The Mulc-Spider is a biological tool that species use when leaving a planet during migration. The spiders are planted in the middle of cities so they can slowly break down the inorganic material and return the location to a natural state. They have an intelligence that can be communicated with telepathically but they have no interest in dialog or really anything beyond their purpose, and even that they approach without passion. 
Mulc-Spiders grow miles long tendrils that can secrete acids to break down stone and metal. The tendrils can produce different chemicals but under normal circumstances there is no reason to do so, nor do the tendrils normally move at a speed that is noticeable to sophants. The spiders can live millions of years but accidents that kill or cripple them are common.
- ↑ Brightness Reef p.143