Maybe you could harvest giant mosquito saliva?
In real life mosquito saliva has anti-coagulative properties, so I don’t see why the giant ones would be very different.
The first use for this that came to my mind was being able to ‘poison’ weapons (knife, arrow, bullet even?) so that when they inflict cutting or piercing damage they induce a ‘bleeding’ effect on your enemy, causing them to lose x health every y turns.
I’m sure there are also medicinal uses for this too.
If this was implemented I’d like to see giant mosquitoes being able to give you the bleeding effect too. And leeches. Cataclysm needs giant leeches for more nightmare fuel.