Milkweed bugs (Oncopeltus fasciatus, Lygaeidae) groom the distal segme
nt of each antenna between the tibial combs of both forelegs. Variable
numbers of grooming strokes compose a grooming bout. A total of 112 s
pontaneous grooming bouts, comprising all developmental stages, were v
ideo-taped, the angular movements of the forelegs were digitized, and
the outcomes computer-analyzed. Individual strokes of forelegs were me
asured by their angular amplitude and mean angular elevation. These tw
o measures are randomly combined in the first instar and then differen
tiate during ontogeny into two distinct types: small amplitude strokes
of high average elevation and large amplitude strokes of lower averag
e 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 patter
ns are more random than mature ones. Some traits do not follow this ru
le 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 th
e less random mature behavior. Developmentally persistent behavioral r
andomness is explained by the behavioral-entropy principle: traits in
behavior vary randomly unless randomness is selected against.