Edema is a swelling, usually of the legs, due to the accumulation of excessive fluid in the tissues. The edema that occurs in diseases of the heart, liver, and kidneys is mainly caused by salt retention, which holds the excess fluid in the body. Biting insects are usually after a meal, and they suck your blood to satisfy their needs. To allow the blood to flow easily, they pass substances that prevent the blood from clotting in the area around the bite. It is the chemicals and proteins that the insect injects under your skin which produce a local reaction on your legs that result in swelling. Your observation that antihistamines help you suggests that you are forming a histamine reaction to the bites. This causes seeping of fluid into the tissues around the site where the insect has injected you. Sometimes people react strongly to the presence of insect venom and protein, and occasionally our defensive cells are not readily able to combat the dirt and bacteria that the insect injects into us. When this happens, the limb will become hot and swollen and red due to the spread of infection. Since you are so prone to react to bites it would be wise for you to take non-sedating antihistamine tablets. Also take all possible precautions to avoid being bitten. These include the wearing of long trousers, a button-up neck-line and socks as dusk approaches, as well as insect repellant.