Introduction

The passalid Odontotaenius striatopunctatus (Percheron, 1835) is among the saproxylophagous organisms widely distributed in Mesoamerica. It inhabits tropical and subtropical humid forests, as well as humid temperate forests, mountain cloud forest and areas of secondary vegetation, between sea level and 2100 m (^{Reyes-Castillo, 1970}). It preferentially colonizes logs in an intermediate state of decomposition, building galleries between the sapwood and heartwood of dead trees, which then become a resource that is used as both habitat and food (^{Castillo & Reyes-Castillo, 2003}).

Odontotaenius striatopunctatus shows a subsocial behavior characterized mainly by biparental breeding care, larval food preparation from a mixture of their excreta and chewed timber innoculated with microorganisms (external rumen). They also build a cocoon to protect the larva before metamorphosis; the cocoon can be repaired by the parents and siblings, in case of damage (^{Valenzuela & Castillo, 1983}; ^{Reyes-Castillo & Halffter, 1984}; ^{Schuster & Schuster, 1997}).

Pseudoscorpions are frequently observed within passalid galleries and their presence has been documented since the late nineteenth century. (^{Vachon, 1940}) quotes: (Waterhouse, 1878, Hagen, 1879 and Ihering, 1893), who recorded a Chelifer sp. on Passalus sp. and (Tullgren, 1912), who lists different species of pseudoscorpions on the passalid Pelopides tridens (Wiedemann, 1823). From his observations in Ecuador, (^{Ohaus, 1909}) cites the presence of pseudoscorpions under the elytra of a passalid. (^{Reyes-Castillo & Hendrichs, 1975}), mentioned pseudoscorpions on passalids collected in night flying, attracted to light traps. (^{Aguiar & Bührnheim, 1992}, ¹⁹⁹⁸, ²⁰¹¹) documented the presence of various species of pseudoscorpions with numerous species of passalids in Brazilian Amazon forest. (^{Villegas-Guzman & Reyes-Castillo, 2005}, ²⁰¹²) cite new records of pseudoscorpions on passalids in the Mexican states of Chiapas and Veracruz.

Encounters between Pseudoscorpiones and Coleoptera have shown interesting interactions, such as those between the cerambycid Acrocinus longimanus (Linnaeus, 1758) and the pseudoscorpion Cordylochernes scorpioides (Linnaeus, 1758) where an interesting phoretic relationship, as well as intense sexual selection occurs in both taxa, (^{Zeh et al., 1992}; ^{Zeh & Zeh, 1992a}, ^1992b, ¹⁹⁹⁴, ¹⁹⁹⁷).

Pseudoscorpions of the genus Lustrochernes Beier have been reported on various insects and even on vertebrates (^{Villegas-Guzmán & Pérez, 2005a}): on Orthoptera: Acrididae (^{Hoff, 1947}), Coleoptera: Cerambycidae (^{Muchmore, 1971a}), Curculionidae (^{Beier, 1948}), Elateridae, (^{Beier, 1948}), Diptera: Calliphoridae (^{Beier, 1948}) and obviously on various species of Passalidae (^{Aguiar & Bührnheim, 1992}, ¹⁹⁹⁸, ²⁰¹¹) (Table 1).

Table 1: Species of passalids and pseudoscorpions exclusive of the American Continent, recorded together, coexisting in decaying logs. (*) It is the only passalid with different habitat, the dumps of ants of the genus Atta. 

Unlike the close relationship between A. longimanus and C. scorpioides, there is no evidence of any specific association pseudoscorpion-passalid; only (^{Beier, 1948}) mentions a relation between the pseudoscorpion Dithella javana (Tullgren) and the passalid P. tridens. In most cases, pseudoscorpions found on passalids are not species-specific phoretics or can be found on members of other Coleoptera families and subfamilies, especially Staphylinidae, Cerambycidae, Elateridae and Platypodinae (^{Reyes-Castillo & Hendrichs, 1975}; ^{Aguiar & Bührnheim, 1998}).

As for Lustrochernes grossus (Banks, 1893), it has been recorded on the spider Olios fasciculatus (^{Hoff & Jennings, 1974}), on cerambycids Ergates spiculatus (^{Muchmore, 1971a}) and Trogosoma chiricahuae (^{Muchmore, 1991}), as well as in rodent's nests (^{Villegas-Guzman & Pérez, 2005b}; ^{Francke & Villegas-Guzman, 2007}). On Passalidae, it has been recorded with the species Heliscus tropicus (Percheron), Passalus interstitialis Eschscholtz and Passalus punctatostriatus Percheron (^{Villegas-Guzmán & Reyes-Castillo, 2005}, ²⁰¹²).

