Measurements from Antweb.org
Measurements of worker and sexual size were done on specimens from Antweb.org; images were downloaded and measured using ImageJ2 v2.3.0 (Rueden et al. 2017) relative to the scale printed on the image. Head width was defined as the width of the head as measured across the eyes.
To ascertain whether the data available on Antweb.org were representative of natural variation, Pogonomyrmex harvester ant samples from Antweb.org were compared to head width measurements that were available from other sources. The overlap of worker size variation as measured from Antweb.org was compared to that reported in a taxonomic overview of the genus in North America (Cole, Arthur 1968). A t-test was used to test whether the proportion of overlap between worker size ranges from Antweb.org and Cole (1984) differed from 1. Note thatP. wheeleri measurements from Antweb.org (N = 2) did not overlap Cole’s range and this 0% overlap was excluded from analysis.
Additional head width measurements were available from four species (N = 382 P. badius , N = 38 P. barbatus, N = 433 P. coarctatus , and N = 431 P. rugosus ). These additional samples were measured optically using an SZX7 Stereomicroscope by Olympus with a Retiga 2000R camera by Q-Imagine, and iSolutions Lite software by iMT Technology; samples were from various colonies and populations sampled as parts of other studies. Overlap of Antweb.org measurements with these more exhaustively sampled species was qualitatively compared.