answer:I am not a doctor or a biologist, so I may be totally full of it. But I believe that I am more or less correct. When the spider but you, your body (skin) saw that as an intrusion (I believe the technical word is “insult”), and it rushed white blood cells to the location of the bites. These would defend the body against biological issues that resulted from the bites. They release histamines to cut down on allergic reactions, among other things. But they don’t know when to stop, and therefore they will create lots of white blood cells – too many, in fact. (Same theory in allergic reactions and why you need anti-histamines during the pollen season) If I am not mistaken, the clear stuff is surplus white blood cells that are not needed at the point of the bite/