Sericinus, , Bryk, 1934

Matsumoto, Kazuma & Orr, Albert G., 2021, The occurrence of a sphragis and genital modification in Hypermnestra, Archon and Sericinus (Lepidoptera: Papilionoidea: Papilionidae), Journal of Natural History 55 (15 - 16), pp. 1033-1057: 1053-1054

publication ID 10.1080/00222933.2021.1930227

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In Sericinus   , the sphragis is basically formed as a lid closing the mouth and lining the wall of the cup-like invagination, at the bottom of which the ostium opens. It may sometimes fill the invagination almost totally. Probably the ancestor of Sericinus   had a flat female genital plate like those of the other three genera of Zerynthiini as well as Luehdorfia   and Archon   . At that stage the ancestral species probably had a sphragis covering the genital plate as in Allancastria   and Archon   , though we cannot postulate the use of male scales with confidence. The present sphragis of Sericinus   is specialised in a unique way. Although externally it looks amorphous, it has a definite structure and is clearly formed with some precision. We therefore consider this to be a true sphragis. It would be worth investigating how the male Sericinus   forms a mostly thin and flat lid by ejaculating semi-fluid material over a hollow space.

The invaginated genital plate of Sericinus   is probably difficult to plug, because it requires a relatively large investment of the plug material, if the hollow space must be blocked by completely filling it. It may have evolved as an anti-plugging device. The lidlike structure of the sphragis is a male counter-adaptation to overcome it while saving sphragidal material. The secretion lining the inner wall, including the bottom where the ostium opens, may not be easily penetrated and extracted. The male accessory glands which produce the sphragis are only moderately hypertrophied ( Orr 1988, 1995) and their capacity to produce sphragidal material is probably quite limited, presumably the reason why the sphragis seldom fills the invagination and why many mated females lack a sphragis. The sharply hooked tips of the robust valvae may be used for removing the sphragis, if the opportunity arises. The short ductus bursae may be a plug-rejecting structure effective in past, when the genital plate was flat. This evidence suggests an arms race between   the two sexes in the past, but we suspect the intersexual conflict in this species is now relaxing, especially because of the somewhat low frequency of the sphragis and high incidence of monoandry in wild females as indicated by the spermatophore counts. In Sericinus   , the sphragis may be becoming redundant.

A relatively semelparous oviposition schedule and short adult life span in female Sericinus   may reduce the need for the sphragis as a sperm guarding device. Monastyrskiy and Kotlobay (1995) reported that female S. montela   from Maritime Territory of Russia began to oviposit 10 min–15 min after copulation and oviposited every other day for 5 days with 65% of the eggs being laid on the first day. Li et al. (2016) reported that female S. montela   of Gansu Province, China mated on the day of eclosion and laid eggs on the same day or the next day and then died. The reason for these somewhat different results is unknown, but both studies indicate that females live rather short lives and lay most of their eggs soon after the mating. In such a situation they would gain little from male nutrients in oogenesis. Our sample also indicates old females are rather sparse in the field, suggesting a short female adult life ( Tables 4 and 5). The female lays eggs in large clusters, probably only a few times during their adult life. Accordingly, the period during which a mated female has high reproductive value ( Fisher 1930) should be short. In this case the sperm guarding value of the sphragis quickly declines with time after mating. Moreover, the females are cryptic and inactive ( Monastyrskiy and Kotlobay 1995; Li et al. 2016), hence not easy to locate by searching males. In this situation, the males may not be selected to invest much in a sphragis nor to produce many sphragides.

It is now confirmed that a sphragis, in some form, occurs in almost all species of all genera in Parnassiinae   . The subfamily includes genera having a protosphragis ( Hypermnestra   ), a vestigial sphragis ( Bhutanitis   and Zerynthia   ), various types of large elaborate sphragides ( Parnassius   , Luehdorfia   , Archon   , Sericinus   and Allancastria   ) as well as those that have probably secondarily lost the sphragis ( Parnassius simo   group). Archon   and Sericinus   may be close to an evolutionary tipping point in terms of losing the sphragis.

Parnassiinae   is thus an important model group for investigating the process of sphragis evolution. As well as the functional morphological analysis presented here, investigations coordinating ecological, behavioural and physiological factors and relating these to the presence or absence of a sphragis are needed. Ecological factors include habitat type, hostplant dispersion, population structure and life tables with oviposition and survivorship schedules; behavioural traits include mate location and mating behaviour and encounter rates; physiological traits chiefly relate to the material benefits females may gain from mating and material costs to males of sphragis production. With such information it would be possible to construct a more holistic model of sphragis evolution and hopefully also gain insight into the circumstances leading to several anomalous forms.