The effect of relative humidity on photoresponse in sympatric species of Drosophila: Long-term exposure to desiccating environments

The photoresponse of isofemale strains of the sympatric species Drosophila immigrans and Drosophila tripunctata were compared at relative humidities of 95, 50 and 5% during an 8-h period. The onset of a change in photobehaviour due to the stress of desiccation was calculated for each strain for the three relative humidities. The two species could be distinguished on the basis of their light responses under the three desiccating conditions. With increased desiccation there was an increased positive response to light after the behavioural shift. The effect was greater and more uniform in D. tripunctata, which is a fungus feeder, than in D. immigrans, possibly because D. tripunctata occupies a narrower niche than D. immigran. The species were observed to have higher activity levels as they desiccated. It is postulated that the increased positive light responses with increased relative humidity are attributable to both desiccation and starvation. The time of onset of the shift in photobehaviour is dependent on the relative humidity.