Most insects sting as a defence by injecting venom into your skin. Symptoms of an insect bite or sting. When an insect bites, it releases saliva that can cause the skin around the bite to become red, swollen and itchy. The venom from a sting often causes a swollen, itchy, red mark (a weal) to form on the skin. This can be painful, but it's harmless in most cases.