Or maybe something insectoid
It couldn't have been sex; we know that from Jahlee's troubles. But what if Krait's mother were already gravid (or about to be, or whatever they do) and the first bite imprinted the foetus (or whatever it is). Maybe it's that first bite.
Now I know I'm up for the Borski prize.
-alga
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.