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Experimental Studies of the Biological Significance of Non-Cryptic Pigmentation with Special Reference to Insects.

Authors

  • E. J. Popham B.Sc., Ph.D., A.R.C.S., F.Z.S.


Summary.

(1) Under the experimental conditions employed, nymphs of Aeschna grandis, adults of Dytiscus marginalis, Common Rudd (Leuciscus erythrophthalmus), Minnows (Phoxinus phoxinus) and the Common Toad (Bufo bufo bufo) were able to distinguish between two types of prey, one brown, the other red, having approximately equal grey values. Owing to the ability of insects to perceive ultra-violet light the experiments with insects predators are of doubtful value as this factor was not controlled. In comparison with the colour vision of man these predators have a very poor sense of colour.

(2) Minnows were fed on three types of Corixa distincta coloured respectively red, yellow and brown and all having approximately the same tonal value. The survival value of each of the three colours of prey depended upon the colour of the background upon which it was placed. In each case the insect with a colour harmonizing with the background was destroyed in the smallest numbers.

(3) When the Minnows were given two types of Corixa distincta painted different tones of red, the type having the greater tonal difference with the background was destroyed in the larger numbers.

(4) Specimens of Corixa distincta painted with three transverse bands of yellow were less conspicuous than evenly coloured yellow insects on a brown background to Minnows, but more conspicuous to Toads.

(5) Some vertebrate and invertebrate predators were offered specimens of insects belonging to the orders Orthoptera, Hemiptera, Coleoptera and Hymenoptera and Diptera. Toads learned to avoid various species of Hymenoptera with stings and in some cases Lady-birds of the species Coccinella 7-punctata. Nymphs of Aeschna grandis learned to avoid Water Mites of the genus Hydra-rachna.

(6) Starlings, Great and Blue Tits were observed to cat various Hymenoptera in such a way as to avoid being stung.

(7) The rate at which Toads learned to avoid a given species of Hymenoptera or Lady-bird depended upon (a) the individual toad, (b) the nature and quantity of other types of food.

(8) Toads trained to avoid any one species of Hymenoptera would accept unrelated species with similar coloration and refuse related species with a different pigmentation. This suggests that to a partially colour-blind predator, such as the Toad, the pigmentation of the prey is of little importance in enabling a predator to recognize it and that factors such as size, shape, habits and movement of the prey are of greater value.

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