The spider in your photo is the silver garden spider, scientific name: Argiope argentata, in the family Araneidae. There is probably no definitive answer on how spiders "keep their cool", because it depends on the species, location and other factors. Spiders are "cold blooded", meaning they can't regulate their temperature physiologically, rather their body temperature just remains close to the ambient air temperature. However, they can do things behaviorally. Many spiders are only active at night, remaining hidden in secluded places during the day. This is mainly to avoid predators, but this would also mean they are not as exposed to daytime temperatures or direct sunlight. A spider that was "overheating" during the day would likely move under a leaf or other object to avoid direct sunlight. Spiders and insects in general can tolerate temperatures up to about 110 degrees Fahrenheit before it becomes lethal. Also, because of their small size, insects and spiders have less surface area that would be exposed to the sunlight, and consequently don't heat up as fast as some larger animals. You can actually put small insects and spiders into a microwave and zap them at high power and they hardly heat up at all, because there is not enough water inside their bodies for the microwaves to act upon. It probably works the same way with direct sunlight. Your silver garden spider has other factors that probably help it. It is primarily a tropical species that barely extends up into California. Therefore, it has probably become more adapted and tolerant of heat than other kinds of spiders from temperate areas. Even though it is sitting out on its web exposed to the direct sunlight, it is far away from other surfaces such as leaves, the ground, etc. that might reflect or rediate a lot of heat. So being in the center of its web, may actually be the coolest place around. Parts of the underside and top side of the spider are covered with silvery hairs that also reflect some of the light and prevent some sunlight and heat contact with parts of the body. So all in all, your spider is sitting in the middle of its web on a hot day in the direct sunlight, mainly because it does not bother it and it is completly tolerant of those conditions. It might take temperatures well above 100 degrees, before it might feel the need to move. Other types of spiders may not be able to do this, and this actually gives silver garden spiders an advantage because it allows them to catch insect prey in their webs when other spiders are not active.