Last time, I posted about a typical day in the life of summer field work. The field mosquito trap processing includes parity analysis. Parity is defined as the number of times a female has given birth (or in the case of the mosquito, whether she has laid eggs). This gives us some idea as to the age of a mosquito. Determining the age structure of a population is important, as older mosquitoes are more likely to transmit diseases since they have had the opportunity to blood feed on potentially infectious hosts.
Since it is impossible to tell how old a wild-caught mosquito is, our best guess relates to parity. If the mosquito has not laid eggs (nulliparous), she is likely relatively young; if she has laid eggs before (parous), she must have had the opportunity to mate and take a blood meal, and is therefore likely older.
Parity assessment can be performed by a technique called ovary tracheation. Tracheoles are tube-like structures that are part of the respiratory system in insects. In mosquitoes, the tracheoles make what looks like a knot around the forming ovarioles. Due to its resemblance to a ball of yarn, this knot is referred to as a skein. The picture below shows the tracheolar skeins in a nulliparous female at 40x and 100x:
Once the eggs become fully developed and the female oviposits them into the environment, these skeins become loosened and never form back into tight knots. Below is a parous female, lacking the tracheolar skeins:
While the two examples above are pretty clear, some mosquitoes have the presence of a moderate number of skeins. For this reason, it can become a bit subjective when determining parity. However, as with any art form, it is possible to become proficient. You can read more about how ovary tracheation compares to another form of parity assessment here.