Caste

Definition

A morphologically and behaviorally distinct subset of individuals within a eusocial insect colony, characterized by specialized roles in reproduction, foraging, defense, or nest maintenance. Castes typically arise through differential gene expression, nutrition, or environmental cues during development, rather than through genetic differences. The major functional divisions are reproductive castes (queens, kings, fertile males) and sterile worker castes, with the latter often further subdivided by size, age, or task specialization.

Etymology

From Portuguese casta 'breed, race, lineage,' from Latin castus 'pure, separated'; applied to social insects by analogy with human social stratification systems.

Example

In the leafcutter ant Atta cephalotes, four distinct worker castes exist—minims, minors, mediae, and majors—ranging from tiny gardeners tending fungal gardens to large soldiers with powerful mandibles specialized for cutting vegetation and colony defense.

Synonyms

• morph
• worker class
• social morph

Related Terms

• eusociality
• Polyphenism
• Queen
• Worker
• soldier
• ergate
• Gyne
• task allocation
• division of labor
• superorganism

Usage Notes

In entomology, 'caste' refers strictly to phenotypic differentiation within a colony, not genetic subpopulations; contrast with 'morph' which may describe any discrete form variant. Some specialists reserve 'caste' for genetically identical individuals (as in Hymenoptera) and use 'class' or 'morph' when genetic differences exist (as in termite reproductives). The term is occasionally extended to non-insect eusocial arthropods such as amblypygid colonies with cooperative brood care, though this usage remains debated.