Would be interesting to know how long ago this trait evolved, and whether the current state of affairs is stable or if adaptations are in progress.

It's not too hard to imagine a mutation from a toxin that causes painful bites, to something more lethal - and this would dramatically increase the amount of 'food' available to these critters.

Hopefully that doesn't happen for at least a million years or more, bc if you thought mosquitoes were bad ..