i893. COLOUR CHANGES IN INSECTS. 289 



light reflected from them. The suppression of the superficial dark 

 pigment thus leads to the green colour of the caterpillar. 



From these experiments, the fact that geometrid larvae have a 

 very considerable power of colour-adaptation to their environment 

 appears to be beyond dispute. There seems no reason to doubt that 

 this power is of considerable protective value to the insects, especially 

 when we consider that the larvae in which it is developed are all of 

 a form closely resembling the twigs of their food-plants, but that 

 they need some power of colour-adaptation to render the resemblance 

 perfect. Poulton lays stress on the fact that all the numerous 

 varieties of A. hctulavia produced by the experiments would be in 

 correspondence with some possible natural environment. The ex- 

 periment of moving caterpillars from one set of conditions to another 

 showed that the early larval stages only are susceptible to any great 

 extent; after the last moult but one, little or no change was produced. 

 Hence, the most sensitive caterpillar has not that power of changing 

 and rechanging its hues which is possessed by many fishes and 

 reptiles ; but then, in natural conditions, the environment of cater- 

 pillars will very rarely be changed. 



In " The Colours of Animals," Poulton gave several instances 

 of variation in, the colour of the cocoons spun by caterpillars before 

 pupation, which appeared to correspond to changes in the environ- 

 ment. The cocoons of Satuvnia pavonia [cavpini), Eriogaster lanestris, 

 and H alias pvasinana were found to be brown among leaves, and white in 

 paper bags. Bateson (3, 4) has, however, now shown that the white 

 cocoons of the two former species are not due to the colour of the 

 surroundings, but to the disturbance of the larvae before spinning. 

 Caterpillars which were removed from their food, when ready to spin, 

 produced white cocoons whether placed in dark or light bags, while 

 when white strips of paper were placed about the food, without 

 disturbing the larvae, they spun dark cocoons. Bateson has further 

 established that the dark hue of the cocoons is due to a brown 

 fluid contained in the alimentary canal, with which the caterpillar 

 colours the silk as it is discharged from the mouth, perhaps, also, 

 staining the finished cocoon by an ejection from the intestine. 

 When a caterpillar is disturbed, this fluid is voided before spin- 

 ning, and hence the cocoon is white. Tutt has discovered (5) a 

 similar cause for the varying colours of the cocoons of Halias cJi/orana, 

 which, however, do vary in hue with their surroundings to some 

 extent. Poulton has now made further experiments on H. prasinana, 

 and, carefully avoiding disturbing the larvae, finds that the cocoons of 

 this species do vary in accord with the colour of the surroundings. 

 The caterpillars seem to exercise choice in the colour of the silk they 

 produce, and to be less irritable than those of Satuvnia pavonia and 

 Eyiogaster lanestris. A larva which had begun to spin a white cocoon 

 in a chip box was removed, an attempt was made to cut out the 

 egg of an ichneumon, and the caterpillar was injured thereby. 



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