The reason is that insect cells are often incubated in very simple incubators that are not humidified (and without CO2 at 24°C to 28°C).

Insect media typically do not need CO2 for buffering pH, so gas exchange is not required. To avoid evaporation of medium in a simple incubator that is not humidified, non-ventilated caps have to be used. If a conventional cell culture incubator with humidification is used, ventilated caps can be used as well.