Culex mosquitoes eggs stick together and are laid in "rafts" that float on the surface of water. In the picture above are four freshly laid egg rafts from our lab mosquito colony, each laid by a single female. They are on the end of a paintbrush that we used to lift them out of the water. They get darker as they age. The two lower rafts are almost the same age; the dark one above them is older and the very light one at the top is less than 1/2 an hour old. To prepare them for microinjection you want to use the very youngest one like the raft at the top. They are seperated from each other and arranged on a glass slide and injected using a microscope and micromanipulator for fine detailed control. They also harden as they age and the light ones can be injected without breaking the thin glass needle that is used. Once they harden you have to use a brief treatment with bleach to remove the out egg surface (chorion) which lowers the rate of survival.