128 



KNOWLEDGE. 



[June 1, 1895. 



THE COLOURS OF BUTTERFLIES. 



By C. P. Maeshall, M.D., B.Sc, F.E.C.S. 



M 



OST people are familiar -with the beautiful and 

 varied colours met with among butterflies ; but 

 the reasons why such colours exist are, perhaps, 

 not known to all readers of Knowledue, even in 

 these post-Darwinian days. There are still, no 

 doubt, persons who believe that the colours of insects, and 

 other beautiful colours met with in nature, were intended 

 for the special benefit of man, to please the eye of the 

 " lord of creation." But any siich idea is at once rendered 

 absurd when we reflect that the most gorgeously coloured 

 butterflies and birds are found in parts of the world where 

 man does not dwell, and where he seldom ventures, viz., 

 the tropical forests of South America and the islands of 

 the Malay Archipelago. 



The explanation of the colours of animals and plants is 

 due chiefly to the investigations of Alfred Kussel Wallace, 

 and the results show that colour is uo accidental quahty, 

 but that in several different ways it benefits its possessor in 

 the struggle for existence. In the case of flowers, it has 

 been shown that the colours and scent serve to attract 

 insects to visit and fertilize them. Therefore, if insects can 

 appreciate the colours of the various flowers, they must be 

 able to distinguish colours in the members of their own 

 and other species. 



Let us now consider briefly the chief groups into which 

 the colours of butterflies are divided, and the purpose 

 served by each different kind of colouring. 



1. Protective Cohntrs. — These enable the butterfly to 

 escape fi-om its enemies, owing to the colouring bearing 

 a strong resemblance to some other object, thus aiding 

 concealment. The resemblance may be to another butterfly, 

 or to a flower or leaf. Protective colouring is usually 

 confined to the under surface of the wings, which is coloured 

 more or less like the leaves or flowers on which the butter- 

 fly usually perches. When, as is their habit after ahghting 

 on their usual plants, the wings are folded up exposing the 

 under surfaces only, the protective is often very striking. 

 In some instances the resemblance is so strong as to 

 deceive the most practised eye. One of the best known 

 instances is that of Kalluna, a butterfly met with in the 

 Malay Archipelago. In this the upper surfaces of the 

 wings are brightly coloured with orange and purple, but 

 the under surfaces bear the most remarkable resemblance 

 to the dead leaves of the plant on which it has the habit 

 of alighting in its flight. Furthermore, in difl'erent 

 specimens, resemblances may be found to dead leaves in 

 almost every stage of decay. In other cases the resemblance 

 involves the whole insect. Phi/lliwn, the "leaf insect," 

 resembles a group of leaves ; here the joints of the limbs 

 are flattened out and coloured hke leaves, in addition to 

 the wings. Loiichodes, the " stick insect," resembles a 

 group of di'ied twigs. 



Protective colours are not confined to the adult insect, 

 but the larvie are also often protectively coloured. The 

 larvcc, or caterpillars of butterflies being soft-bodied, 

 defenceless creatures, are in need of protection, and hence 

 we find in them protective colouring. The usual colour of 

 caterpillars is green, resemblmg the colour of the leaves on 

 which they feed. Caterpillars which have the habit of 

 feeding on grass are protected by longitudinal stripes 

 which facihtate concealment. At the time when the larvse 

 are about to change into pupie, or the chrysalis stage, they 

 usually turn a brown colour to resemble the earth on to 

 which they descend. 



The common "stick caterpillar," found on ivy and other 



plants, is a good example of what is called " special 

 protective resemblance." Anyone who has searched for 

 these caterpillars will have experienced the difliculty there 

 is in distinguishing them from the twigs of the plant on 

 which they feed. 



The explanation of protective colouring is offered by 

 the theory of natural selection. No two butterflies are 

 exactly alike, any more than any other two animals are 

 exactly alike. The butterflies resemble other insects in 

 being the food of birds, lizards, and other animals. Now, 

 any butterfly whose markings and colour render it more like 

 the leaves or other objects on which it perches, will have 

 a better chance of being concealed, and so of escaping the 

 enemies which prey on it. Such butterflies will, therefore, 

 sur\dve and transmit their peculiarities to their offspring. 

 In these, again, some will have colours and markings of a 

 greater protective value than their parents ; these will 

 survive, and in their turn will transmit their advantages to 

 their descendants, and so on, generation after generation, 

 the xwotective colouring becoming more and more marked 

 till the extraordinary resemblances we have mentioned are 

 reached. 



2. Warnhui Colours. — These form a remarkable group 

 of instances in which the colours are conspicuous, for the 

 purpose of warning other insects to keep away. Warning 

 colouring arises from the fact that if an animal, liable to 

 be eaten by others, has a nauseous taste, it is advantageous 

 that it should be quickly recognized, and hence avoided by 

 the animals which would otherwise eat it as food. Some 

 of the best known instances of warning colours are found 

 in butterflies, especially some of those inhabiting the 

 tropical regions. They are brightly coloured on both 

 surfaces of the wings, and the colours are usually the same 

 on both upper and under surfaces of the wings. They 

 all excrete juices of a powerful odour which are offensive 

 to insect-eating animals. 



Warning colours are also frequently found among 

 caterpillars, such as those of the common magpie moth, 

 the cinnabar moth, and the tiger moth. These are all 

 brightly coloured and take no pains to conceal themselves, 

 and if offered to lizards, frogs, and other animals, will be 

 rejected. 



The origin of warning colours is also explained by 

 natural selection, but on a different line to the explanation 

 of the protective colours. In this case those accidentally 

 possessing a nauseous taste will have a better chance of 

 surviving in the struggle for existence. The peculiarity 

 will be transmitted and increased in intensity from gene- 

 ration to generation in the same way as the protective 

 resemblances are increased. In the case of the warning 

 colours, the insect having the most nauseous taste and 

 most gaudy colouring will survive. 



3. Sexual Colours. — These are found when the two sexes 

 differ considerably in colour. As a rule, the male is of the 

 same hue as the female, but of a deeper and stronger 

 colour. In other cases, as in the common orange tip 

 butterfly, patches of colour are found in the male but not 

 in the female. Darwin explained the more brilliant colours 

 of the male as being due to what he termed " sexual 

 selection" — i.e., to the female having preference for a more 

 brightly coloured male, thus giving rise by selection to the 

 development and survival of the more brilliantly coloured 

 males. On the other hand Wallace explained the more 

 sober colours of the female as being protective in order to 

 escape detection while laying eggs — a period during which 

 there is more exposure to attacks from enemies, and so 

 more need for protection. 



4. Mimicry. — Many butterflies escape destruction by 

 mimicking the colours and markings of the uneatable forms. 



