1. The water content of the blood meal of tsetse flies is reduced from 79 to about 55% within the first 3 hr. after feeding.
2. The water which is abstracted from the gut contents serves to bring the tissues of the fly to full hydration, and any excess is excreted.
3. The degree of dehydration of the gut content depends on the amount of blood taken, relatively more water being retained in partial feeds.
4. The water content of faecal matter is about 75% in flies maintained at high relative humidity throughout the hunger cycle, only 35% in flies maintained in dry air. The difference reflects a saving of more than 30% of the total water reserves of the fly under conditions of desiccation.