Egg clutches from two mosquito species were incubated under controlled conditions and the larval development time was recorded. Both the egg raft-laying mosquito Culex quinquefasciatus and the individual egg layer Aedes aegypti displayed significant intraspecific variation between clutches when it came to larval development at the same temperature. Furthermore, embryonic incubation temperature was found to influence subsequent larval growth rate in some cases. These hitherto undocumented phenomena are indicative of development rate plasticity that appears to be conserved across mosquito genera. The derived development rate data were used to calculate the minimum number of egg clutches required to give a representative mean day-degrees value for a population of each species. These values were six and five clutches, respectively. A simple population simulation was given to illustrate the impact of different larval development rates when overlapping generations of problem mosquitoes breed. The implication of this work for mosquito population modelling is that the extent of developmental rate variation needs to be taken into account. Furthermore, such conserved variation in larval development rate illustrates the extent of selective pressures on this life history trait.