Why are mosquitoes larger in spring than in the fall?

This week's question comes from Hugh Campbell in Tobermory, Ont. He asks: 

Why are mosquitoes bigger in the spring and quite a bit smaller in the fall?

Spencer Monckton, a PhD candidate and entomologist at York University said the answer mainly has to do with temperature. It's a well-documented pattern that insects tend to grow larger when they develop at lower temperatures.

This is known as the temperature-size rule, and there have been all sorts of reasons put forward to explain it. Generally these fall along two lines of reasoning. First is that colder temperatures mean insects develop more slowly, and therefore reach a larger body size by the time they're mature. Second, a larger body size can be more advantageous at colder temperatures, because larger insects are less likely to succumb to cold.

In the case of mosquitoes, a larger body size probably also means being able to take larger meals, and to last longer from one meal to the next.

As the days get warmer, subsequent generations of mosquitoes will tend to be smaller, either because they develop more quickly, because a larger body size is no longer worth the investment, or some combination of those factors. The short answer is, if you notice that the mosquitoes are larger in the spring, or in more northern areas, it's likely because they developed in colder temperatures.