Not simply red: Colouration of red wood ant Formica rufa (Hymenoptera: Formicidae) is polymorphic, modular and size-dependent

Abstract

Insects produce pigment and structural colours mainly for camouflage, signaling, physical protection or temperature regulation, and colour patterns can provide information about individual quality. Although the evolutionary function and nature of the variability in colouration are well known for many invertebrate taxa, there is little information on this topic for ants. We studied individual variation in the melanin-based colour traits of workers of the red wood ant, Formica rufa (Hymenoptera: Formicidae), from 20 colonies in Southern Finland and revealed the type of colouration in this species. First, using the threshold approach we distinguished between continuous and discrete variations. Furthermore, the analyses affirmed nine discrete morphs in terms of the colouration on the head and eight on the pronotum, while only continuous variation were found on the other body parts. Measuring the size of a particular colour pattern, the intensity of colour expression (degree of melanization) and statistical analyses allowed an assessment of the intra-individual variation in both discrete and continuous patterns. The results revealed substantial modularity in the above mentioned colouration traits. In workers of F. rufa there were individuals with a dark head and light coloured thorax and vice a versa. Size of the dark pigment colour patterns exhibited less modularity than the degree of melanization. Finally, the interrelation between colouration traits and individual body size revealed their size-dependent origin. Small individuals had relatively larger areas of colour on the head and thorax than big individuals. These results are likely to facilitate further taxonomical and ecological studies on red wood ants, as they show it is possible to assess colouration traits in ants. However, more studies are needed on the function of polymorphism and modular colouration in this group of ants.