Penetration mechanics of elongated female and male genitalia of earwigs

Yoshitaka Kamimura,Chow-Yang Lee,Yoko Matsumura,Stanislav N Gorb,Hamed Rajabi

doi:10.1038/s41598-021-86864-1

Abstract

We unveiled the penile penetration mechanics of two earwig species, Echinosoma horridum, whose intromittent organ, termed virga, is extraordinarily long, and E. denticulatum, whose virga is conversely short. We characterised configuration, geometry, material and bending stiffness for both virga and spermatheca. The short virga of E. denticulatum has a material gradient with the stiffer base, whereas the long virga of E. horridum and the spermathecae of both species are homogeneously sclerotised. The long virga of E. horridum has a lower bending stiffness than the spermatheca. The virga of E. denticulatum is overall less flexible than the spermatheca. We compared our results to a previous study on the penetration mechanics of elongated beetle genitalia. Based on the comparison, we hypothesised that the lower stiffness of the male intromittent organ comparing to the corresponding female structure is a universal prerequisite for the penetration mechanics of the elongated intromittent organ in insects.

Highlights

We aimed to test the generality of the abovementioned functional principles demonstrated and hypothesised based on the cassidine beetle genitalia and addressed the following questions: (1) Are the functional principles limited to the beetles? (2) Are they correlated to the genital length? (3) Do these functional principles depend on the mechanical properties of corresponding female structures? For this purpose, here we chose a distantly related insect taxon, order D ermaptera[19] and compared their genitalia with the cassidine beetle genitalia
We focused on two pygidicranid earwig species Echinosoma horridum Dohrn, 1863, whose intromittent organ, termed virga, is long and narrow[20], and E. denticulatum Hincks, 1959, whose homologous structure is s hort[21,22]
The penis lobe of E. horridum males does not carry any spiny auxiliary sclerites, unlike that of E. denticulatum

Summary

Introduction

We focused on two pygidicranid earwig species Echinosoma horridum Dohrn, 1863, whose intromittent organ, termed virga, is long and narrow[20], and E. denticulatum Hincks, 1959, whose homologous structure is s hort[21,22]. We characterised their geometry and analysed material distributions within male and female interacting genital structures using imaging and microscopy techniques, e.g. scanning electron microscopy and confocal laser scanning microscopy. We determine the general principles of the penetration mechanics of the genitalia in earwigs and compare them with the previous studies on the beetle genital

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Conclusion