Figure 2

Simplified sequence of events from light to diapause in R. pedestris. Light stimulates photoreceptors [9] whose output is integrated by an unknown, complex set of events within the photoperiodic timer and is translated into a long- or short-day signal to the brain. Long days inhibit and short days stimulate the production of neurohomones by neurosecretory cells in the brain [8]. The neurohormones produced by these cells inhibit the corpora allata from secreting juvenile hormone, a terpenoid that is necessary to promote ovarian development. Hence, long days promote ovarian development by blocking inhibition of the corpora allata while short days promote diapause (non-development) by promoting inhibition of the corpora allata. Ikeno et al. [6] show that cycle (cyc) is acting at the level of juvenile hormone secretion by the corpora allata, or at one of many unknown, higher levels in the complex cascade of events initiated by light.