Adult ants cannot digest solid foods. Therefore, everything they eat has to be turned into a liquid before it enters the digestive tract. When the ant eats a piece of food, digestive juices interact with the solid outside of the ant's mouth and dissolve the solid food into digestible liquid. The ant then laps up the food with its tongue.
The food goes through the ant's mouth into a chamber below that houses a filter. The filter makes sure no solids are allowed to progress. The solids in the chamber are compressed into one pellet, which the ant then spits out.
Most of the digestive system is housed in the ant's hindmost section, called the gaster. The gaster is made out of soft tissue and is wrapped in rings that act like rubber bands. When the food leaves the chamber below the mouth, it moves to the crop, which is in the gaster. The flexible gaster allows the crop to expand when it is full of food. When the ant has just eaten, the gaster, or hindquarters may appear longer and thicker than the gaster of an ant that has not eaten for some time. The crop stores the food without digesting it until the ant is ready to digest.
When the ant's body is ready to fully digest the food, it is allowed to leave the crop. The food trickles from the crop slowly into the ant's midgut, which is also in the gaster. In the midgut, the nutrients and energy from the food is absorbed; the waste is eliminated via an opening in the end of the gaster.