Review Article |
Corresponding author: Ana Paula S. Carvalho ( acarvalho@ufl.edu ) Academic editor: Thomas Simonsen
© 2017 Ana Paula S. Carvalho, Albert G. Orr, Akito Y. Kawahara.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Carvalho APS, Orr AG, Kawahara AY (2017) A review of the occurrence and diversity of the sphragis in butterflies (Lepidoptera, Papilionoidea). ZooKeys 694: 41-70. https://doi.org/10.3897/zookeys.694.13097
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Males of many butterfly species secrete long-lasting mating plugs to prevent their mates from copulating with other males, thus ensuring their sperm will fertilize all future eggs laid. Certain species have further developed a greatly enlarged, often spectacular, externalized plug, termed a sphragis. This distinctive structure results from complex adaptations in both male and female genitalia and is qualitatively distinct from the amorphous, internal mating plugs of other species. Intermediate conditions between internal plug and external sphragis are rare. The term sphragis has often been misunderstood in recent years, hence we provide a formal definition based on accepted usage throughout most of the last century. Despite it being a highly apparent trait, neither the incidence nor diversity of the sphragis has been systematically documented. We record a sphragis or related structure in 273 butterfly species, representing 72 species of Papilionidae in 13 genera, and 201 species of Nymphalidae in 9 genera. These figures represent respectively, 13% of Papilionidae, 3% of Nymphalidae, and 1% of known butterfly species. A well-formed sphragis evolved independently in at least five butterfly subfamilies, with a rudimentary structure also occurring in an additional subfamily. The sphragis is probably the plesiomorphic condition in groups such as Parnassius (Papilionidae: Parnassiinae) and many Acraeini (Nymphalidae: Heliconiinae). Some butterflies, such as those belonging to the Parnassius simo group, have apparently lost the structure secondarily. The material cost of producing the sphragis is considerable. It is typically offset by production of a smaller spermatophore, thus reducing the amount of male-derived nutrients donated to the female during mating for use in oogenesis and/or somatic maintenance. The sphragis potentially represents one of the clearest examples of mate conflict known. Investigating its biology should yield testable hypotheses to further our understanding of the selective processes at play in an ‘arms race’ between the sexes. This paper provides an overview, which will inform future study.
Ditrysia , mate conflict, mating plug, Nymphalidae , Papilionidae , sperm guarding, sperm precedence, sexual competition
Male butterflies, like most animals, typically maximize their reproductive success by mating with as many females as possible. Conversely, for females, one copulation is normally sufficient to provide sperm to fertilize all the eggs that they can produce. Female butterflies possess sperm storage organs which can maintain vital sperm from a single mating for their entire life (
When female insects mate more than once, it is often the sperm of the final male to mate which fertilizes most (or all) of the female’s remaining eggs, a process termed “last male sperm precedence” (
Although many strategies are found among insects to prevent females from remating, with females evidently complicit in some cases (
The production of a sphragis, one of the more extreme male strategies that has evolved to prevent the female from remating, occurs only in certain butterflies. It was first clearly described by
The form of the sphragis is consistent within a species, it is shaped by complex adaptations in the male genitalia, and it is wholly or mainly external to the female abdomen. In these respects, sphragides differ qualitatively from smaller amorphous internal mating plugs. In almost all sphragis-bearing species, the female external genitalia are strongly modified from the ditrysian groundplan; these modifications have in turn influenced the form of the sphragis (
Specialized features of sphragides in different species have been identified by several authors (
One oft-cited misconception is that the ‘waxy’ sphragis can dissolve in water (
The sphragis is produced from a viscous secretion which is molded within membranous or sclerotized pockets in the male genitalia, sometimes being extruded gradually as it hardens, so that the final product is far larger than any cavity in the male’s body (
Production of the sphragis is a substantial male material investment, which apparently occurs at the expense of the spermatophore. A great deal of material needs to be repurposed to build the sphragis. The amount of accessory gland secretion (including the spermatophore, and spermatophylax), which females could potentially metabolize and utilize for oogenesis or somatic maintenance (
Percentage of male investment in the sphragis based on male’s weight. Data adapted from
Male investiment on the sphragis (% body weight) | |
---|---|
Parnassius glacialis (Papilionidae: Parnassiinae) | 20.5 ± 2.7 |
Parnassius apollo (Papilionidae: Parnassiinae) | 7.4 ± 2.5 |
Luehdorfia japonica (Papilionidae: Parnassiinae) | 9.8 ± 5.7 |
Cressida cressida (Papilionidae: Papilioninae) | 6.7 ± 1.1 |
Euryades corethrus (Papilionidae: Papilioninae) | 8.6 ± 1.0 |
Parides proneus (Papilionidae: Papilioninae) | 3.1 ± 1.5 |
Acraea serena (Nymphalidae: Heliconiinae) | 3.3 ± 0.4 |
Acraea anemosa (Nymphalidae: Heliconiinae) | 6.1 ± 0.9 |
Acraea andromacha (Nymphalidae: Heliconiinae) | 3.1 ± 0.2 |
Presence of the sphragis is typically associated with specific behavioral patterns. Courtship behavior is absent or rudimentary in almost all sphragis-bearing species (
