The viability of 17,200 Anopheles gambiae eggs from wild females was tested at constant humidity in moist sand by removing batches for hatching at 2-day intervals between 8 and 32 days following oviposition. Eggs stored for 8 and 10 days showed a reduced rate of hatching, while eggs kept for 12 days showed a much lower rate. After 14 to 18 days' storage only a fraction of one per cent appeared viable. None hatched after longer storage.
Tests for viability were also made on nearly 3000 eggs exposed to spontaneous drying in sand or mud. The hatching rate for eggs stored in mud was lower than for those stored in sand for the same period of time. One per cent of the eggs kept in drying sand for 18 days hatched.
The low constant hatching rate after storage for 14 to 18 days following oviposition suggested the existence of a particular type of resistant egg. Among 12,525 eggs oviposited by Anopheles gambiae in the laboratory colonies, were discovered 172 unusual or abnormal eggs covered dorsally with a reticulated exochorion and similar in appearance to the winter resistant egg described by Hurlbut for Anopheles walkeri. Experiments are described in which a high percentage of this abnormal egg could be produced at will by subjecting fertilized females immediately after a blood meal to temperatures of 10° or 13°C for 3 or more days. Transition forms between the normal type and the most abnormal type are described and figured.