ABSTRACT

Social hornets and wasps (Vespinae) build their combs in nature with the cell outlets facing down, in the direction of earth`s gravitational force. The workers of the Oriental hornet spontaneously build combs also under laboratory conditions and here, too, the building is gravity-directed, to an error of a mere 1°. This latter in vitro comb building was examined by us both in a static state, wherein the workers are kept in appropriate stationary artificial breeding boxes (ABBs) while in dimness or darkness, as well as in a dynamic state, that is, with their ABBs affixed to the arms of a moving horizontal or vertical centrifuge. The angle of the building resultant while on a moving centrifuge is computed by the formula: tan Ф = (2πf)² rg = 4π² (25/60)² r/980, where r = radius, g = the gravitation constant and 2πf = the angular frequency. Invariably, the comb building by workers takes place during their first two weeks of life. Both in presence or absence of a queen, the number of cells built depends on the size of the worker group in the ABB, according to the equation cell (building) =  , and this in groups numbering up to 15 workers. In larger groups of 50-60 workers, the workers are seen to split into sub-groups of 8-15 workers, with each such sub-group building a separate comb. In even larger groups, that is, in ABBs housing up to 100 workers, no building occurs.

In the upper part of the inside of each cell in the built comb there is a corpuscle containing a high concentration of iron and titanium, apparently in the form of FeTiO₃. We conjecture that this is a keystone-like crystal inserted in the zenith of the cell and indicating the direction of further building downwards in the cell.

Other species of social wasps/hornets do not usually build new cells in the absence of a queen. The sense of gravitation was investigated also in adult hornets. We found that when placed on a closed horizontal flat, it suffices to tilt the flat by as little as 1° for some of the workers to commence climbing up the slant. The Oriental hornet is found to possess an organ between the frons plate and the protocerebrum. This organ boasts nerve fibers which lend it the appearance of a harp and on these fibers there are rounded, plummet-like bodies. In the interior part of the organ there are abundantly ciliated flats, whose cilia can morphologically be assigned to three different groups. This organ has been named the Ishay organ and it probably serves as the gravitation-sensing organ in hornets. It is encountered in all the Vespinae and, not surprisingly, all the species in this group build in the direction of the gravitational force. We consider the possible intervention in the process of comb building also of the vertical vector of the magnetic field of the earth. We have previously carried out an experiment geared to study hornet comb building in outer space; however, our hornets did not build combs in space, while those returning from their sojourn in space built in our laboratory combs comprised of cell which at first were of greater dimensions than normal and subsequently were of smaller dimensions than normal, but invariably the building was gravity-directed. The time constant for detecting the gravity orientation is a matter of seconds but the change of building orientation is a matter of days.