All experimental evidence (
In the most comprehensive comparative survey to date,
Studies on the biology of the sphragis in living butterflies have mostly focused on C. cressida in the Troidini (
A large body of published information exists on the occurrence and diversity of the sphragis, but it is largely obscure, scattered, old and often published in languages other than English. There is a need for this information to be collated and for several conspicuous gaps to be filled. In this paper we aim to provide an overview of the subject and develop a dataset to inform future investigation. We present the first comprehensive review of the structural variability of the sphragis across all butterflies, illustrating the variety of forms that these can take. All species of butterflies in which a well-defined sphragis is known to occur are listed based on published information and direct observation of museum specimens. Reports of sphragis occurrence in Erebidae (
In addition, as noted previously, sphragis-like formations which may represent incipient sphragis evolution or secondary loss occur in some butterflies (
We examined butterfly specimens in museum collections and searched historical and recent literature for reports, descriptions and illustrations of sphragides or similar external structures (
For each species, we tried to examine mated females of at least ten specimens if possible (in a few cases thousands were available). Generally, the minimum combined sample was five specimens. Overall, we directly examined approximately 80% of all sphragis bearing species. We recorded key traits of the sphragis of each species, especially: the presence of male scales attached to the surface or incorporated into its matrix; whether it was mainly hollow or solid; the presence of projections or other specialized sculpturing; and its size relative to the female abdomen. We also examined structures previously classified as protosphragides or vestigial sphragides, and we define an anomalous form as a ‘hemi-sphragis’. We targeted the following taxa based on published and unpublished information: Nymphalidae: Acraea, Amauris Hübner, 1816, Argynnis Fabricius 1807, Dircenna Doubleday, 1847, Hestina Westwood, 1850, Heteronympha Wallengren, 1858, Hipparcha Fabricius 1807, Pteronymia Butler & Druce, 1872, Sasakia Moore 1896; Papilionidae: Parnassiinae, Cressida Swainson, 1832, Euryades Felder & Felder, 1864, Losaria Moore, [1902], Parides Hübner, 1819, and Trogonoptera Rippon, [1890].
In order to show the extent of interspecific variation in sphragis morphology, we digitally imaged the sphragis of representative species using a Canon 5D MKIII camera body and a Canon MP-E 65mm lens. For each image, 10 to 20 image layers (depending on the size of the sphragis) were taken across a series of close-spaced focal planes, using the Automated Macro Rail for Focus Stacking StackShot. These were later stacked using the software Helicon Focus on a PC computer. Figures were edited and assembled using Adobe Photoshop CS4. The taxonomic classification in this study follows Häuser (2005) for Papilionidae,
• Protosphragis: amorphous, non-species-specific version of the sphragis, often facultative in the groups where it is found, and potentially associated with groups in the early evolution of the sphragis.
• Vestigial sphragis: non-species specific version of the sphragis, irregular in occurrence, associated with groups believed to be in the process of losing the sphragis.
• Hemi-sphragis: semi-internal version of the sphragis, with a complex and regular internal arrangement of lacunae used to increase bulk as well as a regularly striated exposed outer face, associated with complex adaptations in male genitalia, thus equivalent to true sphragides in terms of complexity and regularity of form. Restricted to the troidine genus Trogonoptera and figured in
Based on the traits recorded for each sphragis we developed a system of categorization based on level of complexity. Category 1 (low complexity): a protosphragis or a vestigial sphragis, characterized by being small, amorphous and of facultative occurrence. Category 2 (moderate complexity): a hemi-sphragis, or a well-formed externalized sphragis lacking male scales and essentially solid, of small to medium size. Category 3 (medium complexity): a well formed sphragis incorporating male scales but solid and of simple form, mostly small to medium in size. Category 4 (high complexity): large to very large sphragides, hollow and/or with specialized projections, girdles, or other complex sculpturing, further defined based on absence (Category 4a) or presence (Category 4b) of scales.
A total of 273 butterflies species in two families – Papilionidae (72 species, 13 genera) and Nymphalidae (201 species, 9 genera) – were recorded as having a sphragis, protosphragis, vestigial sphragis, or hemi-sphragis (Suppl. material
A protosphragis was present in 11 species, vestigial sphragides occurred in 14 species, while a hemi-sphragis is found only in the two known species of the Troidini genus Trogonoptera (Suppl. material
For the species where a sphragis occurs (Suppl. material
Category 1 | Category 2 | Category 3 | Category 4 | Uncertain | |
---|---|---|---|---|---|
Number of species | 25 | 35 | 99 | 98 | 16 |
Percentage of species | 9% | 13% | 36% | 36% | 6% |
Four categories of sphragis (including two subcategories) were recognized in terms of structural complexity (Suppl. material
Sphragis of butterfly species, a ventral, b lateral, category of the sphragis in parenthesis. 14 Acraea kraka (2) 15 A. egina (4) 16 A. omrora (4) 17 A. nohara (4) 18 A. oncaea (3) 19 A. zetes (4) 20 A. endoscota (4) 21 A. quirina (3) 22 A. igati (3) 23 A. hamata (2). Scale bar = 1 mm.
