Jurassic scorpionflies (Mecoptera) with swollen first metatarsal segments suggesting sexual dimorphism

Abstract

Background: Sexual dimorphism is widespread in insects. The certain specialized structures may be used as weapons in male-male combats or as ornaments to enhance mating opportunities.

Results: We report striking swollen first tarsal segments in two families, four genera and six species of scorpionflies from the Middle Jurassic Yanliao Biota of Northeastern China. Swollen tarsal segments are restricted to male specimens and to hind leg tarsi. The geometric morphometric analyses reveal that the degree of swelling within the orthophlebiid species possessing swollen first metatarsal segments is species-specific, which can be used as a diagnostic character for taxonomic and phylogenetic studies.

Conclusions: The new findings indicate that swollen first metatarsal segments are relatively common in the family Orthophlebiidae during the Middle Jurassic. The tarsal swellings are considered to be sexually dimorphic, potentially associated with sexually display by males and/or camouflage of a "nuptial gift" in the mating process.

Keywords: Fossil insect; Holcorpidae; Mesozoic; Nuptial gift; Orthophlebiidae; Tarsal swelling; Yanliao Biota.

Fig. 1

Photographs and line drawing of the holotype of Conicholcorpa longa sp. nov. a, b Habitus and line drawing of part for CNU-MEC-NN2015108p. c Habitus of counterpart for CNU-MEC-NN2015108c. d Swollen first segment of the metatarsus of (c). e Under ethanol. f Enlarged part of (e). Scale bars represent: 4 mm in (a–c); 0.5 mm in (d–f). A6–A8: the sixth to eighth abdominal segments

Fig. 2

Comparison of non-swollen versus swollen first metatarsal segments. a, b Non-swollen, Orthophlebia extensa (CNU-MEC-NN2014059). c, d Slightly swollen, Orthophlebia elenae (CNU-MEC-NN2014005). e, f Moderately swollen, Juraphlebia eugeniae (CNU-MEC-NN2014009). g, j Heavily swollen, Mesopanorpa luanpingensis (CNU-MEC-NN2016222). h, i Extremely swollen, O. extensa (CNU-MEC-NN2006046). b, d, f, h, j Habitus, first metatarsal segments of (a, c, e, g, i). Scale bars represent: 4 mm in (a, c, g, h, j); 0.5 mm in (b, d, e, g, i)

Fig. 3

Comparison of non-swollen versus swollen first metatarsal segments. a, b Non-swollen, Mesopanorpa densa (CNU-MEC-NN2016270). c, d Heavily swollen, Mesopanorpa luanpingensis (CNU-MEC-NN2016229). e, f Slightly swollen, Orthophlebia elenae (CNU-MEC-NN2014020). g, h Moderately swollen, Mesopanorpa densa (CNU-MEC-NN2016253). i Artist’s reconstruction of orthophlebiids with the first metatarsal segment slightly swollen (male at left) or extremely swollen (male at center) (credit: Dr. Chen Wang). b, d, f, h Habitus, first metatarsal segments of (a, c, e, g). Scale bars represent: 4 mm for (a, c, e, g); 0.5 mm for (b, d, f, h)

Fig. 4

Rank order plot of the aspect ratio (AR) of each scorpionfly taxon. Note wide variations in AR of swollen tarsi relative to non-swollen ones. The orange lines denote swollen tarsi, and blue lines represent non-swollen ones. s = number of specimens having swollen tarsi; n = number of specimens having non-swollen tarsi. The data for O. longicauda is obtained from Willmann and Novokshonov (1998). y-axis: Species; x-axis: Aspect ratio (AR)

Fig. 5

Geometric morphometric analyses (GMA) of the first metatarsal segments in non-swollen and swollen morphotypes. Swollen metatarsi have higher and more variable values for both (Procrustes distance)² and spline bending energy, and each taxon group has a relatively certain range

Fig. 6

Landmark points for geometric morphometric analyses and graphic representation of geometric morphometric methods. a–f The first metatarsal segment is represented by two curves, each comprising 50 landmark points. g–l Arrows indicate Procrustes distance and Spline bending energy relative to the reference specimen, Juraphlebia eugeniae (CNU-MEC-NN2014038). a, g Non-swollen, Orthophlebia extensa (CNU-MEC-NN2014059). b, h Slightly swollen, O. elenae (CNU-MEC-NN2014005). c, i Moderately swollen, Juraphlebia eugeniae (CNU-MEC-NN2014009R). d, j Heavily swollen, Mesopanorpa luanpingensis (CNU-MEC-NN2016222). e, k Extremely swollen, O. extensa (CNU-MEC-NN2006046). f, l Slightly swollen, the new taxon of Conicholcorpa longa Zhang, Shih & Ren sp. nov. (CNU-MEC-NN2015108)