CASTE AND REPRODUCTION IN ANTS: NOT ALL MATED 1 
EGG-LAYERS ARE “QUEENS”* 
By Christian Peeters and Ross H. Crozier 
School of Biological Science, University of New South Wales, 
P.O. Box 1, Kensington NSW 2033, Australia. 
The existence of two classes of adult females is characteristic of 
the highly-eusocial insects, which comprise termites, ants, various 
bees (Bombus, Apis , Meliponini) and vespine wasps. Queen and 
worker castes differ phenotypically as a result of morphological 
adaptations for efficient reproduction (dispersal, egg-laying) and 
maintenance activities respectively. Reproductive role partitioning 
in highly-eusocial species is specified by caste membership, but 
exceptions exist (for example, ponerine ants without queens). By 
contrast, in primitively-eusocial insects, adult females are all similar 
in form. Individual differences in size often occur as a result of 
environmental variations during larval growth (such as nutrition) 
and, together with age and insemination, are the basis for reproduc- 
tive differentiation (reviewed by Wheeler 1986). Thus, although 
reproductive division of labor is a feature of both primitively- and 
highly-eusocial insects, it is achieved in two distinct ways: role dif- 
ferentiation among monomorphic adults, or production of alterna- 
tive adult phenotypes. This dichotomy is not reflected by the current 
use of “queen,” “worker” and “caste.” Each of these terms has alter- 
native meanings, and this, we suggest, obscures various evolution- 
ary processes associated with eusociality. 
The two meanings of caste 
Dimorphic adult females are produced by divergent developmen- 
tal pathways coordinated by endocrine signals, and this involves the 
expression of different sets of genes (see Wheeler 1986, Craig and 
Crozier 1978, West-Eberhard 1986). “Caste” has been used (as early 
as Latreille 1802) to distinguish these distinct female phenotypes. 
However, “caste” has also become a synonym for the separation of 
reproductive and sterile roles (e.g. Michener 1985: 303; Wilson 1985: 
308; Fletcher and Ross 1985), or it sometimes serves to describe the 
* Manuscript received by the editor July 12, 1988. 
283 
