Filling the gap: a new ant-buttery symbiosis in the forest canopy

Myrmecophilous butteries can establish complex symbiotic relationships with ants. An elusive symbiotic caterpillar wandering among the brood of the aggressive ponerine ant, Neoponera villosa, was found inside the core of a nest built in the myrmecophytic bromeliad Aechmea bracteata. This is the rst caterpillar found in symbiosis with a species of the subfamily Ponerinae. Tissue sample was barcoded, and an integrative approach was used to identify it as Pseudonymphidia agave, a poorly known Pachythonina member in the riodinid myrmecophilous tribe Nymphidiini. The caterpillar has a general tank-like body morphology, without projections or tubercles, and is covered dorsally by specialized at setae that form an armor of small plates. Ant-organs potentially related to the caterpillar-ant signaling, such as perforated cupola organs and tentacle nectary organs, are present. These functional traits, together with the stable social integration, suggest that P. agave is a specialized social parasite of N. villosa. However, several knowledge gaps remain, notably the oviposition site, dependence on bromeliad association, steps to colony integration, and the larval diet through ontogeny. Carnivory has been reported in all known members of the Pachythonina subtribe (caterpillars prey on honeydew-producing hemipterans), and a shift to myrmecophagy inside the ant nests is a possible evolutionary step.


Introduction
Animals of several classes, especially arthropods, establish long term associations with ants and bene t from these facultative or obligate associations in several ways. These so called myrmecophiles (for a detailed de nition, see 1,2 ) circumvent the behavioral and chemical defenses of the ants and thrive in their homeostatic, stable nests or in their surrounding territories 3,4 . Living in concealed environments -the ant nests -, most specialized myrmecophiles are in general rare, restricted to speci c habitats or even microhabitats. Moreover, they are unevenly distributed in time and space, thus presenting challenges for their study 5 .
Symbiotic associations between lepidopteran caterpillars and ants are diverse and widespread, spanning at least six lepidopteran superfamilies: Noctuoidea, Papilionoidea, Pyraloidea, Tineoidea, Gelechioidea, and Zygaenoidea 6-8 . These interactions range from commensalism or mutualism to social parasitism, and relationships may be facultative or obligate, occurring during only a portion or the whole life cycle of the myrmecophile 9,10 . These associations are noticeably prominent in two sister families of butter ies: Lycaenidae and Riodinidae (Papilionoidea) 6 . Myrmecophilous caterpillars of these families typically form symbioses with ants on host plants where the ants, and sometimes also the caterpillars, harvest liquid food from plants (extra oral nectar) and exudates from hemipterans (aphids, treehoppers, scale insects and relatives) [11][12][13] . In trophobiotic interactions, the caterpillars secrete nutritional food rewards for ants and gain potential protection against predators and parasitoids 11,14,15 .
From this ancestral pattern (phytophagous caterpillars), several Lepidopteran lineages have evolved to exploit the ant-colony resources inside the ant nests, either as social parasites that elicit trophallaxis from workers, or as brood predators [16][17][18] . Although more than 99% of lepidopteran caterpillars are phytophagous, transition from feeding on plant parts to aphytophagy has occurred several times, with carnivorous caterpillars found scattered in the butter y phylogeny 19,20 . Predation on ant brood has been documented in Lycaenidae, but not in the Riodinidae; however members of both families exhibit complex myrmecophily with the evolution of speci c ant-organs and behavioral strategies to integrate and exploit the ant host colonies 20 such as chemical, vibrational and tactile mimicry, ant-dependent oviposition, release of brood-carrying behavior, trophallaxis and others [21][22][23] . Those obligate social parasite species are completely dependent on the ants to ful ll their life cycle 24 .
