Among the diverse menagerie of army ant-associated arthropods are numerous species awaiting scientific discovery. Here we have described one of them, the histerid beetle Nymphister kronaueri with an unusual mechanism of phoretic transport (Table 1). The beetles specifically grip the trunk between the ants’ two petiolar nodes (petiole and postpetiole) with their long mandibles (Fig. 1c, d). Possibly, the unique fronto-clypeal structure of this species helps to ‘anchor’ the beetle to the ant tightly (Fig. 1d). The beetles attach preferentially to workers of medium body size. This way they participated as hitchhikers in the frequent colony emigrations of their single host species Eciton mexicanum s. str.

Various myrmecophiles hitchhike in army ant emigrations by either attaching at different ant body parts or by attaching on brood or booty that is being carried by the ants (Table 1) [24, 58, 59]. Similar to N. kronaueri, some myrmecophiles are very specific in the location at which they attach to ants. Mites, in particular, are transportation specialists partitioning the ant bodies into ˈphoretic microhabitatsˈ [59]. For instance, a mite species of the genus Circocylliba Sellnick, 1926 [60] attaches specifically at soldier mandibles while another species, Macrocheles rettenmeyeri Krantz, 1962 [61], is only found at the pulvili of Eciton dulcium legs [59]. We are aware of only one other myrmecophile that attaches at the same position as N. kronaueri, the histerid Latronister rugosus Reichensperger, 1932 [58, 62]. It also attaches between the two petiolar segments of its host workers E. hamatum and E. vagans, but it rides in a ventral position. Interestingly, like N. kronaueri, it rides specifically on medium-sized workers but not on majors or small workers [58]. Irrespective of the exact mechanism, phoretic transport can be interpreted as a more advanced, energy-saving host-following mechanism compared to myrmecophiles following the colony emigrations on foot [3, 15].

To a human eye, attached N. kronaueri beetles are difficult to detect during army ant emigrations as they somewhat resemble the gaster of host ants in size and shape (Fig. 1). If this resemblance fools potential predators and as a consequence increases the beetles’ fitness, it constitutes a case of adaptive resemblance. We cannot exclude this possibility, but it seems unlikely to us. Nymphister kronaueri was exclusively collected during nocturnal ant emigrations and we did not detect it during daily ant raids. Hence, it seems unlikely that nocturnal predators, which barely rely on visual cues, have caused selection pressures on N. kronaueri to resemble a host gaster. In this context, it is also worth noting that other histerids with a generally similar body shape do not attach at this particular position (see for example Ecclisister bickhardti costaericae Reichensperger, 1935 in Fig. 3). Nonetheless, imitating a part of an ant’s body, here a gaster, might hamper the recognition by ants themselves, facilitating social integration of myrmecophiles into ant societies (e.g., [7, 27, 62]; see also discussion about tactile mimicry below).

Besides its unique mechanism of phoresy, Nymphister kronaueri’s external morphology, particularly its cuticular microsculpture and high density of macrosetae, constitutes an exception within the genus Nymphister (Table 2; see for comparison the smooth cuticular surface and lack of macrosetae in N. simplicissimus, Fig. 3). These morphological characters are also found in Ecclisister beetles, another group of Eciton-associated histerids of the tribe Nymphistrini that shows a similar host specificity on Eciton burchellii, and a similar caste specific phoresy on the underside of major worker heads (Table 1). The close morphological similarity of N. kronaueri and Ecclsister beetles led us to speculate about the possibility of convergent evolution deriving from occupying the same selective environment (for a discussion about possible selection pressures acting on myrmecophilous beetles see [8]). Frequent and close contact with army ant host workers, which principally represent potential predators for symbionts, might be responsible for the evolution of a high setal density on the cuticle of these beetle inquilines. This is because one of the primary functions of setae is mechano-reception [63, 64], possibly providing the necessary mechano-sensory input for nest-inhabiting myrmecophiles to avoid being caught by host ants. Frequent host contact might also be related to the second characteristic shared among these beetles, the integumental microsculpture. In our opinion, the microsculpture of N. kronaueri and Ecclisister is fairly similar to that of their host ants, a phenomenon that has been described for numerous myrmecophiles [1, 3, 7], including histerids [35]. Reichensperger noticed that histerid species with a rather loose, facultative association with army ants rather resemble the more typical histerid microsculpture of a smooth and shiny cuticle, whereas obligate myrmecophiles generally show a high degree of microsculpture similarity to host ants [35]. Upon tactile inspection by ant workers, a cuticular microsculpture resembling those of host ants might help to fool the host into accepting the inimical guests as nestmates (tactile mimicry; [1, 7, 65]).

Finally, the analysis of DNA barcodes in N. kronaueri deserves discussion. Sharing the same COI haplotype among species is rare [66], which is the reason why DNA barcoding is a useful tool to screen for species boundaries and to detect cryptic diversity [65–68]. For instance, we discovered morphologically cryptic species in Eciton-associated Vatesus beetles with the help of DNA barcodes (Staphylinidae: Tachyporinae) [30]. In contrast to Vatesus, we did not detect distinct COI clades, i.e. evidence for cryptic diversity, in N. kronaueri. Instead of genetic divergence, we only found a single COI haplotype for 44 analyzed specimens in the population at LSBS, an interesting and exceptional case of missing genetic variability among Eciton myrmecophiles at LSBS ([30, 48], and unpublished data, CvB). A complete lack of variance in COI is unusual in native arthropod populations and the possible causes are diverse [67–74]. A possible explanation is a recent genetic bottleneck, maybe due to a recent dispersal event followed by the expansion of a small genetically uniform founder population, a population genetic pattern typical for invasive species [71–75]. More elaborate population genetic markers and the inclusion of different geographic areas will be required to explore reasons for this lack of variability in N. kronaueri.