Endless forms most bloated: insights on diversity and evolutionary significance of post-imaginal growth in termitophilous beetles

Abstract Ecological interactions have long driven evolutionary diversification, and among the most striking examples are the symbiotic associations between termites and their associated fauna. Within this framework, many termitophiles exhibit extreme morphological specializations, none more emblematic than the phenomenon of physogastry—a post-imaginal modification of the abdomen that occurs in various termitophilous beetles. This study provides a theoretical framework exploring hypotheses on the evolutionary origins and ecological significance of post-imaginal growth in termitophiles, drawing on an extensive review of their morphology, behavior, and natural history. We examine its potential origins as a byproduct of trophallaxis and hormonal ingestion, its adaptive significance in host integration, and its role in shaping phenotypic plasticity in these highly modified beetles. Furthermore, this work briefly discusses the taxonomic and phylogenetic implications of post-imaginal growth, discussing how ontogenetic transformations may obscure species boundaries and affect systematic classification. By synthesizing multiple lines of evidence, we argue that post-imaginal growth represents a key innovation in termitophile evolution, facilitating deep integration into termite societies while also imposing significant trade-offs in dispersal. This process of post-imaginal development ultimately led to the evolution of a diphasic life cycle, characterized by an extranidal stenogastric dispersive morphotype and an intranidal parasitic physogastric morphotype through divergent natural selection.