Copulatory courtship by bushcricket genital titillators revealed by functional morphology, μCT scanning for 3D reconstruction and female sense structures

Abstract

Genitalia are rapidly evolving morphological structures most likely under sexual selection. Due to their internal nature they are often hidden inside the body, thus morpho-functional studies of animal genitalia are broadly lacking. Males of some bushcricket taxa bear paired genital appendices called titillators, the exact function of which is unknown since they are obscured inside the female body during pairing. To investigate titillator morphology and possible function during copulation, we studied the bushcricket Metrioptera roeselii (Hagenbach, 1822) using a novel combination of independent, yet complementary, techniques. Copulating pairs were snap-frozen and scanned by X-ray micro-computed tomography (μCT) to visualize the coupling of male and female genitalia in situ. Video recordings of copulating pairs also showed rhythmical insertion of male titillators into the female's genital chamber, where they percuss a softened structure on the female's subgenital plate. Movements did not induce damage to the female's structure, which lacks any sclerotized genital counterparts. Instead, scanning electron microscopy and histological sections show the female subgenital plate to be covered with two different types of sensory receptors at the contact zone between the male's titillator and the female genital chamber. We interpret the non-harmful function of the titillator processes, the lack of a genital counter-structure and the presence of sensory cells on the female's subgenital plate as indicators of a copulatory courtship function of titillators, subject to sexual selection by female choice.

ErechthisKatydids (Tettigoniidae: Conocephalinae) in the Caribbean: New Species from the Bahamas and Hispaniola

“…Such structural specificity in genitalic structures is commonplace in insects and usually explained in terms of sexual selection. For example, a recent comparative study of phallus diversity among katydids suggests certain specific structures (e.g., titillators and other modified ventral sclerites) may function to ensure female mating via cryptic choice (Chamorro-Rengifo & Lopes-Andrade 2014, Vahed 2015, Wulff et al 2015. Many katydid genitalic structures must also be under natural selection, e.g., for correctly positioning the spermatophore for transfer (Heller & Liu 2015).…”

The evolution of female genitalia

“…The types of morphological and/or cellular detail expected to be seen in female genitalia that have evolved to facilitate and assess male genital structures might be morphologies that are more cooperative than defensive, or those that are composed of sensory cells. Three study systems that have utilized microscopy, histological and electrophysiological techniques to visualize male and female genital structures in such a way as to elucidate function are tsetse flies (Briceño & Eberhard, ), bushcrickets (Wulff & Lehmann, ; Wulff, Lehmann, Hipsley, & Lehmann, ; Wulff, Schöneich, & Lehmann, ; Wulff, van de Kamp, dos Santos Rolo, Baumbach, & Lehmann, ) and damselflies (Córdoba‐Aguilar, , , , ).…”

“…The female subgenital plate is covered by a soft membrane and is composed of numerous sensory cells (Figure ; Wulff & Lehmann, ). The female subgenital plate does not appear to present as a defensive structure and is not harmed by males during copulation (Wulff et al., ). These characteristics might be expected of female genitalia that were involved in Fisherian cryptic female choice.…”

“…(, ) on damselfly genital evolution. Other studies that utilize 3D technology to visualize and study female genitalia include those on millipedes, seed beetles, bushcrickets, mantophasmatodeans, barklouse and spiders (Cheng & Yoshizawa, ; Dougherty & Simmons, ; Küpper, Klass, Uhl, & Eberhard, ; Schendel, Junghanns, Bilde, & Uhl, ; Wojcieszek et al., ; Wulff et al., ). Some studies have pioneered the use of micro‐computed tomography (micro‐CT) to obtain 3D images.…”

“…Not only does 3D technology allow the visualization of the diverse form of female genitalia across species, but it allows us to better understand the way in which female genitalia interact with the genitalia of males and even has the potential to allow the elucidation between different mechanisms of genital evolution, particularly cryptic female choice and sexual conflict. A good example comes from bushcrickets, where scanning electron microscopy allowed the visualization of sensory cells and hairs on the female subgenital plate, indicative of this structure being used to evaluate male stimulatory quality and thus function more as a means of cryptic female choice than as a defensive structure that has resulted from sexual conflict (Figure ; Wulff et al., ).…”