Having laboured like Escoffier over your moggy, do not be alarmed if he or she still insists on chewing grass and weeds at the first opportunity. Grass is good for cats. It contains certain vitamins and also acts as an efficient emetic, helping the animal regurgitate unwanted matter such as fur balls. If you and your cat share a high-rise apartment without ready access to a garden, grow him some grass from seed in a window box. Chewing grass is not a sign that the cat feels ill.
Cats hunt mice, birds, insects, and so on for sport, not as a food source. True, they may on occasion eat part or all of their prey, but basically they are in it for the fun of the game. So you need not think that by underfeeding, or indeed not feeding your cat, he will be encouraged to clear your premises of small rodents. The opposite is true. Well-fed cats are the best mousers. They have the stamina energy, and quick reactions required for the sport.
“Cats that catch flies and eat them go thin” is a very common saying. Although there is the possibility that a cat eating a blue-bottle might also take in disease bacteria, the problem is a minute one. Occasionally worm eggs might be carried by flies from one cat to another, but other than this slight risk, fly-eating cats rarely come to much harm.