Condition | No. of species |
---|---|
Scales | |
Yes | 145 |
No | 113 |
Projections and/or girdle | |
Yes | 60 |
No | 199 |
Structure | |
(Mostly) Hollow | 73 |
(Mostly) Solid | 176 |
Acraea species exhibit great variation in the form and development of the sphragis. In some, such as A. natalica Boisduval, 1847, there is no sphragis or internal mating plug and females mate numerous times with males, which produce small spermatophores (
Sphragis of butterfly species, a ventral b lateral, category of the sphragis in parenthesis. 24 Acraea umbra (4) 25 A. quirinalis (3) 26 A. pharsalus (2) 27 A. serena (3) 28 A. althoffi (3) 29 A. orestia (3) 30 A. pentapolis (3) 31 A. issoria (3) 32 A. rhodope (3) 33 A. ozomene (3). Scale bar = 1 mm.
A girdle occurs in E. corethrus (but not in the related E. duponchelii) (
Almost all butterfly species in our list display structures that meet our definition of the sphragis, however, we recognized a few unusual intermediate forms. The large, semi-exposed plug of the troidine papilionid Trogonotera Rippon, 1890 has internal structure (ordered lacunae) and external, well defined striae, hence we classify it as a ‘hemi-sphragis’. The male has a specialized pouch where the plug material is formed into a broad ribbon, which coalesces into a solid body with lacunae (
Although the sphragis appears to be an effective structure to prevent remating, there are examples where the female might be able to mate again (
There are cases known in Parnassius species where hardened sphragides have been lost by the female or were removed by subsequent males (
The satyrine nymphalid H. penelope is the only species of its genus, and subfamily, bearing a true sphragis (Figure
The evolution of the sphragis was studied in detail by
Furthermore,
When we include Category 1 in our analysis, the sphragis appears to have arisen in at least six subfamilies in butterflies (Figure
The two most complex forms of the sphragis (Categories 3 and 4) are found in five out of six subfamilies where the sphragis is found (Figure
The occurrence of the sphragis in butterfly subfamilies. Dark circles indicate that some species in the clade bear sphragides or a version of it. Numbers inside the dark circles indicate estimation of minimum number of sphragis evolution events. Numbers under butterfly images indicate sphragis category of that species. Tree adapted from the phylogeny of Heikkila et al. (2011).
In a final twist of the hypothesized evolutionary process, it is possible that in some circumstances the sphragis has become secondarily advantageous to females. Females of L. japonica usually mate only once, early in their life (
It is known that in Papilionidae extremely complex female genitalia occur widely, as well as the numerous externalized forms normally associated with bearing a sphragis (
Many questions still remain unanswered, especially regarding the processes involved in the evolution of the sphragis. Additionally, more studies are necessary to investigate how the presence of the sphragis may be related to factors such as ecology, especially habitat type and hostplant dispersion, reproductive behavior, and sperm dynamics, however, these topics go beyond the scope of the present study.
Ventral and parasagittal views of the female genitalia and lateral abdomen with sphragis for Luehdorfia puziloi (a, b, c respectively), and Acraea horta (d, e, f respectively), showing convergence in externalization of female genitalia, reduction in the size of the bursa copulatrix, and how the genitalia is covered by the sphragis. BC, bursa copulatrix, DB, ductus bursae; DS, ductus seminalis; OB, ostium bursa.
This is the first near complete survey of variation and morphological complexity of the sphragis in butterflies to date. This study provides the most comprehensive review on the sphragis, besides suggesting a method of categorization of the structure.
The main outcomes are as follows:
1. A true sphragis was found in 232 species of butterflies from the families Papilionidae and Nymphalidae.
2. The sphragis and related structures can be categorized into roughly four structural types: low complexity (amorphous and facultative), moderate complexity, medium complexity, and high complexity.
3. The sphragis and related structures is found in the Papilionidae subfamilies Parnassiinae and Papilioninae, as well as in the Nymphalidae subfamilies Danainae, Heliconiinae, Apaturinae, and Satyrinae.
4. Based on previously published evidence, the sphragis functions primarily to prevent remating by a secondary male.
We especially thank Kazuma Matsumoto (AIES) for his many contributions on Palearctic and Oriental butterflies, especially for sharing his unpublished data on sphragis-like structures in Apaturinae and Losaria, for providing literature and allowing access to an unpublished manuscript. Kazuma Matsumoto, along with Jim Miller (
Table S1. Sphragis-bearing butterfly species and morphological characteristics of their sphragides
Data type: Microsoft Excel document
Explanation note: G: girdle, VS: vestigial sphragis, PS: protosphragis, HS: hemi-sphragis, *: species is figured in the plates.