OCR Text |
Show 1887.] VALUE OF COLOUR AND MARKINGS IN INSECTS. 263 its imitative resemblance complete by entire quiescence, and it is usually effectually protected in otber ways ; but tbe larva must feed, and at tbe same time is sluggish in its movements, defenceless, and when palatable is more relished than any other stage, for it does not possess the hard investment of tbe one or tbe scaly covering of the other. It has also been seen that an unpleasant taste may arise incidentally at this period. Assuming, then, that tbe great needs of certain larvee have been met in this way, there will be the tendency for the unpleasant quality to pass on by simple continuity into the other stages; and if these are hard pressed, there is always the possibility that such attributes may be made the starting-point of a similar method of defence for them also. Hence I believe we shall nearly always find that conspicuous unpalatable imagos develop from larvse which are also unpalatable and conspicuous, and such a conclusion is entirely borne out by the table. But the unpleasant quality may pass on in the same way into other stages, which hold their own successfully by elaborate and perfect protective resemblances, and then there wdl be no tendency for the quality to be made use of, although it will always remain as a possibility should the species be worsted by its enemies in these stages. It must be remembered that the possession of an unpleasant taste by a protectively coloured species can never be injurious in any way to itself except in so far as it causes tbe destruction of a greater amount of insect-life, inasmuch as the part contributed by the species itself to the total destroved does not count as food under ordinary circumstances. And the species itself remaining on the same protective lines as the great mass of palatable species, it will itself come in for a proportional share of the extra loss which follows from the fact that it is not relished as food. But so long as these unpalatable species remain in a small minority, the reaction of their own inedibility upon themselves will be inappreciable. Mr. W . Esson has kindly expressed the danger actually incurred in a mathematical form, showing that it is inappreciable when the inedible species are relatively few. If there were a practically unlimited number of protectively coloured insects consisting of two sets of species, the one set edible and the other inedible and consisting of individuals in the ratio of 100 : 1, it is reasonable to suppose that in any number n of captures there will be killed of each set a number of individuals proportional to the numbers in the sets themselves; i. e. of the edible -^ and of the inedible -,. The insect-eaters will go on catching the insects until the edible ^ l becomes equal to the number required for their food- a. Therefore ^ = a and n = ^ ; therefore there are caught of the inedible species ^ that is A-. Considering the above-mentioned exceptions among the imagos rather more in detail, it would certainly be difficult to find any species with an appearance more completely the opposite of that produced by the typical warning coloration than the imagos of P. bueephala and 0. antiqua. The special character of the imitative resemblance |