The Origin and Evolution of Polymorphism in Ants

Abstract

Preponderant evidence indicates that in ants sex is detd. by the genetic mechanism of haplo-diploidy, and female castes are detd. by larval nutrition. In complete dimorphism of the female castes there probably exists a critical developmental time at which the imaginal discs assume 1 or the other of 2 alternative specific growth potentials, depending on whether or not the larva has reached a certain threshold size. In incomplete dimorphism, whether in the more primitive types of worker subcaste differentiation, or in the rare cases of queen-worker intergradation, the females form a gradient series which exhibits simple or modified allometry. For this reason polymorphism is defined in terms of allometry and its lower limits are set as follows: allometry occurring over a sufficient range of size variation within a normal mature colony to produce individuals of detectable different form at the extremes of the size range. The trend of the evolution of worker polymorphism is 1st toward allometric differentiation along a gradient size series and 2d toward dimorphism, or the segregation of 2 size groups with no intermediates. The worker caste probably arose from the queen by the same mechanism. Queen-worker intergradation shows diphasic allometry in the alitrunk and abdomen and monophasic allometry in the head. The differential growth underlying at least the gross features of polymorphism in adult ants must occur during the brief period of the proliferation and development of the pupal hypodermis before the last larval ecdysis.