The slake-moth that prevails over its companions and attains the role of mother to their nascent brood happens to be the very one that the Weaver had mauled during the mid-air battle of the previous night. Coincidence? Maybe not: the Weaver seems capable of some astounding perceptions and leaps of insight, and might have recognized which moth was most likely to be the egg-layer and brood-guarder. However unfathomable its mental processes might be, it's plausible that the Weaver targeted that slake-moth over the others, because it knew that one - the most high-estrogen moth of the lot - would produce more eggs if it escaped and was able to mate. If nothing else, the Weaver could weaken it so that the brood would have an injured defender, giving the spider's humanoid allies a better chance of destroying the nest.
The brief scene in which the Mayor speaks to the Ambassador seems like it's merely an opportunity to imply The Worf Effect, by revealing that even daemons are terrified of slake-moths. But it also serves a Magic A Is Magic A purpose, establishing that the advanced techno-magic of this setting can simulate the psychic energies released by a sacrificial victim well enough to serve as an "invitation" to otherworldly beings. Which, in a massively-upgraded form, is exactly how Isaac's crisis-powered trap for the slake-moths will work.
Throughout Perdido Street Station, there are snippets of reports of an eye-snatching serial killer dumping eyeless victims all over the city. Benjamin Flex gets snatched by New Crobuzon's militia, tortured for information, and is later found, dead and eyeless, in the river. A few chapters later, Mayor Rudgutter mentions that his eyes are failing. He'll have to get another pair...
The slake-moths apparently do have a place in the ecosystem they came from, where food is sufficient to sustain their population. Slake-moths can only feed on creatures with a human-like consciousness. So logically, whatever living hell of a habitat they came from, there must be lots of sentient beings living there, who think and feel much like we do, who have to endure slake-moths' depredations every day of their lives...
