The midges Chironomus crassicaudatus and Glyptotendipes paripes were reared in the laboratory on artificial food under constant temperature (30 degrees C) and a 14:10 h light: dark photoperiod for 31 days, from eggs laid by field-collected females. Sequential samples of developing immature stages were taken and measured. Eggs were on average 254 microm long and 102 microm wide at the widest point for C. crassicaudatus and 286 microm long and 113 microm wide at the widest point for G. paripes. Mean larval lengths ranged from 0.9 mm after hatching to 16.3 mm before pupation in C. crassicaudatus and from 0.8 mm after hatching to 9.7 mm before pupation in G. paripes. Mean length of pupae was 8.7 mm and 8.3 mm in C. crassicaudatus and G. paripes, respectively. Four morphometric head parameters (length, width, mentum width, and cephalolabial length) were tested for differentiation of larval instars. All parameters revealed 4 larval instars in both species, with head capsule width apparently the most sensitive indicator for instar differentiation. The cephalolabial length was the most sensitive indicator of sex differentiation in last instar. All investigated morphological parameters of head capsule of both species followed Dyar's law.