Dark-adapted third-stage Trichonema larvae were orthokinetically stimulated by light. The response soon reached a maximum and then gradually decreased, the larvae then being inactive as long as they were kept in light. The rate of response was independent of intensity, and was a constant for any one age. Inactive larvae were activated by a mechanical stimulus, and were therefore only insensitive to light.

The larvae needed 3 h adaptation in darkness before responding fully to a further light stimulus. Dark-adaptation required continuous darkness; larvae exposed to a flashing light responded as if in continuous light. The presence of a light-sensitive mechanism is postulated.

I thank Professor B. G. Peters for discussion of this work, and for reading the manuscript.