What is characteristic of Culex mosquito eggs?

Culex mosquito eggs are indeed characterized by forming individual floating rafts, which consist of clusters of eggs that are typically laid on the surface of standing water. This adaptation allows the eggs to remain buoyant, creating a protective environment for the developing larvae once they hatch. The rafts can be made up of several dozen to several hundred eggs, depending on the species.

This method of egg-laying serves important ecological functions. It protects the eggs from being washed away by water movement and increases the likelihood of the larvae surviving in aquatic environments where they can find food once they emerge. The other options do not accurately describe Culex mosquito eggs: the spiral pattern pertains more to some other mosquito species; sinking immediately after laying would not provide the survival advantage that buoyant rafts do; and instant hatching upon contact with water doesn't occur, as the eggs typically require a certain level of water to begin the hatching process after a brief incubation period.