A method for estimating larval dry weight as a function of tracheal front air sac length and instar was developed for second, third, and fourth instar Chaoborus americanus using linear regression analysis. The following equation: $$W^{0.5} = (L - a)/b,$$ where W = larval dry weight (µg), L = front air sac length (mm), a = the y-intercept, and b = the slope provided accurate estimates of larval dry weight for three C. americanus populations. No detectable change in size occurred in the air sacs of larvae preserved for one year in 5% formalin. This indicates that air sacs can be used to accurately estimate larval dry weight for preserved as well as living specimens.