Photoperiod

Definition

The duration of light in a 24-hour cycle, typically expressed as hours of daylight; the environmental cue that organisms use to track seasonal progression and coordinate life-history events. In arthropods, photoperiod serves as the primary zeitgeber (time-giver) triggering diapause induction, migratory behavior, reproductive maturation, and seasonal morph determination. The critical photoperiod—the specific day length that switches an organism between alternative developmental or behavioral states—varies latitudinally among populations and is measured experimentally under controlled light:dark regimens.

Etymology

From Greek photos (light) + periodos (cycle, going around)

Example

The flesh fly Sarcophaga crassipalpis enters pupal diapause when autumn photoperiods drop below approximately 13.5 hours of light per day at 25°C; southern populations have shorter critical photoperiods than northern populations, reflecting local adaptation to earlier seasonal transitions.

Synonyms

• day length
• light period

Related Terms

• Photoperiodism
• Diapause
• Circadian rhythm
• critical photoperiod
• zeitgeber
• seasonal polyphenism
• scotoperiod

Usage Notes

Distinguished from light intensity or spectral quality; photoperiod is strictly temporal. Entomologists often manipulate photoperiod in growth chambers to simulate seasonal conditions and trigger diapause experimentally. The term is sometimes used loosely for the full light:dark cycle, but strictly refers to the light portion only (scotoperiod = dark portion). Critical photoperiod varies with temperature and geographic origin, so reported values are always conditional.