Group mating in Cretaceous water striders

Abstract

Fossilized mating insects are irreplaceable material for comprehending the evolution of the mating behaviours and life-history traits in the deep-time record of insects as well as the potential sexual conflict. However, cases of mating pairs are particularly rare in fossil insects, especially aquatic or semi-aquatic species. Here, we report the first fossil record of a group of water striders in copulation (including three pairs and a single adult male) based on fossils from the mid-Cretaceous of northern Myanmar. The new taxon, Burmogerris gen. nov., likely represents one of the oldest cases of insects related to the marine environment, such as billabongs formed by the tides. It exhibits conspicuous dimorphism associated with sexual conflict: the male is equipped with a specialized protibial comb as a grasping apparatus, likely representing an adaptation to overcome female resistance during struggles. The paired Burmogerris show smaller males riding on the backs of the females, seemingly recording a scene of copulatory struggles between the sexes. Our discovery reveals a mating system dominated by males and sheds light on the potential sexual conflicts of Burmogerris in the Cretaceous. It indicates the mating behaviour remained stable over long-term geological time in these water-walking insects.

Keywords: Myanmar amber; group mating fossil; mating behaviour; palaeoecology; sexual conflict.

Figure 1.

The paired water striders Burmogerris rarus gen. et sp. nov., from mid-Cretaceous Burmese amber, under bright-field microscopy. (a) An overall view of the amber piece; red and blue arrows indicate adult males and females, respectively; the middle pair consists of the holotype (NIGP201886, male) and the allotype (NIGP201887, female). (b) Paratypes, NIGP201888 and NIGP201889. (c) Paratypes, NIGP201890 and NIGP201891. Scale bars: 5 mm in (a), 1 mm in (b,c).

Figure 2.

Photomicrographs of Burmogerris rarus gen. et sp. nov. (a) Head and pronotum (NIGP201891). (b) Protibia and protarsus (NIGP201892), write arrows indicate each cluster of pegs. (c) Protibial grasping comb (NIGP201886). (d) Protibia of male (NIGP201886), write arrows indicate each cluster of pegs. (e) Protibia of female (NIGP201889), black arrows indicate the trichobothria-like hairs. (f) Trichobothria-like hairs on mesotibia (NIGP201889). (g) Male genitalia (NIGP201888). (h) Male genitalia (NIGP201890) and female genitalia (NIGP201891). (a,c,f) Under confocal microscopy; (b,d,e,g,h) under bright-field microscopy. ce, compound eye; pr, pronotum; pfe, profemur; pti, protibia; pta, protarsus; msti, mesotibia; cl, claw. Scale bars: 0.2 mm in (a,b,d,e,g,h), 0.1 mm in (c,f).

Figure 3.

Ecological reconstruction of Burmogerris rarus gen. et sp. nov. in the Burmese amber forests during the mid-Cretaceous.