Control of Flashing in Fireflies. Iii. Peripheral Excitation

Recordings of flashes from minute, widely separated regions of the lantern indicate that peripheral excitation is rather uniform. 2. Evidence is presented that peripheral conduction is unlikely to account for more than a small proportion of overall response latency. 3. By stimulating lantern tissue directly with high intensity shocks it is possible to evoke a much earlier response instead of or in addition to the normal flash. For example, in Photuris this "quick flash" occurs about 18 msec. after stimulation as compared with about 70 msec. for the normal or slow flash (at 25°). 4. The quick flash resembles the slow in contour, in apparently being produced by the same tissue, in showing a strength-duration effect for threshold and in showing facilitation, summation, adaptation, fatigue and other typical neuroeffector responses. The quick flash differs from the slow in its shorter latency, higher threshold, higher frequency compliance, and in having a relatively temperature-insensitive response latency. 5. Nerve section experiments show that the slow flash depends on nerves, the quick flash does not. 6. In the small larval light organ and in small pieces of adult lantern, very intense stimulation may induce a flash with a latency of the order of 1 msec. Control experiments with wet filter paper indicate that this "quick quick" flash is not entirely an artifact. 7. It is hypothetically proposed that stimulation involves a three-element neuroeffector (nerve terminal, end organ, photocyte) and that increasing strengths of stimulus can by-pass in stepwise fashion certain links in this chain of peripheral excitation. According to this idea the ultrashort ("quick quick") response would represent direct excitation of the photocyte, the quick flash latency would represent delay necessary for excitation of the photocyte by the tracheal end organ, and the difference between slow flash and quick flash latencies would be time occupied in activation of the end organ complex plus a small delay for peripheral nervous conduction. 8. In lanterns subjected to long-continued intense stimulation the flash eventually disappears and is replaced by a steady glow. Under some circumstances intense shocks produce striking momentary double quenchings of this glow, kinetically similar to a flash. Peripheral inhibition is suggested as a possible mechanism.