Metalmark butter ies (Riodinidae) are classi ed into two subfamilies: the exclusively Neotropical Riodininae with ca. 1200 described species, and the Nemeobiinae which include both old-world and Neotropical groups [25][26][27] . Trophobiosis with ants in this family has evolved in parallel to the Lycaenidae 12,20,28,29 . According to Espeland et al. 30 , caterpillar-ant association has evolved once in the Lycaenidae and twice in the Riodinidae (in the tribes Eurybiini and Nymphidiini, both Riodininae).
Immatures of riodinid species for which life histories have been documented are defoliators, nectarivores, lichen feeders, detritivores, exploiters, and predators of hemipterans 28, 31 . Approximately 20% of the known species of Riodinidae are involved in symbiotic associations with ants 30 . Several recent studies have played a role in unveiling the nature of riodinid-ant associations 20,31-33 , con rming the evolution of social parasitism in this family (kleptoparasitism via trophallaxis with ants within their nest) 20 .
Myrmecophagy, though suspected in several instances, remains to be con rmed in the eld 34 . Individuals of several ant subfamilies are frequently observed tending riodinids directly on host plants. Such associations are recurrent in members of the Myrmicinae, Formicinae, Dolichoderinae and Ectatomminae subfamilies, all of which are territorially dominant ants that harvest animal and plant secretions; however, they are very uncommon in the Ponerinae (see Supplementary Table S1). More intimate and stable relationships between riodinids and their host inside the ant nest, have also been described: some riodinid species rest and pupate in shelters constructed by ants, and others have been collected in ant nests of various formicine, myrmicine and dolichoderine species 20,[35][36][37][38] ; however, to the best of our knowledge, there is no record of a riodinid or a lycaenid as a guest within ponerine ant colonies.
Over the course of extensive eldwork and sampling of colonies of the Neotropical ponerine ant, Neoponera villosa, in the southern region of the Yucatan Peninsula 39,40 , we recorded a single case of a butter y caterpillar wandering among the ant brood. In this study we: (1) establish the identity of this symbiont through an integrative approach; (2) present some notes on both the natural history and behavior of the caterpillar; (3) describe and illustrate its external morphology; (4) discuss the putative functional signi cance of morphological traits that may have allowed this species to invade aggressive ant societies; and (5) compare them to other described social parasitic caterpillars.

Results
A myrmecophilous caterpillar ( Fig. 1) was found among the ant brood in a N. villosa colony collected on June 10, 2017 at Ejido Blasillo, Campeche, Mexico (18°7'13.6056" N, 89°19'47.791" W, 263 m asl). The caterpillar was isolated in a small petri dish and provided with a source of humidity. After several days, the larva appeared to be in a poor condition and was preserved in alcohol. Subsequent to the discovery of the myrmecophilous larva, collecting effort was renewed. In total, 19 nests were collected in the same locality 40 , but no other riodinid larva was found.
Caterpillar identi cation. The tree obtained placed our caterpillar barcode sequence within the Pachythonina subtribe (Fig. 2, Supplementary Table S2), sister to other previously sequenced Pseudonymphidia agave from Mexico (C. Pozo et al., unpubl. data). Indeed, the sequences of specimen MAL-05054 and of the collected caterpillar belong to the same barcode haplotype.
Since its original description as Lemonias agave Godman and Salvin, 1886, this species has not been revised. Presently, two subspecies of P. agave are recognized: nominal plus leucogonia (Stichel, 1911).
Based on similarities in genital morphology, antennal length and wing pattern, Hall and Harvey 42 temporarily placed agave and leucogonia in the genus Pseudonymphidia. This genus forms a monophyletic clade along with Pachythone, Roeberella and Pixus in the phylogenetic hypothesis of Seraphim et al. 26 , who erected the Pachythonina subtribe that included several genera placed as incertae sedis in previous revisions of the family. According to the phylogenetic tree obtained here (Fig. 2), the closest relative to the Pseudonymphidia genus recovered in this study is Archaeonympha drepana, a genus that was absent from the sampling used by Seraphim et al. 26 but that should also be placed within the Pachythonina subtribe (N. Seraphim, unpubl. data).
