The soldiers in societies: defense, regulation, and evolution

Area of Science:

• Evolutionary Biology
• Social Insects
• Behavioral Ecology

Background:

• Eusociality is characterized by reproductively altruistic castes, with sterile castes driving its evolution.
• Eusocial lineages are categorized as worker-first or soldier-first, based on the primary adaptive function of the initial sterile caste.
• Worker-first lineages (e.g., Hymenoptera) focus on housekeeping, while soldier-first lineages (e.g., termites) prioritize colony defense.

Purpose of the Study:

• To review the current knowledge on the biology, morphology, adaptive functions, and caste regulation of the soldier caste in eusocial insects.
• To highlight the understudied soldier-first eusocial lineages, including termites, eusocial aphids, gall-dwelling thrips, and snapping shrimp.
• To discuss factors contributing to the evolution of distinct caste systems in eusocial lineages.

Main Methods:

• Literature review synthesizing existing research on soldier castes.
• Comparative analysis of worker-first and soldier-first eusocial lineages.
• Discussion of biological, ecological, and genetic factors influencing caste evolution.

Main Results:

• The distinct functional roles of worker and soldier castes suggest different evolutionary drivers for worker-first and soldier-first eusociality.
• Research on eusocial evolution has predominantly focused on worker-first Hymenoptera, with limited understanding of soldier-first lineages.
• The review consolidates information on soldier caste biology, morphology, function, and regulation.

Conclusions:

• Understanding soldier-first lineages is crucial for a comprehensive view of eusocial evolution.
• Distinct biological, ecological, and genetic factors likely shape the evolution of different caste systems.
• Further research into soldier-first eusocial societies is needed to elucidate the diverse pathways of eusocial evolution.