That particular membrane conductances are selected for expression to e
nable the efficient coding of biologically relevant signals is illustr
ated by recent work on insect photoreceptors. These studies exploit th
e richness of insect vision and the accessibility of insect photorecep
tors to cellular analysis in both intact animal and isolated cell prep
arations. The distribution of voltage-gated conductances among photore
ceptors of different species correlates with visual ecology. Delayed-r
ectifier K+ channels are found in the rapidly responding photoreceptor
s of fast-flying flies. The conductance's activation range and dynamic
s match light-induced signals, and enable a rapid response by reducing
the membrane time constant. Slow-moving flies have slowly responding
photoreceptors that lack the delayed rectifier, but express an inactiv
ating K+ conductance that is metabolically less demanding, Complementi
ng these findings, locust photoreceptor membranes are modulated diurna
lly. The delayed rectifier is exhibited during the day and the inactiv
ating K+ current is exhibited at night. Insect photoreceptors also dem
onstrate the amplification of signals by voltage-gated Na+ channels. I
n drone-bee photoreceptors, voltage-gated Na+ channels combine with K channels to enhance the small transient signals produced by the image
of a queen bee passing over the retina. This subthreshold amplifier o
perates most effectively over the range of light intensities at which
drones pursue queens,