Euborellia arcanum, Matzke, Danilo & Kocarek, Petr, 2015

Matzke, Danilo & Kocarek, Petr, 2015, Description and biology of Euborellia arcanum sp. nov., an alien earwig occupying greenhouses in Germany and Austria (Dermaptera: Anisolabididae), Zootaxa 3956 (1), pp. 131-139 : 136-137

Biology of Euborellia arcanum sp. nov.

Occupying habitats, burrowing behaviour. E. arcanum sp. nov. lives in underground galleries in tubes and chambers of varying lengths and shapes; burrowed under stones and trunks; or in moist soil. The species is active at night, when it leaves the underground galleries and searches for food on the soil surface. Nymphs of instars 2‒5 show a similar pattern of burrowing behaviour as adults.

Mating, oviposition and hatching. Copulation outside the galleries has not been observed and appears to be confined to the underground galleries. Mating can occur immediately after moulting to the adult stage. Sperm from a copulation is stored in a receptaculum seminis, and the sperm of a single male can fertilize several clutches of eggs laid over time (see Günther & Herter 1974). Males of E. arcanum sp. nov. have anextraordinarily long virga with an extended tip, which is used in the Anisolabididae to remove rival sperm of the preceding copulation from the spermatheca (see Kamimura 2014 and Discussion). Approximately 16 days after copulation, females build a breeding chamber and begin laying eggs. Females lay 35 to 65 eggs in each batch. The eggs are regularly cleaned, and unfertilized or damaged eggs are eliminated (eaten) by the female. After hatching, females care for the nymphs for the next 8‒12 days. From the moult up to the 2nd nymphal stage, nymphs leaving the breeding chamber and start to live independently from the mother. After 43‒55 days, the female usually lays the next clutch of eggs. Exceptionally, as in one case, females may lay a 3rd clutch of eggs. In another case, a clutch contained 20‒ 30 eggs, but no nymphs had hatched after 44 days.

Postembryonic development. The nymphs of E. arcanum sp. nov. undergo five developmental stages, wherein instars 1‒3 are relatively short compared instars 4 and 5 ( Table 2 ). In abreeding culture with temperatures of 23‒26°C, postembryonic development, from hatch to adult moult, lasted 98–293 days ( Table 2 ). The nymphs of the individual developmental stages are morphologically almost indistinguishable but can be separated by the size and number of antennal segments ( Tab. 1). After becoming independent at the second developmental stage, nymphs build their own burrows and live individually inside the tubes. During development, nymphs are very agile and can also be observed outside the galleries during the day.

Feeding behaviour. In breeding culture, nymphs and adults fed on dried Gammarus sp., apples and banana pieces. It was observed that the food is not usually eaten immediately, but that the earwigs retracted a small piece of food into the tube and ate it there undisturbed. This behaviour is independent of trophic pressure from other nymphs because it was also observed in nymphs breeding in isolation. Cannibalism was rare, but the earwigs often fed on conspecific carcasses. In addition, cannibalism was observed at lower humidity (<65%) or under conditions of high population density in the container.

Anisolabididae is a widespread and cosmopolitan family, containing predominantly wingless earwigs with dark and non-contrasting coloration. One specific characteristic of Anisolabididae is the male genital armature bearing two genital lobes, with one lobe directed distally and the second directed basally ( Steinmann 1989b, Srivastava 1999). Based on recent phylogenetic studies, the family appears to be monophyletic ( Haas 1995, Jarvis et al. 2005, Tworzydlo et al. 2010, Kocarek et al. 2013), but these authors included only a few species from the Anisolabidinae in their studies. Anisolabididae is subdivided into 13 subfamilies (sensu Srivastava 1999) and comprises approximately 400 species in 31 genera (Steinmann 1989; Srivastava 1999). Subfamily Anisolabidinae , to which the described species belongs, comprises 277 species in 13 genera ( Anisyutkin 1998a, b, 2004, Srivastava 1990, 1993a, 1999, 2003a, b, Nishikawa 2008, Kocarek 2011a, b). The species are difficult to distinguish based on external morphology because of the uniform habitus and relatively large variability ( Steinmann 1989b). The generic classification is based exclusively on the shape of the male genitalia, particularly the shape and size of the parameres ( Srivastava 1999). Euborellia (Burr, 1910) is a widespread cosmopolitan genus encompassing primarily small earwigs that are usually dark with non-contrasting coloration. This genus currently has 54 identified species, of which approximately 10 are Palearctic ( Steinmann 1989a, b, Anisyutkin 1998a, Srivastava 1993b, 1999, 2003b, Kocarek 2011a, b). Euborellia annulipes (Lucas, 1847) is the only cosmopolitan species.

