The effect of photoperiod on wing length, body weight, and relationship between wing length and body weight was investigated in the mosquito species Anopheles quadrimaculatus. Individuals reared under a short photoperiod (8 h light: 16 h dark) had longer wings and larger weights than did those reared under a long photoperiod (16 h light: 8 h dark). Covariance analysis showed that photoperiod and wing length interacted so that photoperiod did not have a uniform effect on body weight at all wing lengths. At small mosquito sizes, body weight was higher in short than in long-photoperiod individuals of the same wing length, but at large mosquito sizes, body weight was higher in long than in short-photoperiod individuals of the same wing length. Thus, among smaller mosquitoes of this species, wings were disproportionately longer in long-photoperiod individuals, but among larger mosquitoes, wings were disproportionately longer in short-photoperiod individuals. These results, together with previous studies, suggest that photoperiod and temperature have similar effects on the developing insect.