The communication systems of fireflies are especially instructive because both the sending and the receiving use a signal modality almost unique among animals: flashes of visible light; because they are usually coded in readily quantitated parameters; and because much of the associated behavior can be referred directly to physiological events in the nervous system. During several years we studied flash control in species of Oriental firefly that have the peculiarity of flashing synchronously in large congregations and in which the synchronization occurs because the photic response has a latency approximating the free flashing cycle i.e., stimulation at any phase of the cycle sets the pacemaker back to zero time. In recent years we have investigated two American species of firefly which synchronize on a much more limited scale and which the entrainment mechanism has seemed to depend on direct reflex evocation of premature flashing (i.e., response with a latency much shorter than the free run period). Analysis of video field recordings of photic triggering in the species Photinus pyralis showed that premature flashing (350 ms latency) is only evoked by photic signals in the latter half of the flashing cycle (from about 2.8 to 5.6s). However, in P. consisus, a closely related species in which the free run cycle is 2s rather than 6s, we find that males trigger males at a latency of 600 ms. the same as the female's normal response latency, and apparently cannot distinguish real from simulated female answers. This species thus presents a major behavioral puzzle.