Authors such as (^{Szymkowiak et al., 2007} and ^{Tizo-Pedroso & Del-Claro, 2007}) consider that due to the small size of pseudoscorpions (2-12 mm), their long distance travelling is limited, which is why phoresy has proven to be an effective mechanism for transport and colonization of new habitats. Phoresy is considered a non-parasitic partnership between a small animal (phoront) and a large one (host), which results in phoront transportation (^{Muchmore, 1971a}). (^{Poinar et al., 1998} and ^{Tizo-Pedroso & Del-Claro, 2007}) suggest advantages and disadvantages for this kind of relationship, and (^{Zeh & Zeh, 1992c}) considered predation behavior as the origin of phoresy. There are two characteristic types of phoresy: Active phoresy, when the phoront holds or clings to any structure of the host's body and passive phoresy, when the phoront places itself beneath or within any structure or cavity of the host (^{Vachon, 1940}; ^{Athias-Binche, 1994}).

We document the presence of the pseudoscorpion L. grossus, coexisting with the passalid O. striatopunctatus through an observational study in captivity and to discuss the phoretic relationship between the two species.

Materials and methods

In September 2000 and in October 2001, 20 adults of O. striatopunctatus were collected in a remnant of cloud forest, located 3 km southwest of the city of Xalapa, Veracruz, Mexico (19° 30' 38'' N, 96° 57' 03'' W) at 1300 m asl. The climate in the area is humid temperate (^{Peel et al., 2007}), the annual average temperature is 18 ºC, with an annual rainfall of 1 490.5 mm a factor that helps maintain high humidity.

In order to obtain offspring of passalids, 16 breeding terrariums were set, (six in the first year and ten in the second year), placing a couple (male and female) of O. striatopunctatus in a decaying log segment about 30 cm long and 15-20 cm in diameter. The logs were previously segmented in longitudinal sections (2.5-3 cm thick) with the aid of an electric saw, and tied with rubber bands to hold them together; this design facilitates monthly observations inside the galleries without disturbing the passalids (^{Hernández-Martínez & Castillo, 2003}). Throughout the handling process the logs showed no organisms, and were kept individually in plastic containers (37 cm × 28 cm × 20 cm) at 27 °C and 80% relative humidity, with a 14-hour light period.

Two reproduction periods of nine months each were conducted: the first one began on January 20^th and ended on October 11^th 2001, and the second was launched on October 17^th 2001 and ended on July 9^th 2002. In the second period ten couples were made with some of the young adult passalids obtained from the first period and adults from the second collection (Table 2).

Table 2: Presence of Lustrochernes grossus in nests of Odontotaenius striatopunctatus. 

Monthly observations of passalid family groups were conducted to record their stage of development, which consisted of removing the rubber bands, separating the "log segments" carefully and examining the contents of the galleries; during observations the presence of pseudoscorpions in the nests was registered with ad libitum sampling (^{Martin & Bateson, 1986}). To avoid disrupting the passalids and their reproduction, no direct observations were made under their elytra.

For identification, three female and three male pseudoscorpions were prepared using the (^{Hoff, 1949}) technique with modifications by (^{Wirth & Marston, 1968}). The specimens are deposited in the National Arachnids Collection of the Institute of Biology of the National Autonomous University of Mexico. Voucher specimens of O. striatopuncatatus are deposited in the collection of insects (IEXA) of the Institute of Ecology in Xalapa, Veracruz, Mexico.

Results

Pseudoscorpions were found in 11 of 16 passalids nests, in the two breeding periods. In the first period they were present in half of the nests (three out of six), and in the second, pseudoscorpions occurred in eight of ten nests (Table 2). Fifty one observations were made in the 11 nests where pseudoscorpions were present. In 17 observations (33%) from one to many were seen. These pseudoscorpions were L. grossus, being the first pseudoscorpion recorded on O. striatopunctatus.

When starting each of the two breeding periods, no pseudoscorpions were found on passalids or on the breeding device. In all cases, individuals of L. grossus became evident within the nests when adults of O. striatopunctatus were successfully established inside the log. Establishment was evident through the active building of galleries, the preparing of food for the offspring, the increased amount of saw dust, debris, space inside the trunk, and of course, the presence of offspring.

