Temperature-size rule is a well-known empirical tenet concerning animal body size. I studied seasonal changes in the body size of the multivoltine black fungus gnat Ctenosciara japonica, whose larvae feed on soil humus. Adults of C. japonica were sampled twice per month during four years (2002 and 2005-2007) at two sites in Yokohama, Japan: a broad-leaved forest (site 1) and a coniferous forest (site 2). Mean abundance per year was 208 at site 1 and 340 at site 2. The wing length of sampled individuals was measured, which reached a minimum in August and a maximum in December at both sites. Monthly mean air and soil temperatures (2005-2008) and the dry weight of monthly leaf litter input (2005-2006) were measured using data loggers and litterfall traps at these sites. The dry weight of soil humus was compared between August and December in 2010; it was found to slightly decrease (site 1) or significantly increase (site 2) in December.

This study illustrates that the pattern of seasonal changes in the body size of C. japonica was similar between two sites in which seasonal fluctuations in the quantity of food resources were different, and that body size was inversely related to temperature at both sites. Although the quality of soil humus was not evaluated, the results shown here suggest that the body size of C. japonica follows temperature-size rule.