What is characteristic of Culex mosquito eggs?

Culex mosquito eggs are known for their unique characteristic of being laid in rafts that float on the water surface. These rafts usually consist of several eggs that stick together, forming a cohesive cluster that helps them remain buoyant. This floating behavior provides several advantages for the developing embryos, such as access to oxygen from the air above and protection from certain predators that may feed on individual eggs.

The other options do not accurately describe the nature of Culex mosquito eggs. For example, laying in a spiral pattern is not typical for Culex, which instead produces these distinctive rafts. Additionally, the eggs do not sink immediately after laying, nor do they hatch instantly; the eggs require a certain amount of time and favorable conditions before they hatch into larvae when submerged in water.