Midge larvae possess giant polytene chromosomes. Genes on these chromosomes undergoing transcription are visible as puffs. The nucleolar organizer (NOR), an especially large puff, shrinks when a larva is stressed. Two feeding experiments were conducted to examine how NOR size changes as a function of Chironomus riparius growth. NOR size was linearly related to an individual's recent growth rate, independent of its body size.

Chironomids were collected from wetlands constructed with oil sands mine water and tailings (OSPM), and reference wetlands to evaluate the utility of the NOR as a field-based measure of larval growth and condition. Small larvae (<9mm) did not yield sufficient quality preparations. Five karyotypes were identified, revealing cryptic diversity in the wetlands. NOR size was measured in a single karyotype; no measures were obtained from OSPM-affected wetlands. NOR size has potential as a surrogate growth measure that can facilitate estimation of chironomid secondary production.