Termites are social cockroaches. Because non-termite cockroaches are larger than basal termite lineages, which themselves include large termite species, it has been proposed that termites experienced a unidirectional body size reduction since they evolved eusociality. However, the validity of this hypothesis remains untested in a phylogenetic framework. Here, we reconstructed termite body size evolution using head width measurements of 1638 modern and fossil termite species. We found that the unidirectional body size reduction model was only supported by analyses excluding fossil species. Analyses including fossil species suggested that body size diversified along with speciation events and estimated that the size of the common ancestor of modern termites was comparable to that of modern species. Our analyses further revealed that body size variability among species, but not body size reduction, is associated with features attributed to advanced termite societies. Our results suggest that miniaturization took place at the origin of termites, while subsequent complexification of termite societies did not lead to further body size reduction.

Data S1. The list of literature for head width measurements. 

Data S2. Datasets for head width measurements. 

Data S3. The list of mitochondrial genome sources for phylogeny.

Data S4. The nexus file with MrBayes block to reproduce the phylogeny.

Data S5. The phylogenetic tree used in the analyses.

Data S6. R source codes and Rdata for the analyses, which includes 210222_Functions.R, 210611_analysis-code.R, ColonySize.rda, HeadWidthData.rda.