Prevalence, natural history, and behavioral observations.
Out of 82 N. villosa colonies collected between January 2016 and April 2018 in several sites in the southern part of the Yucatan Peninsula, only one colony was infested with a riodinid larva, even though 19 of the colonies were from the same locality (Ejido Blasillo) and its surroundings. The caterpillar of P. agave was wandering among the host cocoons and larvae while workers were present, and no aggression was detected during a 3h observation period. The host colony where the riodinid caterpillar was discovered nested in an A. bracteata bromeliad established at a height of 4 m on a Haematoxylum campechianum tree (Fabaceae), situated in a patch of low deciduous forest. The colony was composed of 11 dealated queens, three gynes, 165 workers, 265 larvae, 173 cocoons, and plenty of eggs. Larvae and pupae (in cocoons) were not parasitized by endo-or ectoparasites. Apart from the riodinid larva, very few other myrmecophiles were hosted by the colony: only the cleptoparasitic mite Cosmolaelaps sp. (present on ant larvae, N = 5) and two species of staphylinid beetles (Myrmigaster sp. and Tyropsis sp., N = 2 and N = 1, respectively) were found in the nest chambers. Additionally, 17 adults of an unidenti ed species of Nitidulidae (Coleoptera), and 16 pseudoscorpions (Chelodamus mexicolens) were found in the nest refuse.
Distribution records for Pseudonymphidia agave adults.
Pseudonymphidia agave, the white-trailed metalmark or agave metalmark, is a rare species, with records only from a few localities. Based on museum specimens, P. agave has been recorded from as far north as San Luis Potosi in Mexico to Colombia in South America (Fig. 4). Specimens have been collected between 0 and 1 600 m of altitude 43 . This species seems restricted to speci c habitats in tropical forests; data of specimens correspond to adults collected in evergreen, semi-evergreen, deciduous, semideciduous, and cloud forest. Adults have been collected on rambutan owers, Nephelium lappaceum (Sapindaceae), in a commercial plantation 43 , but caterpillar host plants are unknown and natural history information was not available prior to this study. Only 52 individuals from Mexico are hosted in collections around the world (see Supplementary   Tables S3, S4). Regarding the museum specimens, ight activities are recorded almost the whole year, with a signi cant peak in June and a less important one in October ( Supplementary Fig. S1), a pattern previously observed for the riodinid family and other small-sized butter ies whereby the largest populations have been recorded during the wettest season of the year 44 .

Discussion
The biodiversity in the canopy of tropical forests is a frontier of knowledge that can still reveal great surprises 45 . In these environments, ants are pervasive in terms of richness and abundance connecting species and trophic levels through antagonistic and mutualistic direct and indirect interactions (reviewed in 46 ). Our endeavor to describe this speci c type of biodiversity unveiled an unexpected symbiotic interaction between a butter y caterpillar and an aggressive ant. Our report that Pseudonymphidia agave caterpillars are guests in the nests of Neoponera villosa is, to the best of our knowledge, the rst case of a butter y caterpillar in intimate symbiosis with a ponerine ant. Our nding is the rst to provide information on the life cycle of a member of the genus Pseudonymphidia, of which the immature stages were previously unknown, and sheds light upon the possible evolutionary steps of social parasitism in riodinids.
In recent years, a substantial new information related to the immature stages of Riodinidae has been revealed 20,47-52 . Nevertheless, some lineages remain completely unknown, principally in Nymphidiini, suggesting that these taxa may exhibit specialized association with ants, including long periods inside the ant nests 20,28,52 . The discovery of the P. agave caterpillar reinforces this supposition. This species belongs to the recently erected subtribe Pachythonina (Nymphidiini), a clade diagnosed based on molecular characters 26 . This group comprises rare and restricted taxa associated with tropical forests and is the latest subtribe of Nymphidiini whose natural history information has been published 31 .
