Rediscovering a species not seen for a hundred years, Stathmopodatacita (Meyrick, 1913) (Lepidoptera, Stathmopodidae), with its unusual fern-spore-feeding life history

Abstract Background Despite being the second largest group of vascular plants, ferns are scarcely reported being fed by insects when compared to angiosperms. Within these fern-feeding insects, lepidopterans are poorly represented and are restricted only to specific groups in this speciose order. The consumers specialising on fern spores are even scarcer in the order, with the majority being consumers of vegetative structures. Amongst the fern-spore-feeding Lepidoptera, Stathmopodidae is the family with the highest species diversity, even with a subfamily, Cyprininae Sinev, 2015, specialising on fern spores. However, fern-spore-feeding habit is not restricted to this subfamily. To understand the evolution of fern-spore-feeding within this family and to increase our knowledge of insect-fern evolution, detailed studies on fern-spore feeding stathmopodids are essential. New information The present study rediscovered a rare, fern-spore-feeding, stathmopodid micro-moth, Stathmopodatacita (Meyrick, 1913), which has not been formally recorded or identified for more than 100 years. We documented the life history of this species and identified several species of Pyrrosia (Polypodiaceae, Platycerioideae) as host for the moth’s larvae. A re-description of the fern-feeding moth is also provided as the original description is obscure in terms of character diagnosis.


Introduction
The family Stathmopodidae (Lepidoptera, Gelechiidae) is distributed throughout the world with most species reported in Asia and Oceania and the members of this group of microlepidoptera can be recognised by the characteristic rosettes of long and rigid bristles on hind leg segments (Sinev 2015). Just like many minute insects, the diversity of Stathmopodidae is underestimated, lacking comprehensive study. Even though more than 350 species have been described in this family, some species have not been recorded after their original description.
Stathmopoda tacita (Meyrick, 1913) represents such a case. This mysterious species was firstly described by Edward Meyrick in 1913, based on material from Assam, India. Meyrick also established a new genus Agrioscelis, Meyrick (1913), with this species as type species. However, none of the diagnostic characters in Meyrick's original description is diagnostic for species in the genus. Sixty years later, Kasy (1973) transferred this species to Stathmopoda after dissecting and examining the genitalia of the specimens collected by Meyrick. Afterwards, this species was not reported or mentioned until 2016, when two unidentified specimens from Japan were tentatively labelled as "Stathmopoda sp.5" by Terada (2016), who pointed out that that female genitalia was highly similar to S. tacita. However, a taxonomic conclusion could not be reached due to the lack of male specimens.
Here, we present the rediscovery of S. tacita and found that they utilise spores and the mesophyll of Polypodiosida, an unusual host for Lepidoptera, as larval diet. Fern feeding is uncommon in Lepidoptera, only occurring in a few groups of moths (Fuentes-Jacques et al. 2022a). Consumption of fern spores is even rarer, with this feeding habit only found in three families: Stathmopodidae, Micropterigidae and Tineidae (Lees andZilli 2019, Fuentes-Jacques et al. 2022a). Amongst these three fern-spore-feeding families, Stathmopodidae has the highest fern-feeding species diversity. Although the subfamily Stathmopodinae represents the highest species richness in Stathmopodidae, there are only three fern-spore-feeding species known in this subfamily, with most of the remaining fern-feeding species restricted to the subfamily Cyprininae Sinev, 2015 (Terada 2016, Shen and Hsu 2020, Shen et al. 2022, Robinson et al. 2022. The recorded fern-spore-feeding species in Stathmopoda (S. aenea (Braun, 1918), S. elyella (Busck, 1909) are only known to be distributed in the Nearctic Region (Braun 1918, Needham 1947, with an undescribed Stathmopoda species reported by Fuentes-Jacques et al. (2022b) in Mexico. The discovery of the fern-spore-feeding habit in S. tacita not only increases the record of the uncommon feeding habit in Stathmopoda, but also represents the first record of a fernspore-feeding species of this genus in the Old World.

Diagnosis
Stathmopoda tacita is similar to S. masinissa Meyrick, 1906and S. maritimicola Terada & Sakamaki, 2011(see Terada et al. (2011), but can be distinguished from these two species by the colour and shape of fascia near apex of forewing. The fascia is oval, ochrous in colour in female and vestigial in male in S. tacita, whereas the fascia is creamy-white in colour and triangular in both sexes of S. masinissa. For S. maritimicola , the fascia is triangular, yellow in colour in both sexes and sometimes vestigial in male.
In genitalia, these species can be distinguished by the presence of cervix bursae. Cervix bursae are absent in S. tacita, but well-developed in the other two species.

Biology
Larvae were found in January, March, April, October, November and December. Larvae (Fig. 2B) constructed tunnel-like shelters and live gregariously on the underside of the sporophyll ( Fig. 2A). The early instar larvae constructed shelters inside the mid-rib of the host plant and retreated back into the shelters when disturbed. The larvae not only fed on the spores, but also ingested the mesophyll. The pupae were enclosed in loose cocoons within larval shelters. Pupae are oval-shaped, brown and glossy in colour (Fig.  2C). Adult moths (Fig. 2D) emerged about one month after pupation, suggesting that the species is multivoltine.

Remarks
Kasy (1973) described male genitalia of the species, stating cornutus is absent in the phallus. However, the slightly sclerotised cornutus was actually recognisable in the illustration in his work (see pl. 94), conforming to the specimens from Taiwan. Based on this observation, we consider that a weakly-developed cornutus is present in the male phallus of the species.

Discussion
Fern-feeding is an uncommon feeding habit in Lepidoptera and is restricted to only a very few groups (Lees and Zilli 2019, Sawamura et al. 2009, Fuentes-Jacques et al. 2022a). Compared to the other fern-feeding lepidopterans, the specialist spore feeders are even rarer. Moreover, spore-feeders are even rarer and largely restricted to the family Stathmopodidae (Fuentes-Jacques et al. 2022a). Understanding how this rare feeding pattern evolved and determining the evolutionary relationships between these species and their fern hosts is of great interest and will require further investigation. To solve these intriguing questions, understanding the systematic relationships of this family is indispensable. We therefore suggest that future studies focus efforts on investigating the biology and systematics of this family. Only with a comprehensive understanding of the intra-family relationships can the study of Stathmopodidae be enhanced.