The biodiversity in the canopy of tropical forests is a frontier of knowledge that can still reveal great surprises45. 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 in46). Our endeavor to describe this specific type of biodiversity unveiled an unexpected symbiotic interaction between a butterfly 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 first case of a butterfly caterpillar in intimate symbiosis with a ponerine ant. Our finding is the first 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 revealed20,47-52. 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 nests20,28,52. 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 characters26. This group comprises rare and restricted taxa associated with tropical forests and is the latest subtribe of Nymphidiini whose natural history information has been published31. 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 specific ants as oviposition cues; e.g., Minstrellus grandis33, 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 stingers3. Most species are specialized or generalist predators, occupying a high trophic position54,55. The use of liquid foods on foliage, however, has been recorded for some species such as N. villosa which harvest liquid secretions from extrafloral nectaries, exudates from honeydew-producing hemipterans and secretions of some facultative myrmecophilous caterpillars56-58 (Supplementary Table S1). According to DeVries11 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 caterpillars11,15,59,60. This hypothesis can be proved both in ecological time61 and evolutionary time if the specialization between P. agave caterpillars on N. villosa ants is confirmed.
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 ants57,62. The first 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. villosa57; 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 liquid57. 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 carnivory63,64. Carnivory in riodinid caterpillars has been documented in five lineages of Nymphidiini preying on ant-tended hemipterans on plants31; however, myrmecophagy has only been recorded under artificial conditions34,65.
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 xanthe31). Although phytophagous, Menander caterpillars are covered with a prominent carapace that flares outward to the substrate and covers the body and legs; they are thus heavily armored and possess a complete set of ant-organs28,66. 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 traits31. 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 nest20. This seems to be related to the ontogenetic moment when ant-organs and chemical compounds that trigger ant adoption are produced67. 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 inflorescences of the bromeliad is not known but seems possible. As suggested for other specific lycaenid species associated with ants68, the scarcity of this butterfly might be explained by its highly specialized life history, making the butterfly distribution dependent on the N. villosa-bromeliad association. Finding answers to the questions raised in this study can only be possible with more field effort and new data on natural history.