Guide-line and gravity orientation of blind termites foraging in the open (Termitidae:Macrotermes, Hospitalitermes)

Abstract

1. Once in several days large numbers ofHospitalitermes sharpi, like other species of this oriental genus, leave their nests for a foraging trip to replenish food stores. Foraging columns are continuous, well ordered files of marching termites, up to 300 meters long and sometimes longer, stretching without interruption all the way from the nest to the grazing area on living tree bark. Topography permitting, the foraging columns follow pre-existing guide-lines, the crest-lines on the tops of sticks, roots and other elongate objects (=crest-line trailing). On vertical substrate, like cliffs or standing tree trunks, they either proceed horizontally or vertically. During any such foraging period each individual soldier and worker leaves the nest only once. 2. Homebound traffic inHospitalitermes marching columns occupies the central lanes, outbound traffic the lateral lanes. Soldiers lead the advancing column, trail the retreating column and stand on either side of every column, facing away from the column's edge. 3. The odor trails laid down by the foraging columns are bidirectional, i.e. homebound and outbound direction are indistinguishable for the termites. Misdirected homing termites restore their correct orientation after series of head-on collisions with correctly oriented companions. 4. No outside directional stimuli, in particular the directions of light and gravity, are used byHospitalitermes to make up for the directional ambivalence of the odor trail. 5. The orientation mechanism underlying crest-line trailing, the predominant type of orientation prior to the formation of an odor trail (=primary orientation), is a geoklinotaxis: the advancing termites on a crest-line, say the top of a horizontal stick, oscillate to the sides, turning back up-slope after reaching a particular angle of downward inclination. In 50% of the cases measured withHospitalitermes umbrinus, this angle is 12–13° or less from horizontal. 6. InHospitalitermes rufus, H. sharpi, andMacrotermes carbonarius, the orientation angle between the slope direction upward or downward and the direction of walking (=angle of geomenotaxis) decreases with increasing slope inclination. InMacrotermes carbonarius body weight is one factor mediating gravity perception. 7. The orientation strategy and marching column organization of the orientalHospitalitermes-species bear some striking resemblances to those of some neotropicalEciton-species (=army ants).