Recently, the light reactions of three species of tent caterpillars were described (3). The present paper contains a similar description of the responses of larvae of the spotless fall webworm, Hyphantria textor Harr. This species was chosen for comparative study because, like tent caterpillars, it is colonial through most of its larval stage, during which it forms extensive webs.
Larvae of all six instars were studied. Some of the behaviour described here was first observed during the summers of 1949-50 at Sault Ste. Marie, Ont. During July and August of 1953, several of the laboratory tests were repeated at Sault Ste. Marie, and, in addition, some new reactions were observed for the first time.