Mosquitoes in the subfamily Anophelinae have a peptide called anophelin in their saliva. (The mosquitoes that transmit the malaria parasite belong to this subfamily.) Anophelin inhibits thrombin, preventing blood coagulation. Mosquitoes in the subfamily Culicinae have an anticoagulant in their saliva that inhibits the coagulation or clotting factor known as FXa. It’s referred to as an “FXa-directed anticoagulant”.
The saliva of mosquitoes isn’t well characterized. It may contain additional chemicals that affect blood clotting and make obtaining the liquid more efficient. Only female mosquitoes feed on the liquid. They need blood proteins in order to make their eggs.