting of the Entomological Society of London (see Proc. Ent. Soc. ser. 3, v. p. lxxx, 1867) Wallace made his important suggestion as to the biological value of conspicuous and gaudy colours in caterpillars. It is obvious tbat the question of the value of such colours in the larval stage is almost the same as in other stages, and it was chiefly from the determination of the use in the latter case (due originally and principally to Bates) that Wallace suggested that a similar solution would be found to apply to the former also. Nevertheless there are reasons why such a method of defence is especially applicable to tbe laival stage. I have shown that there is a special reason in the anatomical construction of larvae which explains why these organisms require to be defended from slight injuries (see Trans. Ent. Soc. Lond. 1885, pt. ii. Aug. pp. 321-323). A larva "may be described as a soft-walled cylindrical tube which owes its firmness, and indeed the maintenance of its shape, to the fact that it contains fluid under pressure. The pressure is exerted by the muscular parietes of the body. The advantage of this construction is as obvious as its danger; the larva possesses a motive force which can be applied to any movable part of the surface throuo-h the medium of'thefluid.". . . " Thiscon-truction isextremely dangerous ; for a slight wound entails great loss of blood, while a moderate injury must prove fatal. The larvae of Smerinthus ocellatus (and many others) nibble off each other's horns, and the wounded larvse (although they do not seem to be aware of the injury) |