Geographic patterns in the flight ability of a monophagous beetle.

The migratory ability of a monophagous insect, Tetraopes tetraophthalmus (Coleoptera: Cerambycidae), was measured in ten populations in each of two regional types. In the first region, a river valley, populations were abundant and close together; in the second regional type, mountainous and peripheral areas, populations were uncommon and far apart. The data revealed that regional variation in migratory behavior occurred along two dimensions. Regions differed not only in the mean migratory ability of individuals (1st dimension) but in the amount of variation between populations (2nd dimension). Whereas river valley populations were found to be uniformly sedentary, mountainous and peripheral populations were found to be heterogeneous, some being very vagile, others being very sedentary. In the mountainous and peripheral areas, small and sparse populations were more vagile than large dense populations. The site to site variation in the mountainous and peripheral areas was due exclusively to females. In these areas, the mean migratory ability of females in a population was positively correlated with mean body size of the females, and inversely correlated with populations density. The data are consistent with the FLY hypothesis for this species (long distance migraters=Females, Large, and Young). Taken together, the geographical patterns support the "founding hypothesis" of migration. According to this theory, the inevitable selection against migration within populations is balanced by periodic extinctions of the populations and the founding of new populations by migrating individuals.