My friend at work was bitten by a spider or something a couple nights ago. Bitten on her calf. The area is swollen and painful to touch, with a bit of purplish discoloration. The doctor says that it's a bacterial infection, but didn't know what the animal was that caused it. There's nary a mark, maybe one mark. Now, the black widow spider leaves two marks when it bites. Are there any insects that can cause such a reaction that leave only one mark?