Current knowledge of anisolabidine biology is based on only a few species studied in breeding culture, such as Anisolabis maritima (Bonelli, 1832) , A. littorea (White, 1846) , Euborellia cincticollis (Gerstaecker, 1883) , E. plebeja (Dohrn, 1863) , E. annulata (Fabricius, 1793) , and E. annulipes (Lucas, 1847) ( Klostermeyer 1942, Giles 1953, Herter 1960, Knabke & Grigarick 1971, Baijal & Srivastava 1974, Matzke & Klass 2005, Nonci 2005). All species studied, including E. arcanum sp. nov., pass through five developmental stages and show similar patterns of biology. However, the feeding behaviour observed in this study, in which food is retracted into an underground tube, is specific to E. arcanum sp. nov.

Males of E. arcanum sp. nov. have an extraordinarily long virga, more than 2.5 times longer than the entire body and likely one of the longest among all earwigs (see Kamimura 2014). Conspicuously elongated genitalia are found sporadically in Spongiphoridae and Anisolabididae and are used for the removal of rival sperm from the preceding copulation in the spermatheca ( Kamimura 2014). The mechanism of sperm removal and the significance of such behaviour had been studied in E. plebeja (Dohrn, 1863) and Mongolabis brunneri (Dohrn, 1864) . The males of these species insert the virga into the spermatheca without ejaculating and then extract the rival sperm using a fringe-like projection on the virgal tip while simultaneously ejaculating semen ( Kamimura 2000, 2014). Both species studied are highly promiscuous, and, due to multiple matings, their offspring show mixed paternity ( Kamimura 2005, van Lieshout & Elgar 2011).

By commerce, earwigs are frequently imported into Europe via plants and goods, as documented by Weidner (1974), who studied the importation of Dermaptera into Hamburg. The spread between botanical gardens and other greenhouses is likely facilitated by the trade and exchange of seedlings and other plant material. Thigmotactic and sciophilous earwigs usually live in potting soil, and their eggs are laid in wet soil. In Europe, alien earwigs in human habitats have been documented in Germany, United Kingdom, the Netherlands, Sweden, and Ukraine ( Harz & Kaltenbach 1976, Brindle 1977, Joost & Klausnitzer 1986, Wallaschek 1998, Borisch 2002, Heller & Haas 2013). The most frequently observed synanthropic earwig is E. annulipes ( Kocarek et al. 2015) .

According to tropical greenhouse employees, the majority of the plants in our study were imported from Florida. This was particularly true of the Biosphere Potsdam where almost all the plants originated in Florida (J. Neumann, pers. comm.). We can therefore assume that E. arcanum was introduced to Europe along with plant material from South Florida´s greenhouses or nurseries. Because the only native species of Euborellia in Florida seem to be E. ambigua (Borelli, 1906) ( Hebard 1921, Steinmann 1989a, 1989b), the source population is probably also alien, and the region of origin remains undetermined. The most probable region of origin could be Neotropical, due to the distribution of the morphologically similar and likely related species E. peregrina . In the past, two species of Euborellia , E. janeirensis (Dohrn, 1864) and E. peregrina , were accidentally transported with plant material to Europe from South America, but these species did not become established ( Weidner 1974).

Tables

TABLE 2. Duration of the individual nymphal stages and duration of development from hatching to the adult moult (in days).

Origin	n	1	2	3	4	5	Total
Biosphere Potsdam	30	17–21	7–13	11–26	21–119	89–114	145–293
Gondwanaland, Leipzig	30	22–41	10–14	6–22	28–67	32–74	98–218
Discussion