Egg-masses of Simulium damnosum s.l. were investigated to explain the frequent presence of undeveloped eggs in the lower layers. Natural development within egg-masses depended on whether eggs were laid (a) all at once or (b) were added to on consecutive evenings. In the field where the development of eggs from situation (a) was followed, top layer (TL) eggs hatched in 2 days when unexposed bottom layer (BL) eggs were still white. BL eggs uncovered 12 hr after oviposition were undeveloped, then hatched in 3 days. BL (a) eggs uncovered after 5 days were still untanned and undeveloped but were not further investigated.

In the laboratory, portions of egg-masses—intact or with the BLs exposed—were incubated in Petri dishes with and without preceding KOH egg separation treatment, or in flowing water. Other samples of BL eggs were incubated at different O2 levels. In Petri dishes, most TL eggs hatched within 24 hr of setting up but exposed BL eggs hatched (if at all) after much delay in 6–14 days. No unexposed BL eggs hatched even after TLs above them had done so. Transfer to flowing water or KOH treatment increased hatching and reduced the delay. Hatch rates of exposed BL eggs rose from 2.7 to 63% with increased O2 levels.

Implications with regard to laboratory culture and control operations are discussed. Mortality of covered eggs could be a potent population regulating factor within and perhaps even between species of the S. damnosum complex.