After a brief review of larval polyphenism with regard to colouration and behaviour of E. ello, experiments bearing on the influence of environmental factors upon the determination of the various color forms are described. 2. Larvae growing up singly develop into the green, the brown, or, rarely, into the greengrey form. With higher population densities the proportion of the green and greengrey larvae decreases while the proportion of brown ones increases; in addition, a blue and a greenblue type appear. In still more crowded populations the blue form gradually replaces all others (Fig. 8) 3. The occurrence of brown larvae is crucially influenced by the food quality and obviously not by contacts among the larvae; on fresh Poinsettia shoots 20% of singly bred larvae turn brown; however, on nearly defoliated shoots their proportion rises to 70% (Fig. 3). The latter effect probably explains the increase of the brown form in moderately dense populations, where the foodplant is also heavily damaged by the larvae. On another foodplant, Euphorbia cotinifolia, as many as 90% of single caterpillars are of the brown type. 4. By contrast, the development of the blue and greenblue forms in very dense populations is largely determined by stimuli from the larvae themselves. The nature of these stimuli is still unknown. Mechanical irritation with a brush and withdrawal from or feeding to the larvae the juice they regurgitate when disturbed failed to change the color of singly raised individuals (Fig. 5). 5. The different color forms exhibit varying degrees of flexibility of change into one or the other form, including reversal of some of the developmental pathways. Only the brown color form, which also differs from the others in a peculiar resting behaviour, is irreversibly determined after reaching the fourth instar (Fig. 7).
