Ah, DF evolution again...
The problem with "evolution" in DF is that this game is not complex enough to model realistic costs of adaptations - if any creature without blood is far more likely to survive a fight, as it means only decapitations or dissections will kill them, why wouldn't everything evolve to lose their heads, lower torsos, and blood, and become essentially like the materials-based HFS?
In real life, many aspects of a creature's biologies are actually trade-offs. Spiders (as species, not the entire class of creatures), for example, are believed to have evolved constantly back and forth between being nearly-blind, slow-moving web-spinners, where much of their biological energy is taken OUT of eyesight and mobility in favor of being more energy-efficient, and relying upon their webs to catch flying insects, and changing into "wolf spiders", which have good eyesight, move quickly to outrun prey, and devolve their web spinning abilities because they rely upon out-running land insects.
Evolution isn't a matter of becoming "better" at everything, it's a matter of optimizing yourself to the one particular survival strategy that works for the niche you have carved out right this moment. Spiders can't BOTH have good eyesight and running speed and have web-spinning, it costs too much, and spiders often starve from lack of nutrition to upkeep such body parts. The spider that specializes is the spider that lives to breed, which is where evolution pushes them.
This, again, is something way beyond DF's current modeling capabilities.