This person was bitten by a spider ,but little did he know that it had been a recluse spider! Leaving it untreated allowed the venom to cause these lesions . Scientists say this venom contains a rare protein that can cause a lesion as well as a much less common, but more dangerous systemic reaction in humans.. The most common response is inflammation, which ,aftr one to two days can develop into a "dark lesion" surrounding the bite site. The blackining, or " necrosis" ,of the skin is dead skin cells, evidence of the immune systems effort to prevent the spread of the toxin by preventing blood flow to the affected area.