Conservative whole‐organ scaling contrasts with highly labile suborgan scaling differences among compound eyes of closely related Formica ants

Abstract

Static allometries determine how organ size scales in relation to body mass. The extent to which these allometric relationships are free to evolve, and how they differ among closely related species, has been debated extensively and remains unclear; changes in intercept appear common, but changes in slope are far rarer. Here, we compare the scaling relationships that govern the structure of compound eyes of four closely related ant species from the genus Formica. Comparison among these species revealed changes in intercept but not slope in the allometric scaling relationships governing eye area, facet number, and mean facet diameter. Moreover, the scaling between facet diameter and number was conserved across all four species. In contrast, facet diameters from distinct regions of the compound eye differed in both intercept and slope within a single species and when comparing homologous regions among species. Thus, even when species are conservative in the scaling of whole organs, they can differ substantially in regional scaling within organs. This, at least partly, explains how species can produce organs that adhere to genus wide scaling relationships while still being able to invest differentially in particular regions of organs to produce specific features that match their ecology.

Substantial variability in morphological scaling among bumblebee colonies

“…Allometric intercepts seem to follow more consistent patterns, with only a single difference in mean antenna size. All three organs scaled with negative allometry, and this is consistent with data from many other investigations into insect allometry [ 4 , 42 – 44 ].…”

Differences Between the Allometric Rules Governing Two Formica lugubris Zetterstedt, 1838 (Hymenoptera: Formicidae) Ants Populations in Northern Greece

“…These relationships occur in all nests we studied in both study areas. Seifert (2016) found similar results for F. lugubris, while Perl, Rossoni, & Niven (2017) found grade shifts in the allometric scaling of the compound eye among four Formica species in terms of eye scaling, but no grade or slope shifts in the scaling of mean facet diameter. Also, similar relationships have been observed in other ants such as e.g.…”

Static allometries do not reflect evolutionary allometry in exaggerated weaponry of male New Zealand sheetweb spiders (Cambridgea spp.)

“…Our results, therefore, also suggest that males of smaller species of Cambridgea may be under weaker sexual selection. Differences in allometric slope and intercept across species, likely result from selection acting to differing degrees upon the characteristics of each species' scaling relationships (Eberhard et al, 2018 ; Rico‐Guevara & Hurme, 2019 ) rather than on absolute trait sizes, resulting in changes in slope, intercept and shape (Perl et al, 2017 ). Indeed, changes in scaling relationships can occur over a small number of generations as demonstrated by artificially selecting for specific ratios of trait to body size in fruit flies ( Drosophila melanogaster , Weber, 1990 ), stalk‐eyed flies ( Cyrtodiopsis dalmanni , Wilkinson, 1993 ) and dung beetles ( Onthophagus acuminatus , Emlen, 1996 ).…”