Why does it leave a red mark if you squash a mosquitoe?

When you squash a mosquito, it can leave a red mark due to the rupture of small blood vessels near the skin's surface, resulting in localized bleeding or a bruise. Additionally, the mosquito's saliva, which contains anticoagulants to prevent blood clotting while it feeds, can also trigger an inflammatory response, causing redness and swelling in the area. This reaction is part of the body's immune response to foreign substances.