They may also regurgitate food to masticate again, similar to cud-chewing by cattle. They may also regurgitate food to masticate again, similar to cud-chewing by a cow. Most platyhelminths have no anus and regurgitate undigested material through the mouth. When caught they often regurgitate the contents of their stomachs. Some caterpillars regurgitate acidic digestive juices at attacking enemies. Keratin resists digestion, which is why cats regurgitate hairballs. Another ploy is to regurgitate their stomach contents when harassed, repelling potential predators. Horses are unable to regurgitate food, except from the esophagus. The workers also regurgitate food for other ants to consume. They regurgitate food in the center of the nest rather than feeding each chick individually. Similarly, larvae of the noctuid moth regurgitate when disturbed by ants. Ruminants regurgitate their food as a normal part of digestion. However, not all infected snakes may regurgitate. Some birds such as pigeons create a "crop milk" which they similarly regurgitate. Patients usually are able to regurgitate or force through the food material and resume eating.