Species in the Pachythonina for which some life history information is known, present armored carnivorous caterpillars specialized in preying on honeydew-producing hemipterans (e.g., Pachythone spp. 31,53 ) and are thought to carry out ant-mediated oviposition on harmful ant-plant symbioses (i.e., use speci c ants as oviposition cues; e.g., Minstrellus grandis 33 , or prey on ant brood (e.g., P. agave; this study). Although scattered, the available information indicates that the Pachythonina species demonstrate morphological and behavioral traits that allow coexistence with extremely aggressive ants, including unusual associations with pseudomyrmecine and ponerine ants (Supplementary Table S1).
Ponerine ants are a group of large aggressive ants with powerful stingers 3 . Most species are specialized or generalist predators, occupying a high trophic position 54,55 . The use of liquid foods on foliage, however, has been recorded for some species such as N. villosa which harvest liquid secretions from extra oral nectaries, exudates from honeydew-producing hemipterans and secretions of some facultative myrmecophilous caterpillars 56-58 (Supplementary Table S1). According to DeVries 11 the evolution of symbiosis between caterpillars and ants is associated with genera of ants that harvest liquid food on vegetation. The aggressiveness and conspicuous appearance of N. villosa ants associated with liquid feeding on plants may have generated an ecological opportunity for the evolution of exploitation by preadapted caterpillars 11,15,59,60 . This hypothesis can be proved both in ecological time 61 and evolutionary time if the specialization between P. agave caterpillars on N. villosa ants is con rmed.
Detailed behavioral interactions between the P. agave caterpillar and N. villosa ants could not be analyzed. However, numerous evidence suggest that the nature of the relationship between caterpillars of this riodinid and N. villosa is not facultative or simply casual: (i) the larva was found among the brood in the core of the ant nest, (ii) it has a general protective morphology, with the head retracted or maintained under the shield-like, thick cuticle of the body, (iii) it had a slow, gradual, slug-like movement behavior, and (iv) ants did not behave aggressively. These traits indicate a well-integrated myrmecophile species.
Exchange of liquid food through regurgitation (stomodeal trophallaxis) is a highly evolved form of social food sharing, but it is infrequent in ponerine ants 57,62 . The rst case of social parasitism in riodinids (exploitation of the colony resources through trophallaxis inside the ant host nest) has been recently reported in Aricoris arenarum (Riodininae: Lemoniadina) 20 , and a similar "cuckoo" life-style could be possible for the P. agave caterpillar. However, stomodeal trophallaxis is not known to occur in N. villosa 57 ; instead, workers share liquid food by means of mandibular pseudo-trophallaxis: workers gather and transport liquid substances with surface tension to maintain a drop between the mandibles. In the nest, they offer the liquid to other workers who "spoon" some of the liquid 57 . Since the colony had no other source of food for the caterpillar but the ants themselves, and the general rigid morphology of the former likely prevents obtaining food by pseudo-trophallaxis, we further hypothesize that the caterpillar is most probably a brood predator (myrmecophagy), at least during the last instar. Furthermore, in adults of P. agave, the wings present a greasy appearance, a trait that has been considered as a potential sign of carnivory 63,64 . Carnivory in riodinid caterpillars has been documented in ve lineages of Nymphidiini preying on ant-tended hemipterans on plants 31 ; however, myrmecophagy has only been recorded under arti cial conditions 34,65 .
The larva of P. agave differs from all other known riodinid caterpillars, though the general tank-like appearance is similar to other riodinids in the Lemoniadina (e.g., Menander spp. 28,66 ), and Pachythonina subtribes (e.g., Pachythone xanthe 31 ). Although phytophagous, Menander caterpillars are covered with a prominent carapace that ares outward to the substrate and covers the body and legs; they are thus heavily armored and possess a complete set of ant-organs 28,66 . In P. xanthe caterpillars, the prothoracic shield divided vertically into two movable plates, the absence of vibratory papillae and a carapace that protects the head and appendages are undoubtedly the most remarkable traits 31 . Some of these larval characters such as the body shape, the morphology of the lateral fringe setae, and the positioning of the spiracles, are shared with P. agave, suggesting that they may be morphological synapomorphies for the subtribe. In fact, these morphological characters led us to classify the caterpillar as a potential Pachythonina before corroboration through molecular data.
