Models of Enallagma cerci.
Damselflies, like all odonates, have a very interesting breeding system. Before they actually copulate, males and females of many groups spend some considerable time in physical contact with one another, presumably in an effort to assess their potential mate. In the figure to the left, a male Enallagma aspersum is holding a female E. aspersum. This physical contact is made by the male clasping the female's thorax with the four terminal appendages on the end of his abdomen.
The male's terminal abdominal appendages consist of two superior cerci and two inferior paraprocts. The upper figure to the right is a computer generated model of the tenth abdominal segment of a male E. ebrium viewed from the posterior. These four appendages grasp the female's thorax while they are in tandem. The lower figure to the right is a computer generated model of the females thorax viewed from above. The male's paraprocts contact her prothorax and the male's cerci fit into the two plates on the anterior surface of her mesothorax - the mesothoracic plates. These mesothoracic plates are thought to be used by the female to assess the species identity and potentially the suitability of the male that is grasping her in tandem.
Field studies have shown that females discriminate among males based on cerci shape (Paulson 1974, Robertson and Paterson 1982, Fincke et al. 2006). Also, males with experimentally altered cerci are rejected by conspecific females (Robertson and Paterson 1982). Thus, the morphologies of these male and female structures seem to be critical for mate recognition, and so the evolution of these structures may play a vital role in speciation in Enallagma. These structures are akin to a lock-and-key mechanism, with each sex potenitially evaluating the fit between the male's cerci with the contours of the female's mesothoracic plates.
In our current studies we are using computer tomographic (CT) imaging to generate 3-dimensional models of male cerci and female thoraces, and building virtual 3D models of these parts using spherical harmonics to understand how male and female damselflies make mating decisions. The spherical harmonic model provides a quantitiative description of a 3D shape in a multidimensional phenotypic space.
Our first analysis is to reconstruct the evolution of the male cerci over the 15 million year history of the Enallagma clade. We have applied Evolutionary Contrasts Analyses to the spherical harmonic models to reconstruct the shapes of cerci across the phylogeny of the Enallagma, and to estimate the rates of evolution in 3D shape across the history of the clade. You can see and interact with these models here.
Fincke, O. M., A. Fargevieille, and T. D. Schultz. 2007. Lack of innate preference for morph and species identity in mate-searching Enallagma damselflies. Behavioral Ecology and Sociobiology 61:1121-1131.
Paulson, D. R. 1974. Reproductive isolation in damselflies. Systematic Zoology 23: 40-49.
Robertson, H. M. and H. E. H. Paterson. 1982. Mate recognition and mechanical isolation in Enallagma damselflies (Odonata: Coenagrionidae). Evolution 36:243-250.