100 THE EVOLUTION THEORY 



Euploeidse in the tropical regions of the Old World enjoy a certain 

 immunity on account of their repulsive odour and taste. Here, too, 

 observation and experiment have shown that birds, lizards, and 

 predaceous insects leave the butterflies of these families unmolested. 

 I need only mention the observation of Trimen that, under an acacia 

 much visited by butterflies, on which Mantides — the so-called 

 praying-insects — caught and devoured large numbers, the wings of an 

 Acroea or a Danais were never found. These unpalatable butter- 

 flies also possess a motley or at least striking dress, recognizable from 

 afar, and alike on both surfaces ; and they also have a slow flight, by 

 which they are readily recognized. They, too, usually assemble in 

 large swarms, and both sexes are alike, or resemble each other 

 closely in colouring, or at least they are both equally conspicuous. 

 But even these cases do not complete the list of butterflies which are 

 protected by their unpalatability ; among the otherwise much- 

 persecuted and therefore palatable Pierida3 (Whites) there is an 

 Asiatic genus, Delias, which in all probability belongs to the immune 

 butterflies, as their gaily coloured under surface indicates, and among 

 the nocturnal Lepidoptera of different countries and families there are 

 isolated generations which are very gaily and conspicuously coloured, 

 and which are rejected by birds, their unpleasant odour being 

 perceptible at a distance of several feet (Chalcosiidse and Eusemiidge). 

 The latter no longer fly under cover of night, like their relatives, but 

 have assumed diurnal habits. 



It is to be supposed that the repulsiveness of such ' unpalatable ' 

 butterflies is associated with the food-plant on which the caterpillar 

 lives. Acrid, nauseous, astringent, and actually poisonous substances 

 are produced in many plants, and we shall see later that this is to their 

 own advantage ; these substances pass into the insect, and they do so 

 probably in part unaltered, in part certainly altered, but still they are 

 protective, perhaps even in an increased degree. This is borne out by 

 the fact that many caterpillars of immune butterflies live on more or 

 less poisonous plants : the Acr«3ida3 and Heliconiidas on Passiflores, 

 which contain nauseous substances ; the Danaida3 on the poisonous 

 Asclepiadae, which are rich in milky juice or latex; the Euploea3 on 

 the poisonous species of Ficus, the Neotropinae on the Solanaceas, and 

 so on. But there are many genera, rich in species, and distributed 

 over the whole earth, the caterpillars of which live on plants of very 

 various families and characters, and of these the majority of species 

 are palatable, though a few are repulsive in taste and odour, and 

 therefore immune. This is the case in the genus Papilio. As far 

 back as the sixties Wallace discovered that there were immune 



