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Show 204 MR. E. B. POULTON ON THE PROTECTIVE [Mar. 1, true of gregarious larvae. Thus a group of phytophagous Hymeno-pterous larvas may remain inconspicuous while uniisturbed, but nevertheless the approach of an enemy determines united movements in the colony which render the whole strikingly conspicuous, and which may be attended later by the emission of an offensive smell from the numerous ventral glands of all the individuals simultaneously (e. g. Croesus septentrionalis). In the other larvae which suddenly assume a terrifying attitude " the effects produced approximate somewhat to an intensely exaggerated caricature of a sort of generalized vertebrate appearance, probably of the serpent type (at any rate in Charocampa), such as would be most efficacious in the case of birds. It is likely that the terrifying appearance of our own larvae in temperate latitudes first arose in the tropics, where the imitated cause of alarm to the enemies of the larva is real and obvious. And it is probable that the success of the same method in countries where the reptilian fauna cannot be said to constitute a source of alarm is due to inherited memories of a tropical life which live on, as that instinctive fear of anything snake-like which is so commonly exhibited by the higher land-vertebrates including ourselves." (Poulton, Trans. Ent. Soc. Lond. 1886, pt. ii. June, pp. 156, 157). The success of this combination of defensive measures depends on the extraordinary sensitiveness of the larvae, so that the transition from the one method to the other is instantaneous, and in the case of the suddenly assumed terrifying attitudes, the enemy is additionally alarmed by tbe way in which some dreaded object seems, as it were, to spring into existence. It is very unfortunate that so few experiments have been made upon this most interesting group of larvae. Just as it was suggested that insect-eating Vertebrates might, under the influence of hunger, be induced to eat and finally to relish distasteful larvse, so we must expect that the same cause would in the end prevent this elaborate system of intimidation from being successful. In this case, however, there is no prejudice against an unpleasant taste or smell to be overcome, and it is most probable that the larvse would be in great danger as soon as the imposition was detected. It is perhaps on this account that these methods are adopted by an exceedingly small proportion of larvae, but also because a certain size is necessary for any cbance of success. Nevertheless this size is less than might be anticipated, for the anterior part of the body with large eye-like marks is generally swollen out into a resemblance to the head of a serpent, while the larval body is partially concealed among the leaves of the food-plant, and, in many positions, what is seen merely serves to suggest a far more extended length than that which actually exists. Wallace has suggested that it is very probable that the " spectacles " of the Cobra are terrifying marks, which warn the enemy against approach, and it is most interesting to note that the Charocampa-l&rvse mimic the terrifying eye-like marks of a Cobralike serpent, and not the real eyes of a serpent, which are relatively small. (Table II., pp. 206, 207). Having thus tabulated the results of experiments upon undoubtedly |
