Is there a better way to "Evolve" units?

i'm making a scenario where the race is spiders for example. instead of training units from the barracks, i made an ability based on the undead cryptlords carrion beetles. every so many seconds, a spider is created from a modified create corpses ability. that spider has an ability based on the elven demon hunter metamorphisis. once the spiders mana reaches the appropriate level, the spell can be used to evolve the spider into a more power version. the problem is that because the spiders are spawned, and not trained, there is no way to specify a worker unit, so the spiders eventually evolve into one. the only problem is that the worker unit only builds when i place it on the map initially. when the unit comes into play through evolution, the worker simply walks up to the shadow of the structure and does nothing. is this a problem with the metamorphisis spell, or have i overlooked something?

Im not sure, but i might to work if you modify the Chaos spell, its like meta but perminent so it should work like that.

When a unit is upgraded or morphed from another unit, usually the new one cannot use any abilities that the old one did not have. So, if you're trying to build a building but don't have the necessary build and repair abilities, you can't finish it. Try just replacing the worker with itself via trigger whenever it's morphed to. That should fix it, if I've identified the problem correctly.

that sounds like a good idea panto, but im not very experienced with trigger enhanced abilities. should i use something like "spider finishes using an ability, ability = evolve, then replace that spider with the worker unit?

If the "Evolve" ability morphs the spider to the worker, then just have a trigger that, when a unit enters the map, or is summoned, or something similar, and it's a worker, turn off the trigger, replace the worker with a worker, and turn the trigger on.

I'll move you to the Trigger Haven, wherein are the ninjas.

thanks a lot panto