A Change of Phototaxis and a Tropotactic Response by Larvæ of the Mustard Beetle

Abstract

De Ruiter and van der Horn¹ have reported on changes in phototaxis during the larval life of Smerinthus ocellata L. and mentioned the problem of functional significance. Preliminary work by me on the olfactory responses of the larvæ of the mustard beetle, Phaedon cochleariae Fab., were hampered for a time by complete reversals of results obtained in earlier replicate olfactometer tests. The cause proved to be a response to light, and experiments similar to those of de Ruiter and van der Horn gave results comparable with those obtained by these authors, with larvæ generally positively phototactic at the beginning of an instar and negatively phototactic at the end. Observations of larval behaviour in the rearing cages showed that larvæ about to moult moved to the darker areas of the food-plant, mainly the underside of the leaf, so that the negative phototaxis shown by the larvæ is probably a response which gives protection during moulting.