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.

Taxonomy of the ant genus Carebara Westwood (Formicidae, Myrmicinae) in the Malagasy Region

“…n. and C. nosindambo ). We did not report any major worker smaller than the minor worker, but this phenomenon may occur in other species of Carebara (Baroni Urbani 1998). The main differences in the morphology of these intermediates include: size differences in the head, posterolateral corners of head, and thorax; larger major workers with one to three ocelli (ocelli are reduced compared to the queen caste); reduced flight sclerites; and eye size differences.…”

How ecology shapes caste evolution: linking resource use, morphology, performance and fitness in a superorganism

“…the exit hole of the original occupant; Creighton, 1963; Powell, 2008). Cephalotes defend their cavity entrances by blocking them with their armoured heads, both individually and co‐operatively, and extant species span four discrete character states in the evolution of a soldier caste specialized for this task (Creighton, 1963; Creighton & Nutting, 1965; Baroni Urbani, 1998; de Andrade & Baroni Urbani, 1999; Powell, 2008). These character states, in the order of increasing morphological specialization are: (1) no soldier (ancestral), (2) soldiers with a simple domed head, (3) soldiers with an incomplete head disc that retains characteristics of the domed head and (4) soldiers with an elaborate and complete head disc (Baroni Urbani, 1998; de Andrade & Baroni Urbani, 1999).…”

“…a soldier caste specialized for defence). They have evolved independently in the ants (Hölldobler & Wilson, 1990; Baroni Urbani, 1998), aphids (Stern, 1994), polyembryonic wasps (Cruz, 1981), termites (Thorne et al. , 2003; Thompson et al.…”

Ontogeny of worker body size distribution in bumble bee (Bombus impatiens) colonies

“…In some insects, such as the honey bee ( Apis mellifera ), workers are similar in size and shape but differ in physiological attributes, like brain organisation, hormone levels and gene expression levels related to age (Robinson, 1987; Farris et al , 2001; Ben‐Shahar et al , 2002; Whitfield et al , 2003), all of which contribute to different behaviors (Lindauer, 1953; Seeley, 1982; Robinson et al , 1994; Fjerdingstad & Crozier, 2006). In other insects, like some ants (Family: Formicidae), workers differ in more obvious morphological ways (Noirot & Pasteels, 1987; Hölldobler & Wilson, 1990; Stern & Foster, 1997), such as the dome‐shaped head of some workers of the ant Cephalotes (Baroni Urbani, 1998; Powell, 2008) or the differently‐sized workers within a colony of leaf cutter ants (Hölldobler & Wilson, 1990). All of the differentiations above contribute to variation in behaviours, which ultimately lead to division of labour (Beshers & Fewell, 2001) and to the functioning of the colony as a complex system.…”