Often the cause of purple skin around a bug bite is your body’s reaction to the saliva or venom from the insect. For example, when a mosquito sucks your blood, it injects saliva into your skin. This fluid exchange produces an itchy rash in most people, and it can also lead to a bruised appearance.
Other times, like with a bee sting or spider bite, when the venom releases into the skin and surrounding tissue, it causes pain and inflammation. These reactions can sometimes lead to bruising.
Some insect vectors (those responsible for spreading human disease), like ticks, may cause a rash that appears purple. These rashes are unlike other bug bite reactions because they indicate a systemic (body-wide) infection rather than a localized skin reaction.