Host specificity is a fundamental characteristic in symbiont communities. It falls along a continuum with, at one end, host-specific symbiont species that target one or few narrowly related hosts and, at the other end, host-generalist species that engage with many, unrelated hosts . It is expected that symbiont species may co-exist by a trade-off between host range and the average fitness achieved on different host species. This “jack of all trades is master of none” model predicts that generalist species may associate with different host species, but have a lower average fitness on the hosts shared with specialists .
Macrosymbionts employ a wide range of behavioural, chemical, acoustical and morphological tactics to approach, attract, manipulate and even deceive their host [3, 4]. There has been a strong focus on the chemical deception strategies of very specialized parasites, especially those targeting Hymenoptera. Several studies showed that these symbionts may hack the chemical signalling system of the host. A spectacular example can be found in a blister beetle, whose larvae lure and climb on the male of their solitary bee host by mimicking the sex pheromone of the female bee. Eventually, they pass on to the female during mating and are transported to the nest . By contrast, behavioural strategies of symbionts that facilitate the association with the host have been poorly studied so far. However, more and more it appears that symbiont behaviour plays a pivotal role, either on its own or in combination with other tactics, for a successful host association of host specialists and generalists [6, 7]. Studying different tactics simultaneously in host symbiont communities may hint at how symbionts employ a range of different tactics concurrently and whether the relative importance of the tactics changes with increasing host specificity.
It has been argued that increasing host specificity within symbiont clades is accompanied by specialization [8, 9]. The specific association of organisms with a particular host environment imposes selection for specialization of traits . This link between host specificity and trait specialization has mainly been studied in insect herbivores , but some studies tested this association in symbiont lineages as well . For example, the attachment structures in monogenean parasites are more specific and specialized with increasing host specificity . Another striking example is oviposition site selection of parasitoid wasps, with endoparasitoids being more host-specific than ectoparasitoids .
A remarkable gradation of host specificity can be found in the different groups of arthropods associated with ants. Some species of these ant guests or myrmecophiles only target a single ant species or genus, whereas, at the other extreme, some may associate with all ant species in their distribution range . In line with the positive association between host specificity in other symbiont systems [11, 12], high host specificity in myrmecophiles is often linked with increased specialization in chemical ecology, behaviour and morphology [14–17]. This increased trait specialization enables them to approach their specific host and intimately interact with them (integrated myrmecophiles, see ). The degree of myrmecophile specialization is most prominent in the divergence of chemical and behavioural strategies. Ant colonies have an intricate nestmate recognition system, which is based on a unique blend of cuticular hydrocarbons, the so-called “nest odour” . Ants tend to reject or attack individuals when their odour deviates from the familiar colony odour . Many myrmecophiles dupe the host by chemical disguise of the nest’s odour, which enables them to stay undetected in the nest. They either actively produce (chemical mimicry) or passively acquire the chemical odour (chemical camouflage) of their host colony [20, 21]. Active production of hydrocarbons is often found in well-integrated myrmecophiles with a narrow host range, whereas passive acquisition is more flexible and allows host species switching [20–22]. An alternative form of chemical disguise is when ant associates suppress the concentration of hydrocarbons below detectability for the host. This makes them odourless (chemical insignificance) and thus virtually undetectable . Chemical insignificance on its own is probably a general strategy employed by non-integrated (low trait specialization, and tend to avoid host interaction, sensu Kistner 1979 ) social insect associates with a broad host range [21, 23–25]. Crucially, cuticular deception strategies are not mutually exclusive and often used in concert. Several studies showed that social insect parasites make first use of insignificance or chemical mimicry to approach and invade the host colony. Once integrated, they finetune the chemical deception by passively acquiring the host-colony specific cues [20, 26, 27]. Highly integrated myrmecophiles often secrete substances that manipulate the behaviour of the ant host in concert with chemical deception . They also show advanced behaviour to facilitate their integration in the host colony. Their behavioural repertoire consists of approaching the host, climbing on the host worker, allowing inspection, exchanging of food and grooming with workers [15, 16, 29]. By contrast, the chemical and behavioural strategies of a large group of non-integrated myrmecophiles lack adaptations to their cuticular profile. They resemble their free-living relatives and are detected as intruders . Ants display aggressive behaviour towards them, but they might survive the hostile nest environment by displaying unspecialized behaviour, such as fleeing, hiding, ducking, feigning death or repellent secretes . Non-integrated species often have a broader host range than integrated species [15, 16], although some of these non-integrated species tend to have a narrow host range [13, 30]. These species are probably attracted to particular nest conditions or food sources only found in nests of related ants (e.g., myrmecophiles in organic mounds of Formica ants , carton nest in Lasius fuliginosus ).
A large number of silverfish (order Zygentoma) within the families Nicoletiidae and Lepismatidae made the transition from free-living soil dwellers to facultative guests of ant and termite nests and ultimately to permanent social insect associates [29, 33–36]. Silverfish are wingless, primitive insects with distinctive scales on their body and are particularly species-rich in the Iberian peninsula [36, 37]. Based on their dependency on ants, silverfish can be categorized as (1) unassociated (free-living) species, found away from ants (2) occasional or facultative myrmecophiles (occur in ant nests, but can also be found away from ants) and (3) obligate or strict myrmecophiles (always found in ant nests). The latter group spans a gradient of host specificity with species showing no specific host association, to host-specific species that are mainly restricted to the nests of a single ant genus, especially Messor or Aphaenogaster . Previous research suggested the use of different chemical deception strategies in myrmecophilous silverfish with Malayatelura ponerophila displaying chemical mimicry and two unidentified species employing chemical insignificance [24, 35, 38]. In contrast to other myrmecophilous groups such as beetles and flies , there is apparently limited morphological divergence between host-specialist and generalist species and even to free-living relatives. The most defining morphological features are the yellow colour of most myrmecophilous species, a reduction of the length of terminal filaments and a trend to a limuloid shape, with the lateral areas of the thorax expanded .
Currently, few studies compared the behavioural and/or chemical integration and deception mechanisms within a single lineage of myrmecophiles with different degrees of host specificity. They either focused on two species at the extremes of the host specificity gradient (crickets in ) or compared host-specialized species (associated with one genus) with extreme host specialists (associated with one species, rove beetles in ). Our aim was to study behavioural and chemical strategies along the different stages of ant host specificity in a large group of European silverfish (species belonging to the order Zygentoma: families Nicoletiidae and Ateluridae) encompassing unassociated free-living species, facultative ant-associates, obligate ant associates with a broad host range (generalists) and host-specific species (host specialists). Furthermore, we analysed the CHC profile of freshly moulted individuals of host generalist and host specialist silverfish species that chemically mimic their host. This allowed us to infer whether the CHCs were passively acquired (= chemical camouflage, moulted individuals would then lose the host profile) or actively produced (= chemical mimicry, moulted individuals would still carry the host profile even in absence of the host).
We hypothesized that facultative myrmecophiles have an idiosyncratic cuticular profile, deviating from their host and display similar behaviour than unassociated silverfish. We further predicted that obligate myrmecophilous species show different strategies, where species with broader host ranges relying on chemical insignificance, generalist species passively acquiring the host’s profile and the host specialists relying on chemical mimicry. In parallel, we predicted that unassociated and facultative species elicit high levels of aggression and display avoidance behaviour, whereas host specialist species provoke little or no aggression and tend to approach their host.