Due to the fact that no direct observations were made under the passalid's elytra to avoid disturbing them, and initially the logs didn't have these organisms, we believe that pseudoscorpions were under the elytra of adult passalids from the moment they were collected. We substantiated this assumption during the second period of passalid collection when splitting a log with an axe, we accidentally hurt two adult males and observed three pseudoscorpions of the same species under their elytra, one male and two female adults (Fig. 1). As is common with passalids, mites were also found on them (^{Hunter, 1993}; ^{Villegas-Guzmán et al., 2012}).

Figure 1: Adult male of Odontotaenius striatopunctatus and two specimens of Lustrochernes grossus damaged during collection. Scale bar = 3 mm. 

During the two nine month passalid nests observation periods, breeding was successful in 13 of the 16 nests. The presence of pseudoscorpions was evident in nine of them, which shows that the presence of L. grossus was no impediment to the successful reproduction of the host (Fischer exact test: P = 0.71, n = 16) (Table 2). Similarly, the number of passalids offspring was not affected by the presence of L. grossus individuals (Mann Whitney U = 11.50, P = 0.2553) (Table 2).

On three occasions, at the beginning of spring, females of L. grossus carrying their egg sac adhered to the ventral abdomen were observed: in April 2001 (nest I-5) and in March and April 2002 (nests II-7 and II-9), females were observed in the passalid galleries, 35 mm from the adult passalids. One of the egg sacs had 19 eggs (nest II-7). No silk chambers remnants were observed.

In the second breeding period, pseudoscorpions were found in eight of the ten nests and in four of these, the passalids came from the first breeding period, where L. grossus was present, while in the other four cases they could come from passalids collected in the field and/or from the first breeding period (Table 2). In all cases, we believe that pseudoscorpions came under the elytra of their guests, the founding passalids.

On several occasions throughout the observations of nests, we could observe the presence of pseudoscorpions in the galleries and their proximity to the passalids (15-45 mm) some also clinging onto adults and larvae, and even under the misshapen elytra of a juvenile passalid (Figs. 2 and ³).

Figure 2: Adult couple of Odontotaenius striatopunctatus in their nest next to a specimen of Lustrochernes grossus. Scale bar = 1.5 cm. 

Figure 3: Lustrochernes grossus in the nest under construction, together with Odontotaenius striatopunctatus, close up. Scale bar = 7.5 mm. 

Discussion

The association of the pseudoscorpion L. grossus with its host O. striatopunctatus was recorded for the first time. Cited cases of pseudoscorpions and passalids in the Americas thus increase to 36 (Table 1). Individuals of L. grossus travel under the elytra of passalids, just as other pseudoscorpions of the same genus do, with their hosts (^{Aguiar & Bührnheim, 1992}). Apparently, pseudoscorpions get off the passalids to get into the decaying logs, where beetles begin building their nest. In the nests there is much of debris produced by passalids, which favors the presence of mites and other micro-arthropods that can serve as food for the psudoscorpions (^{Weygoldt, 1969}).

Odontotaenius striatopunctatus successfully bred within the nests unaffected by L. grossus. The presence of pseudoscorpions increased from the initial three to the eight nests set for the second breeding period, which meant their establishment in this type of habitat and their passing from the parent passalids to their offspring, the presence of L. grossus does not seem to affect them (Table 2).

The presence of L. grossus females with brood sac was evident in three passalids nests with different stages of family development: a) in the presence of eggs, b) with larvae of different instars c) with young adults. It is worth mentioning that passalids are subsocial Coleoptera (^{Wilson, 1971}), they may show overlapping generations in their nests, a situation that allows extending the life of the colony for a longer time inside the dead tree. This may allow pseudoscorpion nymphs that have not yet matured to do so, and to be able to disperse among a new generation of passalids.

Based on observations made by (^{Aguiar & Bührnheim, 1992}) in Amazons on developmental stages of pseudoscorpion species traveling on passalids, a low frequency of phoronts in immature stages was found (tritonymphs), as well as gravid females. Most phoretic pseudoscorpions are adults of both sexes; there is in particular a higher proportion of males for three species of pseudoscorpions: Tridenchtonius mexicanus, (Chamberlin & Chamberlin, 1945), Lustrochernes intermedius (Balzan, 1891) and L. aff. reimoseri. (Beier, 1932). The same is reported by (^{Tizo-Pedroso & Del-Claro, 2007}) studying Paratemnoides nidificator (Balzan, 1888) where they also found adult pseudoscorpions of both sexes. In this study, we found one male and two female adults that confirm the suggestion by (^{Vachon, 1940}), regarding pseudoscorpions traveling under the elytra of Coleoptera, indicated that individuals of both sexes have been found.

