Research Article |
Corresponding author: Chengquan Cao ( chqcao1314@163.com ) Academic editor: Ben Price
© 2019 Chengquan Cao, Pei Yu, Fumio Hayashi.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Cao C, Yu P, Hayashi F (2019) Allometry and morphological trait relationship in the sexually dimorphic Chinese dobsonfly, Acanthacorydalis asiatica (Wood-Mason, 1884) (Megaloptera, Corydalidae). ZooKeys 854: 119-129. https://doi.org/10.3897/zookeys.854.32897
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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. Acanthacorydalis asiatica 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.
Male genitalia, morphological trade-off, sexually selected trait, static allometry
Many groups of animals develop secondary sexual traits mostly in males but a few in females (reviewed by
The order Megaloptera is a minor insect group including only two families, 35 genera, and 397 species in the world (
We also examined the allometric relationships of male genital size of this species. In Megaloptera, Corydalus bidenticulatus Contreras-Ramos, 1998 is the only species for which the relationship between body and genital size has been studied, and a negative allometry is reported although based on only nine males and statistically marginal at P = 0.05 in correlation analysis (
The genus Acanthacorydalis includes some of the most remarkable dobsonflies in the world by their large body size and elongated male mandibles (
Adult A. asiatica were obtained by rearing large larvae collected from Panzhihua, Sichuan Province, China. These larvae were collected on 12 April 2015 and brought to the laboratory to be kept in large plastic tanks (40 cm wide, 60 cm long, and 20 cm high) in which water obtained from underground was circulated 10 cm in depth. Chironomid larvae and shrimps were made available as food. Fully-grown larvae were replaced to the same-sized tanks but filled with wet soil (5 cm deep) for pupation. These tanks were covered with nets to prevent larvae escaping. When adults emerged, they were kept in a large rearing cage covered with fine wire nets (8 m wide, 25 m long, and 3.5 m high) in which several trees and grasses were planted and an artificial pond was set. Fruit (broken water melon) was given as food because the adult megalopterans can be reared by giving sugar solution, fermented milk, and/or fruit (
The mean value (± SD) and CV (%) were calculated for all measured parameters. As in
Males were larger than females on the average and there was a great size variation in male morphological traits; CV was greater in males than females (Table
The prothorax length (PL), head width (HW), mandible length (ML), wing length (WL), and genital length (GL) of male and female Acanthacorydalis asiatica.
Sex | Males | Females | |||||||
---|---|---|---|---|---|---|---|---|---|
Body part | PL (mm) | HW (mm) | ML (mm) | WL (mm) | GL (mm) | PL (mm) | HW (mm) | ML (mm) | WL (mm) |
N | 31 | 31 | 29 | 30 | 28 | 33 | 33 | 33 | 33 |
Mean | 10.56 | 11.44 | 19.27 | 43.43 | 3.38 | 8.37 | 10.53 | 7.02 | 45.36 |
SD | 1.86 | 1.70 | 5.93 | 4.51 | 0.33 | 0.87 | 0.85 | 0.82 | 4.19 |
CV% | 17.63 | 14.83 | 30.76 | 10.38 | 9.73 | 10.37 | 8.07 | 11.74 | 9.23 |
HW was correlated linearly with PL in log-log relationship both in males (R2 = 0.946, P < 0.0001) and females (R2 = 0.813, P < 0.0001), and the slope of the regression line was 0.829 in males and 0.773 in females (Fig.
ML was always greater in males than females (Fig.
WL was correlated linearly with PL in log-log relationship both in males (R2 = 0.774, P < 0.0001) and females (R2 = 0.664, P < 0.0001), and the slope of the regression line was 0.582, significantly lower than1 (P < 0.01) in males and 0.903, not different from 1 (P > 0.05), in females (Fig.
GL was correlated linearly with PL in log-log relationship in males (R2 = 0.336, P < 0.005), and the slope of the regression line was 0.605 which was significantly lower than1 (P < 0.01), suggesting the ectoproct length shows the negative allometry with male body size (Fig.
Log-log relationships between the prothorax length (PL) and the head width (HW in A), mandible length (ML in B), wing length (WL in C), and genital (ectoproct) length (GL in D) in male and female Acanthacorydalis asiatica. Regression lines: A y = 0.829 x + 0.211 in males and y = 0.773 x + 0.310 in females B y = 1.663 x − 0.428 in males and y = 1.113 x − 0.181 in females C y = 0.582 x + 1.046 in males and y = 0.903 x + 0.823 in females D y = 0.605 x − 0.094 in males. For statistical tests, see the text.
Males of Acanthacorydalis species combat each other for access to females or limited resources (tree sap) that attract females (
For insects, resources used for adult body development are limited to those acquired during larval periods. Males suffer in how they allocate the limited resources to weapons and other body parts of adults. Much allocation to weapons is costly and, hence, trade-offs occur between weapons and other male traits such as wing size (e.g.,
Male genital size of Acanthacorydalis asiatica was only slightly influenced by variation in body size as supported by its lowest CV, and the allometric relationship was negative between body size and genital size. Although based on small sample size, the negative allometry was also obtained in American Corydalus bidenticulatus (
We thank Tamotsu Kusano for helping statistical tests, and Suting Li and Yaqian Fang for assisting field sampling and laboratory rearing. We also thank Atilano Contreras-Ramos for his invaluable comments on the early version of this manuscript.