The number of castes in ants, where major is smaller than minor and queens wear the shield of the soldiers

Abstract

The traditional hypothesis that ant soldiers originate from large size workers is rejected in favour of their direct origin from gynes. This conclusion is supported by the first report of soldiers smaller than workers, by an intercaste morphometric comparative analysis by means of D'Arcy Thompson's transformation grids and by phylogenetic studies on Cephalotes showing that the cephalic shield appeared ancestrally among soldiers and only later among gynes. The same conclusion flows from facts already known but hitherto misunderstood and stressed in this paper such as: 1) there are species of Cephalotes with soldiers with incomplete cephalic shield and no gyne shield and other species with complete soldier shield but only incomplete gyne shield; 2) the phragmotic behaviour is exercised by workers and soldiers and not by gynes but its morphological correlate, i.e., the shield-shaped head, occurs among soldiers and gynes and not in workers. These observations prove that the selection pressure for the shield morphology acts on workers and soldiers and not on gynes but its morphological correlate actually appears first in soldiers, later in gynes and never in workers. These data sharply contrast with a worker origin of soldiers and are perfectly consistent with the hypothesis of a separate origin of soldiers directly from gynes.