Why adult mayflies of Cloeon dipterum (Ephemeroptera: Baetidae) become smaller as temperature warms

We reared Cloeon dipterum from egg hatch to adult at 10 constant temperatures (12.1–33.57C) to test 3 hypotheses (thermal equilibrium hypothesis, temperature size rule [TSR], and O2and capacity-limited thermal tolerance [OCLTT]) that account for variation in life-history traits across thermal gradients. Male and female adult size declined ~67 and 78% and larval development time declined ~88% with warming; chronic survivorship (thermal limit for population growth) was highest from 16.2 to 23.97C (mean 5 85%) and declined to 0 at 33.57C; thresholds for 0 growth and development were 10.0 and 10.77C, respectively; peak rate of population increase (r) occurred at 27.87C; rates of growth and development were maximal at 307C; fecundity was greatest at 12.17C; and between 14.3 and 307C, growth and development rates increased linearly and the number of degree days (>10.77C) to complete development was nearly constant (mean5 271). Acute survivorship during short-term thermal ramping was 0 at 407C.Warming temperature caused development rate to increase proportionately faster than growth rate; male and female adult size to decrease as per TSR, with adult females ~5 larger at 12.1 than 31.77C; adult size to decrease proportionately more for females than males; and fecundity to decrease proportionately more than adult female size. TSR was related to differences in the responses of growth and development rates at temperatures above thresholds rather than to thresholds for growth or development per se. Respirometry suggested that OCLTT is more applicable to acute than chronic thermal limits. Cloeon dipterum appears to have a thermal ‘acclimation zone’ between 14.3 and 307C where development and growth rates change linearly and degree-day requirements to complete metamorphosis are constant. The optimum temperature is ~27.87C where r is maximum. We propose 5 hypotheses to explain these patterns.