As for phoronts preference for host age, (^{Poinar et al., 1998} and ^{Eickwort, 1990}) indicate a preference for adult hosts, since they have greater mobility and are potential builders of new nests. The fact that L grossus remain for some time in the nests with the passalids means that it is here where they make contact with their lignicolous hosts, sharing the microhabitats, which enables them to determine the right time to ride on them before dispersal, as suggested by (^{Beier, 1948} and ^{Muchmore, 1971a}, ^b).

It was common to see L. grossus cohabiting in the nests and even on both, adult individuals and larvae of O. striatopunctatus (Fig. 2). Phoronts may well get close to their hosts waiting for the moment of dispersion. Something similar happens between A. longimanus and C. scorpioides, since three to five generations of pseudoscorpions remain within the dead tree, until a cerambycid larva completes its development and they can then disperse (^{Zeh & Zeh, 1997}). In the case of passalids, young adults remain inside the log for several weeks, until they reach maturity. Another example occurs between the pseudoscorpion Dinocheirus arizonensis (Banks, 1901) and the fly Odontoloxozus longicornis Coquillett, which inhabits decaying saguaro cactus (^{Zeh & Zeh, 1992b}).

According to (^{Athias-Binche, 1994} and ^{Vachon's, 1940}) classification, L. grossus performs passive phoresy on O. striatopunctatus, as it travels under its elytra. For (^{Beier, 1948}) in addition to phoresy, this relationship is considered phagophilic as (^{Vachon, 1940}) emphasizes that not only do they seek transportation but they also feed on mites found on the host, although in this study there is not direct evidence of feeding on the host.

(^{Weygoldt, 1969}) has observed this behavior pattern in pseudoscorpions of the Chernetidae, where they sanitize their carriers by removing phoretic mites from their body surface. In this study, mites were found on O. striatopunctatus; thus, it is possible that pseudoscorpions had fed on some. (^{Villegas-Guzmán & Reyes-Castillo, 2005}) observed in Chiapas, an individual of Lustrochernes feeding on a mite beneath the elytra of a passalid.

The relationship between L. grossus and O. striatopunctatus might be phoretic-phagophilic providing mutual potential benefit between the two organisms; O. striatopunctatus, having mites and parasite control within their galleries, and L. grossus having a proper place to live, with the possibility of finding food facilitating its reproduction and ensuring transportation for later dispersal.

The relationship between pseudoscorpions and passalids is facultative, since both organisms can live independently from each other in decaying logs; therefore, it is not surprising that L. grossus chooses other lignicolous organisms as possible dispersing agents, as in the case of cerambycids (^{Mahnert & Adis, 1985}).

Lustrochernes grossus is a very plastic species that has been reported on various arthropods (^{Muchmore, 1971a}, ¹⁹⁹¹; ^{Hoff & Jennings, 1974}) and even on small mammals (^{Villegas-Guzmán & Pérez, 2005b}). Nonetheless, the lignicolous or corticicolous microhabitat plays a very important role in order for the encounter between phoront and host to take place.

The records between different species of the genus Lustrochernes and passalids (Table 1) show that the relationship between the two organisms is frequent and apparently nonspecific, resulting in the sinhospitality phenomenon, in which a kind of phoront uses more than one host species. Extremely specific phorethic associations might be counterproductive; undoubtedly they involve high risks for the specialization of the phoront with its hosts. From tertiary amber records, we know that, 40 million years ago, phoresy was already established (^{Ross, 1997}; ^{Poinar et al., 1998}), and cases of specificity are scarce; in extant cases many lignicolous host can be use by the pseudoscorpions.

As reported by (^{Reyes-Castillo & Hendrichs, 1975}), passalids that have been found with pseudoscorpions are species of wide geographical distribution and good flying habits, at least among neotropical species. Such characteristics are present in O. striatopunctatus, which favors the dispersion of pseudoscorpions, making phoresy an effective method for their distribution, Lustrochernes is widely distributed in the Americas and its family Chernetidae is one of the most diverse of the Pseudoscorpionida in which it is common to find cases of phoresy (^{Beier, 1948}; ^{Muchmore, 1971a}, ^1971b).