Among myrmecophilous caterpillars, two strategies can be discerned: (i) free-living caterpillars that establish commensal or trophobiotic associations with ants on plants; and (ii) social parasite caterpillars that at some point of their development gain access to the ant nest 20 . This seems to be related to the ontogenetic moment when ant-organs and chemical compounds that trigger ant adoption are produced 67 . The life cycle of P. agave appears to conform to the second strategy, but details of the access process to the ant colony are still unknown. In summary, our data strongly suggest that P. agave is an obligate symbiont in N. villosa nests and that the last larval instar is possibly myrmecophagous. Whether females lay eggs directly on the external leaves or in orescences of the bromeliad is not known but seems possible. As suggested for other speci c lycaenid species associated with ants 68 , the scarcity of this butter y might be explained by its highly specialized life history, making the butter y distribution dependent on the N. villosa-bromeliad association. Finding answers to the questions raised in this study can only be possible with more eld effort and new data on natural history.

Material And Methods
The ant-plant symbiotic system. Neoponera villosa is widely distributed from Mexico to Argentina 69 . This is the largest of the Mexican ant species; workers measure 1.2 to 1.3 cm 70 and are provided with a powerful and painful sting. They are generalist predators and forage mainly in the canopy collecting at liquid carbohydrate food sources 56-58 . This species nests opportunistically in pre-existing cavities in live and dead trees 71,72 ; in the southern part of the Yucatan Peninsula, Mexico, it preferentially uses the myrmecophytic tank-bromeliad Aechmea bracteata as a nest site [39][40][41]71 . In the A. bracteata microcosmos, a diverse array of specialized and facultative myrmecophiles, mostly antagonists, establish complex trophic interactions with N. villosa 40 . Up to date, various invertebrates have been recorded in direct association with the brood of this species and can be considered as true myrmecophiles 40 . Three of them are brood parasitoids: an unidenti ed species of Kapala Cameron (Hymenoptera: Eucharitidae) 40 , an unidenti ed species of Blanchardiscus de Santis (Hymenoptera: Encyrtidae) 73 , and the hover y Hypselosyrphus trigonus Hull (Diptera: Syrphidae) 73,74 . Two species (the pseudoscorpion, Chelodamus mexicolens, and an unidenti ed tenebrionid beetle possibly of the Alleculinae subfamily) are brood predators, while two mites (an unidenti ed oplitid species of the genus Oplitis and an unidenti ed galumnid species) are phoretic on the host larvae and a third unidenti ed mite species (of the genus Cosmolaelaps) is cleptoparasite on the larvae; nally two other species (a staphylinid beetle of the genus Myrmigaster and a diapriid wasp of the genus Trichopria) have unclear relationships but have been found wandering on the cocoons with their antennae in direct contact with the host cocoon surface 40 .

Sampling.
Ant colonies (N = 82) nesting in the core of the tank-bromeliad A. bracteata (Figs. 1A, B) were collected between January 2016 and April 2018 in several sites in the southern part of the Yucatan Peninsula as part of a larger project 39,40 . Brie y, A. bracteata bromeliads were examined for the presence of N. villosa and a ramet of the epiphyte, housing the ants' colony, was cut off from the supporting branch and dismantled leaf-by-leaf; ants, their brood, and any invertebrate myrmecophile found in the nest were collected and preserved in 96° alcohol. Ants and their associated fauna were collected under Mexican government research permit granted to GP-L (SEMARNAT, DGVS, License number FAUT-0277). DNA barcoding.
DNA extraction was performed on two of the thoracic legs of the larva using a standard glass ber method 75

Supplementary Files
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