MATE RECOGNITION AND MECHANICAL ISOLATION IN ENALLAGMA DAMSELFLIES (ODONATA: COENAGRIONIDAE).

Abstract

The concept of mechanical isolation, first mooted by Dufour (1844), became popular when Dobzhansky (1940, 1951) developed fully the theory of speciation by reinforcement. Very little empirical evidence was produced however, and as a result, both Mayr (1963, 1970) and Dobzhansky (1970) concluded that mechanical isolation was relatively unimportant in the maintenance of species integrity. This concept remains valid, however, within the theory of speciation by reinforcement, and the isolation concept of species (Paterson, 1980). Furthermore, it is still discussed and sometimes advocated by recent writers of books on speciation and evolution (e.g., Maynard Smith, 1975; Dobzhansky et al., 1977; Grant, 1977; Stansfield, 1977; White, 1978; Futuyma, 1979). The mating behavior of many Odonata has previously been explained in terms of mechanical isolation (Williamson, 1906; Johnson, 1962a; Paulson, 1974). This group of insects is particularly suited to this explanation because their copulation involves two separate points of contact. When a male encounters a female, he grasps her thorax with his legs and then clasps the female's prothorax with his anal appendages, thus achieving the tandem position (Fig. 1). The second stage of mating involves female choice or recognition, when she cooperates with the male in bending her abdomen under him to make contact with his accessory genitalia on the second abdominal segment, and so achieves the copula position (Fig. 2). Mechanical isolation may operate at either stage, but the attainment of tandem is usually thought to be the more probable site. The best evidence for this theory was provided by Paulson (1974). He pinned live females of a number of species of coenagrionid damselflies to sedge stems and presented them to males of the various species. He then scored whether the male approached and attempted to clasp the female, and whether he was successful in clasping her. The degree of mechanical isolation could then be expressed as the proportion of attempted tandem linkages which were unsuccessful. Of the seven intrageneric species-pairs (in which the males almost always attempted to clasp the female), four showed nearly complete, two partial and one very little mechanical isolation. Thus, for most of these species, reproductive isolation is normally maintained by the inability of males to clasp non-conspecific females with their anal appendages, presumably because their differently shaped and sized appendages are not compatible with the thoracic structures of the non-conspecific females. Since Paulson only discussed how males are reproductively isolated from non-conspecific females, he did not offer an explanation for how the integrity of the speciespair which showed very little mechanical isolation is maintained in the sympatric situation he studied. This might be explained by the work of Loibl (1958) and Krieger and Krieger-Loibl (1958). They, especially Loibl, concluded that in the genera Lestes (Lestidae) and Ischnura (Coenagrionidae), females recognized conspecific males by the tactile stimulation of the anal appendages when clasped in tandem. They observed that in both genera males would readily attempt and sometimes attain tandem linkage with non-conspecific females. These females then refused to cooperate in achieving copulation, and were eventually released. Furthermore, Loibl found that in Lestes, females