Allometry and morphological trait relationship in the sexually dimorphic Chinese dobsonfly, Acanthacorydalis asiatica (Wood-Mason, 1884) (Megaloptera, Corydalidae)

Abstract

Male insects with large weapons such as horns and elongate mandibles would be expected to invest more on such structures than other parts of the body for advantages in male to male competition for mating. In male genitalia, however, intermediate size provides a better fit for more females than small or large sizes, and such a male would leave more offspring regardless of their body size. These predictions were tested using a static allometry analysis between body size and other trait sizes. Acanthacorydalisasiatica is a large dobsonfly (Megalotera) and males have conspicuously large mandibles used as weapons. We examined the hypothesis that the male mandibles of this sexually dimorphic species are sexually selected to enlarge, whereas the male genitalia are stable to be intermediate regardless of a great variation in body size. The results, as predicted, showed positive allometry between male body size and mandible length but negative allometry between male body size and ectoproct length (a male grasping structure). Sperm are transferred through a small spermatophore attached externally to the female genital opening, so it may be evolutionarily unnecessary to develop an enlarged male genital size. In contrast, there may be a trade-off between male mandible size and wing length, because of negative allometry between body size and wing length in males but isometry between them in females.

Allometry of exaggerated mandibular tusks in the burrowing mayfly Rhoenanthus coreanus (Ephemeroptera: Potamanthidae)

“…Measurements of the female larvae were consistently larger than those of the male larvae, which is a common trend among arthropods (Teder & Tammaru 2005;Kelly 2014;McCambridge et al 2018). In most insects, exaggerated traits form because of positive allometry (Petrie 1992;Cao et al 2019;Ramírez-Ponce et al 2017). In R. coreanus, positive allometry was only observed in female larvae, thereby indicating that tusks in females exhibit disproportionately higher rates of growth (Painting & Holwell 2013).…”

“…In most insects, exaggerated traits form because of positive allometry (Petrie 1992; Cao et al . 2019; Ramírez‐Ponce et al . 2017).…”

“…1) were measured to the nearest 0.01 mm. The PL measurement was used as an indicator of body size since it has been widely and successfully used in other allometry studies on insects and arthropods (Palestrini et al 2000;Ramírez-Ponce et al 2017;McCambridge et al 2018;Cao et al 2019).…”

“…Interestingly, insect growth can sometimes progress disproportionately and, thus, result in the generation of exaggerated traits (i.e. positive allometry; Painting & Holwell 2013;Lavine et al 2015;Ramírez-Ponce et al 2017;McCambridge et al 2018;Cao et al 2019). Furthermore, in many animals, the development of such exaggerated morphological traits (e.g.…”

“…2018; Cao et al . 2019). Furthermore, in many animals, the development of such exaggerated morphological traits (e.g.…”