The influence of water and light upon the pigmentary system in the common frog Rana temporaria.

Many factors, both internal and external, influence the chromatic changes of coldblooded vertebrates, and of these, three external variables are recognized as significant—light, heat and moisture. The frog can change its colour, from brown or black through intermediate tints to pale lemon, by changing the contraction and expansion of three types of chromatophores—the epidermal and dermal melanophores and the xantholeucophores. The activity of the dermal melanophores, however, dominates these macroscopic changes. It is already established that an illuminated light background, dryness and moderate temperatures, favour the pallor associated with the contraction of the dermal melanophores, whilst an illuminated dark background, moisture and low temperatures induce their expansion giving darkening. The influence of light has received most attention, ' indeed almost the whole of the available data based on refined measurements of the behaviour of the dermal melanophores refers to the influence of light' (Hogben, 1942). Our knowledge of the effect of moisture is based more upon general observations than critical experiments, and still less is known of the combined influence of two or more factors. Hogben (1924) concludes that one of the most important factors involved in colour response of adult Anura is moisture—humidity promoting darkening. However, the relative importance of these external agents appears to vary from group to group and between different species of the same groups. Thus Parker & Scatterty (1937) see no reason to assume that with the spotted frog, Rana pipiens, either differences in temperature or humidity (jars dry—jars 10 cm. of water) played any part whatsoever in the final responses. More precise information therefore is needed about the effect of moisture upon the dermal melanophores, both as an independent agent and in relation to other factors. MATERIALS AND METHOD