Visual attraction cues associated with tachinid pollination: insights from colour variation in Succisella microcephala (Caprifoliaceae)

Pollination syndromes describe convergent floral traits linked to specific pollinator groups. While conceptually useful, the application of this framework to diverse insect assemblages, such as Diptera, remains challenging due to their functional heterogeneity. Recent studies have therefore proposed narrowing dipteran pollination into more precise syndromes. In this context, preliminary field observations of flower visitors to Succisella microcephala suggest a potential case of specialisation towards tachinid flies. Despite its generalist-like floral morphology, S. microcephala exhibits geographic variation in corolla colour and contrasting pigmentation between the corolla tube and lobes. Field observations revealed that populations with darker, more contrasting, flowers were predominantly visited by tachinid flies, which were particularly abundant at higher elevation sites, where Deschampsia cespitosa, an important food plant for moth larvae – the main larval hosts of tachinids – was also abundant. These observations suggest a potential adaptive relationship between floral pigmentation and tachinid attraction. Comparative evidence from other European taxa (e.g., Neotinea ustulata) further supports the hypothesis of shared, visually mediated traits favouring tachinid attraction and pollination. Additionally, the proximity between flowering and fruiting structures, and the striking resemblance between the dark red immature diaspores of S. microcephala and the similarly pigmented globular floral structures in plants associated with tachinid pollination, raise the novel possibility of diaspore-mediated pollinator attraction. Here, we suggest that dark colour structures may be associated with tachinid pollination and tachinid flies may act as potential drivers of an as-yet undescribed pollination syndrome. Succisella microcephala represents a promising system to investigate the potential convergence of floral and diaspore traits under pollinator-driven selection and tachinid sensory ecology.