Milkweed bugs (Oncopeltus fasciatus, Lygaeidae) groom the distal segment of each antenna between the tibial combs of both forelegs. Variable numbers of grooming strokes compose a grooming bout. A total of 112 spontaneous grooming bouts, comprising all developmental stages, were videotaped, the angular movements of the forelegs were digitized, and the outcomes computer-analyzed. Individual strokes of forelegs were measured by their angular amplitude and mean angular elevation. These two measures are randomly combined in the first instar and then differentiate during ontogeny into two distinct types: small amplitude strokes of high average elevation and large amplitude strokes of lower average elevation. The sequencing of small and large strokes within bouts is largely (97%) random in all developmental stages. The outcome of the analysis fits the general empirical rule that premature action patterns are more random than mature ones. Some traits do not follow this rule and are persistently highly random. It is suggested that excessive randomness in premature behavior is due to genetic underdetermination, and that incorporation of non-genetic information helps to specify the less random mature behavior. Developmentally persistent behavioral randomness is explained by the behavioral-entropy principle: traits in behavior vary randomly unless randomness is selected against.