Established populations of the indoor silverfish Lepisma saccharinum (Insecta: Zygentoma) in red wood ant nests

1 Terrestrial Ecology Unit (TEREC), Department of Biology, Ghent University, K.L. Ledeganckstraat 35, B-9000 Gent, Belgium. 2 Depto. de Zoología, University of Córdoba, E-14071 Córdoba, Spain. 3 Research Unit of Environmental and Evolutionary Biology, Namur Institute of Complex Systems, and Institute of Life, Earth, and the Environment, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium.

(Atelura formicaria) which lives outdoors in strict association with ants [12]. Lepisma saccharinum Linnaeus, 1758 is a cosmopolitan silverfish that feeds on materials rich in starch and cellulose. It is frequently found indoors in humid places, such as attics, bathrooms, and basements and can become a pest species. In warmer regions, such as in the Mediterranean area, it can also occur outdoors under stones and in leaf litter. In Spain, L. saccharinum has also been recorded in nests of different ant species of the genera Lasius, Aphaenogaster, Messor, Tapinoma and Tetramorium. An overview of the habitats of L. saccharinum reported in the literature is given in Table 1. Crucially, L. saccharinum is typically bound to urban settings in temperate Europe, and only a few individuals were reported outside buildings so far ( [10], Table 1). Mendes [13] revised every citation of L. saccharinum known up to that date and wrote that, "with the exception of the Mediterranean subregion", this species "seems exclusively synanthropic, except for rare cases of colonizing natural habitats." In the framework of a detailed monitoring campaign of arthropods associated with red wood ants (Formica rufa Linnaeus, 1761 and Formica polyctena Förster, 1850), we found over a three-year period (2018-2021) large numbers of L. saccharinum in and at the periphery of these ants' nests ( Fig. 1). The identification of L. saccharinum was verified by checking the redescription of Mendes [14]. The absence of a setal collar on the pronotum, the types of specialized antennal sensilla and the type of paramera place it within the subfamily Lepismatinae. The urotergal chaetotaxy, the type of notal trichobothrial areas (all anterior open and all posterior closed) and the hyperdeveloped paramera are diagnostic characters of the genus Lepisma [15]. Lepisma saccharinum has a characteristic combination of characters (pigmentation, structure of infralateral groups of setae of urotergites, shape of the tenth urotergite, length and number of divisions of the ovipositor, etc.) that distinguishes it from other species of the genus [15,16].
We used plastic boxes (Sunware Q-Line Box: 27 × 8.4 × 9 cm, volume: 1.3 L) with a 1 cm layer of moist plaster on the bottom as traps to collect the silverfish. The sides of these traps were too slippery for the silverfish to escape from, but ants could easily climb out of these boxes. The traps at the periphery of the nest were buried so that their top rim was level with the soil surface. We covered these traps with a plastic roof to prevent rain falling into them. The roof was positioned 2 cm above the opening of the traps by attaching plastic caps in the corners of the roof (details and pictures see [17]). Traps were left for one week and then emptied. We used the same type of traps with roofs to assess the diversity in the nests. These traps were completely buried in the organic mound of the nest. As ants gradually filled the Figure 1 -Interaction between Formica rufa (red wood ants) and Lepisma saccharinum in the organic mound nest. The worker in the right figure tries to grab the agile silverfish. Habitats of Lepisma saccharinum according to literature on Lepismatidae. Only certain identifications have been included (*). Abbreviations: A = with ants; S = synanthropic; F = free-living (in natural habitats; N = no data. When the number of citations given in a work for a certain habitat is not mentioned, the occurrence of the species is indicated only with an X.  inside traps with nest material, these boxes had to be emptied every 1-2 days. Silverfish were collected to avoid double counting and traps were put back inside the mound. Between one and four traps were placed in and/or at the periphery of a nest at the same time (details of period of sampling and number of traps used are given in Table 2).
A maximum of 186 unique individuals was collected in nest A in 2020, 68 individuals were found in this nest in 2021. Nest A is located in a deciduous forest in Poperinge, Belgium (Table 2) Table 2).
In a next step, we were interested to investigate how the ants interacted with the silverfish. Some arthropods associated with red wood ants provoke little or no aggression whereas others are heavily persecuted [18]. We brought some silverfish and F. rufa workers from nest A to the lab to conduct behavioral assays. We prepared a circular arena with a moist plaster bottom and with the walls coated with an anti-escape layer (Fluon). Ten ant workers were put into the arena and were able to acclimatize for 30 minutes. Then we introduced a silverfish, waited for 10 s, and subsequently scored the aggression of the ants in the first twenty interactions with the silverfish. We replicated this with ten different silverfish individuals and replaced the workers for each trial.
Red wood ants responded very aggressively, 79.4% of the interactions were aggressive (biting, opening of the mandibles, chasing) on average, 8.9% of the interactions were biting (Fig. 1). The silverfish mostly tried to avoid contact with the red wood ants and ran away when detected. During the experiment, none of the tested silverfish were killed.
We recorded L. saccharinum in several outdoor ant nests. The very high abundances of the silverfish in nest A are especially intriguing. The presence of all stages over a three-year period indicated that a stable population has been established in the very hostile red wood ant environment. Although the red wood ants readily detected and attacked the silverfish, they may survive by hiding in the organic material, similar to some strongly attacked obligate ant associates [19,20]. The silverfish are likely attracted to the thermoregulated and humidity-controlled environment and the ample food resources. In that way, heated ant nests can serve as a stepping stone for thermophilous organisms to colonize other outdoor niches. It can be expected that L. saccharinum will increasingly explore outdoor habitats in temperate regions as an effect of global warming.