Typically, a spider eats a fly by first injecting a paralyzing venom with its fangs to subdue its prey, while regurgitating digestive juices into the fly's body at the same time. These juices begin to dissolve the fly's internal organs. Once the fly is mostly liquefied, the spider uses its jaws and mouth to break down the tissue further. It then sucks the liquefied body parts out of the fly.
A spider's stomach is encircled by muscles which allow the abdomen to expand and compress. This squeezing action allows a spider to expel digestive juices on or into its prey and to suck the liquid from its victim. While some spiders are known to immobilize their victims, wrap them in silk and save them to eat at a later time, this is not typical. Usually spiders consume their prey as soon as they are able.
Since spiders are cold-blooded, they are able to survive on limited amounts of food by remaining inactive for long periods of time. This comes in handy when local food sources dwindle. Most spiders are predators that eat insects and other spiders. Some very large spiders also eat lizards, tadpoles, frogs, fish and even small rodents and snakes.