There is no sort of damage transfer effect on the stirges. We shall assume attacks are made cautiously enough to rather go wide than hit an ally [@The Harbinger of Ferocity]. If the designers had intended there to be a significant risk, I reckon they would have mentioned that in the monster stat block.

I am half tempted to say that a critical failure might cause such an effect, but my common sense disagrees. Thus, nothing special apart from their automatic hit on survival to next turn.