During the summer ol 1983 I noticed that the males of the four most common libellulids at my study pond in the University of Wisconsin Arboretum, Madison had clearly different colour patterns, partly involvine the distribution of pruinosity. Pachydiplax longipennis has a lightly pruinose abdomen dorsum and clear wings; Plathemis lydia has a heavily pruinose abdomen and a black band on the wings; Libellula luctuosa has a dark abdomen and single black and pruinose bands on the wings; and Libellula pulchella has a lightly pruinose abdomen and three black and two pruinose patches on each wing. This patterning suggests that in addition to possible excretory and thermoregulatory functions, pruinosity may serve a communicatory function, as has already been demonstrated for P. lydia by M.E. JACOBS (1955, Ecology 36: 566-586). Because dragonflies are well known to be capable of seeing U.V., 1 examined the reflectance of these and other structures using a spectrophotometer (Fig. 1). Clearly pruinosity strongly reflects U.V., often more strongly than light in our visible range. There might be a difference between that exuded on the body and wings. Other white coloration on odonates may not reflect U.V. as illustrated by the pterostigma of Calopteryx maculata females. 1 suggest that wherever it is found, a possible communicatory function involving U.V. reflectance be considered for pruinosity.