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[Vol. 95 
partitioning of non-reproductive activities among workers (e.g. 
brood care, foraging). In his major contribution (outlined in Wilson 
1985) to the study of the ergonomic design of colony organization in 
ants, E. O. Wilson has adhered to a functional concept of caste: “a 
set of colony members. . .that specialize on particular tasks for pro- 
longed periods of time.” This definition stems from the need, for the 
purposes of optimization studies, to define age groups (= “temporal 
castes”) as equivalent to morphological castes (Wilson 1968). Thus 
“caste,” which originally denoted alternative female phenotypes, is 
now also used solely to describe role. This leads to ambiguity in the 
literature, because to some authors “caste differentiation” refers 
simply to reproductive division of labor, while to others it refers to 
morphological dimorphism. We suggest that it is useful to restrict 
“caste” to denote groups of female adults which have distinct pheno- 
types following pre-adult differentiation. This usage will give proper 
emphasis to the significance of morphological specialization, which 
is characteristic of the highly-eusocial insects. “Caste” should not be 
used to describe groups of workers whose behavior is age- 
correlated, or fertile as opposed to sterile females. Age-correlated 
behavior occurs throughout the animal world (Caro and Bateson 
1986) , and should not be equated with dimorphism in morphology. 
A terminology based on form or function? 
The equivocal use of “caste” is paralleled by that of “queen” and 
“worker”. In highly-eusocial insects, “queen” denotes the existence 
of a developmentally-distinct reproductive caste with specialized 
morphological traits, except that in various bumblebees queen- 
worker dimorphism is limited to a set of physiological changes (Ro- 
seler 1977). In contrast, Michener (1974: 373), Fletcher and Ross 
(1985) and others use “queen” to describe role (“colony member that 
is primarily active in egg-laying and relatively or totally inactive in 
foraging”). The use of this operational criterion is common in 
primitively-eusocial bees and wasps, and thus authors studying dif- 
ferent taxonomic groups differ in their use of “queen”. This needs 
not be ambiguous to non-specialist readers provided that the 
absence of (phenotypic) castes is made explicit. We are concerned 
however that “queen” is also used in a functional sense in various 
highly-eusocial species in which secondary modifications have 
resulted in caste and reproductive role being no longer concurrent. 
