The sphragis of Heteronympha penelope Waterhouse (Lepidoptera: Satyridae): its structure, formation and role in sperm guarding

Abstract

The external mating plug (sphragis) of the alpine Australian satyrid butterfly, Heteronympha penelope Waterhouse, is described and figured. Its formation is described and the capacity of males to produce sphragides is shown to be limited to one or two in their lifetime. Male specializations are identified including an expanded intersegmental membrane between the genitalia and the 8th abdominal segment, which forms a mould for the sphragis, and hypertrophied paired accessory glands, the probable source of the sphragidal fluid. It is shown that the male genitalia, especially the uncus and valvae are much larger and more robust than in related species which do not form a sphragis, a probable adaptation for sphragis removal. Descriptions are provided of mating behaviour and interactions between males and mated females, including sphragis removal, in the field and in captivity. The frequency of the sphragis in wild populations of females of different ages was found to be 62%, with a slightly lower frequency among older individuals. Spermatophore counts indicate that females frequently mate twice, sometimes up to three times especially when older. Individuals of all ages bearing an intact sphragis are far more likely to have mated only once than those lacking a sphragis. It is shown that most males mate at least once in their life, probably producing at least one robust sphragis, hence it is concluded that rates of sphragis removal may be as high as 50%. Unlike most other sphragis-bearing butterflies, females of H. penelope possess no specializations in their genitalia which might serve to reject the sphragis. It is concluded that in this species, unlike other sphragis-bearing butterflies, the sphragis arose as a result of escalated sexual conflict between males alone.