Heads up: evolution of exaggerated head length in the minute litter bug genus <Emphasis Type="Italic">Nannocoris</Emphasis> Reuter (Hemiptera: Schizopteridae)

Evolutionary biologists have long been intrigued by exaggerated morphologies tied to sexual or natural selection. In insects, relatively few studies have investigated the evolution of such traits at the genus level and above and have used comparative phylogenetic methods to do so. We here investigate the interspecific evolution of head length in the minute litter bug genus Nannocoris Reuter based on the first phylogenetic hypothesis of the group (25 ingroup species, five gene regions, 3409 bp) and ancestral state reconstruction. Head lengths in this speciose genus range from approximately one sixth of the total body length to more than a quarter of the body length, while the head and mouthpart (rostrum) lengths are correlated. Different species therefore possess a markedly different reach of the rostrum when extended. The analyses show that head length evolution in Nannocoris is plastic, with head length elongations and reductions occurring in several clades, derived from ancestors with moderately elongated heads. Evidence is provided that exaggerated head lengths evolved through elongation of either the genal (pricei group) or the tip (arimensis group) region of the head. The biology of species in the genus Nannocoris is unknown, but given the lack of sexual dimorphism of head lengths, we speculate that head evolution in this genus may be driven by natural selection, potentially in the context of prey capture.