Can't we just pasture spiders?
Unfortunately, this doesn't work. The method for creating creatures in a reaction marks them a "friendly" rather than "tame."
I am aware of the limitations this presents, and I am working on a way to get better constructs into the tech tree. Right now, though, this is the only way I can actually CREATE a new creature from a reaction. The next constructs will probably require a tame animal as a "platform" for construction. I plan to do this through granting an interaction to the reaction-performing dwarf that transforms the target "platform" creature. The bad news is that this will require the platform creature to be nearby, which will require a bit of pen/pasture micromanagement on your part, but the good news is that even ritual units should be able to perform this reaction successfully.
Things may end up working a fair bit like weapon upgrades, "researching" new constructs, creating blueprints at the scientific library, and finally assembling them. Constructs will be "advanced" as follows.... note that all of this is purely conceptual (except for the bronze spiders).
Bronze Dwarven Spider (rudimentary AI, cannot pen/pasture, must be "deployed" on site, size 7000)
Iron Dwarven Spider (same as bronze, but stronger and requires different reagents)
Iron Dwarven Mantis (a flying construct with a pair of needle-like appendages, same limitations as spiders, size 8000)
Envenomed Dwarven Mantis (same as an iron mantis, but carries a poison attack)
Iron Dwarven Tarantula (a larger version of the spider that can be penned/pastured, size 15000)
Steel Dwarven Tarantula (same as iron tarantula, but stronger)
Mithril Dwarven Tarantula (same as steel tarantula, but stronger, has an extra ability, and more expensive)
Damascus Steel Dwarven Tarantula (same as steel tarantula, but much stronger and more expensive)
Steel Dwarven Scarab (size 20000, can be penned/pastured and war trained)
Damascus Steel Dwarven Scarab (same as steel scarab, but stronger and more expensive)
Dwarven Void Hornet (size 17000, war-trainable flying construct with a ranged attack, VERY expensive)
Steel Golem (requires dwarven sacrifice, size 70000, functions as a military unit)
Mithril Golem (same as steel golem, but stronger, has an extra ability due to mithril construction materials)
... It's possible that I'll go on beyond that, but that's a good overview of where I'm standing from a conceptual standpoint.
So, the first release of this arc is getting close. The spiders haven't been tested as thoroughly as I would like, but it will take me a few more days to get that done. I've been testing them with a modified copy that removes all the reagents involved, so I haven't really checked resource balancing in-game yet. I'm concerned that they will be too easy to produce, but I might prove myself wrong.
Current (not finalized) reagents for a (bronze) spider...
2 bronze bars
2 copper bars
2 capacitors
4 advanced mechanisms
1 piece of raw clear glass
2 lustrum rocks
1 bronze pipe section