OCR Text |
Show 1887.J VALUE OP COLOUR AND .MARKINGS IN INSECTS. 211 of undue influence and thus throws out of adjustment the preexisting condition of comparative equilibrium. Another conclusion which is demonstrated very completely by the tables is that a Caterpillar may be eaten by one insect-eating Vertebrate although refused by another. I believe, however, that the acquisition of an unpleasant taste and of conspicuous colours appealed, at any rate at first, to a large number, probably all, the vertebrate foes ; for if this were not so, if the species became unpalatable and conspicuous to (say) half its enemies, and became conspicuous but remained palatable to the remainder, it seems only reasonable to conclude that immunity from the attacks of one set of foes would be counterbalanced, or perhaps more than counterbalanced, by the facilities afforded to the other set. On this account and for other reasons which will be given below, I think it probable that the differences observed between the enemies of insects in this respect are of recent date as compared with the acquisition of this mode of protection, and have arisen out of the great competition for food ; but in most instances the change of habit has not become so far confirmed that the previously distasteful food is eaten with avidity and pleasure. The first table of highly conspicuous larvae (including Crcesus and Nematus from the second table) can be shortly analyzed to show in it the various stages of transition from the most utter disregard to the opposite extreme of conduct, indistinguishable from that observed when the larvse are known to be relished. The intervening stages are furnished by the details given by the different observers, and are described in tbe headings of the vertical columns between those numbered I. & V. (see page 212). It must be remembered that these analyses represent a comparison between the results of experiments carried out under different systems and with the use of an incomplete number of Vertebrates in all cases. Hence many of the insects would doubtless have to be shifted into other columns after being offered to other Vertebrates, or to those actually employed, if it were certain that they were thoroughly hungry, Allowing for this, however, the analyses provide us with numerous instances of transition through all conditions of failure in the protective efficacy of the method we are discussing. At the same time one can see at a glance the relative behaviour of different insect-eaters as far as they have been tested in the case of each larva. Jenner Weir's suggestion that the hairs of certain larvae act as a warning of other unpleasant qualities can also be tested by the examination of the former tables. There are altogether fourteen larvse which may be called hairy, out of a total of twenty-seven (omitting the two terrifying species). Of these, two (L. rubi and P. fuliginosa) were eaten, as far as any observations are recorded ; one of the former and five others (L. rubi, L. quercus, L. pini, P. auriflua, A. caja, and O. potatoria) are either known to possess irritating hairs or are believed to possess them ; as many as five are gregarious (V. io, V. urtica, P. bueephala, E. lanestris, C. neustria), and this habit, together with the colour, is by far the most important factor |