[Bill.D.Katt. (OP)]

A while ago I read about how scientists were extracting DNA from blood found in dead fleas/lice stuck a comb that was dated to several thousand years ago. Obviously preserving blood this way isn't the best method of mass preservation as we have better methods now, but can all hematophages preserve blood like this? And if so, why? A protein in their stomachs prohibiting decomposition?

[CliffordK]

Sounds like the theory behind Jurassic Park.
Extract Dino Blood & DNA from Dino Mosquitoes.

Mosquitoes use anticoagulants, but they would eventually digest the blood.

My guess it has nothing to do with the quality of preservation, but rather giving another source for recovering the blood & DNA.

Keep in mind that RBC's don't carry DNA.  Perhaps the lice are better suited to protecting the DNA than hair strands.