The nature of allometry in an exaggerated trait: The postocular flange in Platyneuromus Weele (Insecta: Megaloptera)

Abstract

The origin and function of exaggerated traits exhibited by a great number of species with sexual dimorphism remain largely unexplored. The usual model considered as the evolutionary mechanism for the development of these structures is sexual selection. The nature of growth of the postocular flange (POF) in three species of the dobsonfly genus Platyneuromus (Megaloptera, Corydalidae, Corydalinae) is analyzed to explore sexual size dimorphism and allometric scaling. Results involve positive allometry of POF in males of two species, and negative allometry in males of one species, in general with a female-biased sexual dimorphism. We suggest an ancestral condition of dual incipient ornamentation in Platyneuromus, with a subsequent departure of size and shape of POF in males, triggered by sexual selection. Different sexual selection intensities may explain the parallel or divergent growth of POF within the scheme of dual ornamentation. Empirical behavioral data as well as a phylogenetic framework are necessary to clarify possible causes of phenotypic development, time of origin, and evolution of the POF.

Fig 1. Head of Platyneuromus spp.

1a-c) P. honduranus, 2a-c) P. soror, 3a-c) P. reflexus.

Fig 2. Measures and landmarks.

a) Measures: BODY: interantennal distance (IAD), interocular distance (IOD), anterior wing length (AWL); POSTOCULAR FLANGE: mesial width (MW), diagonal length (DL), and postocular spine length (SL). b) Landmarks in yellow.

Fig 3. PCA of the three Platyneuromus species.

Measurements of body and postocular flange are included for both sexes.

Fig 4. Analysis of pairwise correlation.

Body measures are IAD, IOD, and AWL; postocular flange measures are MW, DL, and SL.

Fig 5. Allometric growth in the males of the genus Platyneuromus.

Fig 6. Allometric growth in the females of the genus Platyneuromus.

Fig 7. Allometric growth in P. honduranus.

The developmental strategies correspond to a divergent growth trajectory. Linear fit male MW = -0.528809 + 2.4817574*IAD; female MW = -0.219867 + 0.9724537* IAD.

Fig 8. Allometric growth in P. soror.

The developmental strategies correspond to a divergent growth trajectory. Linear fit male MW = -0.729184 + 2.2514065* IAD; female MW = -0.014324 + 0.6217066* IAD.

Fig 9. Allometric growth in P. reflexus.

The developmental strategies correspond to a nearly parallel growth trajectory. Linear fit male MW = -0.023069 + 0.8791506* IAD; female MW = -0.34204 + 1.3501466* IAD.

Fig 10. ANOVAs between all traits in each species with respect to sex.

In the measures of body size there is a generally female-biased pattern of sexual size dimorphism, while in the POF measures, males are generally larger.

Fig 11. Geometric morphometric regression between centroid size and Procrustes coordinates.

The lines show the fit of each species and the thin plate splines represent the Procrustes deformation of the POF from the smallest females (open circles) to the biggest males (closed circles). The POF is represented by six landmark coordinates.