Micromegistus bakeri, , Tragardh, 1948

K, Nia Riggins, Young K, Monica R., RennerK, Tanya, K, Frédéric Beaulieu & K, Michael Skvarla, 2024, New records of Micromegistus bakeri, Trägårdh 1948 (Acari: Mesostigmata: Parantennulidae), a mite symbiotic on carabid beetles, and notes on the species’ distribution and host specificity, Acarologia 1948 (2), pp. 575-591 : 585-587

Micromegistus bakeri feeding behavior and relationship with the host

Scarites subterraneus are morphologically convergent with passalid beetles, sharing with them a large robust body, a stalked prothorax, strong mandibles, and a burrowing behavior. The species burrows under logs, rocks or litter, where it hides during the day, mostly motionless, and hunts at night, bringing their prey underground before feeding ( Hlavac 1967 ; Larochelle and Larivière 2003). It occurs in anthropogenic and open natural habitats, such as lawns, pastures,

open forests, and near water bodies. Their burrows can be as deep as 15 cm ( Larochelle and

Larivière 2003). Larvae live in the adult’s burrows, but also leave the burrows at night for hunting. It overwinters as adults in burrows ( Hlavac 1967).

Nickel and Elzinga’s (1970a) study showed that Micromegistus bakeri essentially completes its entire life cycle on S. subterraneus . Female mites carry two large eggs within the opisthosoma which develop concurrently and are born viviparously as larvae. In laboratory conditions, it takes about two weeks before two other larvae are born. Nickel and Elzinga (1970a) observed the birth of a larva, and stated that after emergence, the larva clung onto its mother’s venter for a few minutes, and then to its dorsum for a similar period of time. Finally, the larva “descended to the beetle and wandered about for several minutes on the carabid’s venter before settling near the junction of trochanter and femur II”. This indicates that larvae are born while the adult female is on the host. As much as 30% of all M. bakeri collected from S. subterraneus by

Nickel and Elzinga (1970a) being larvae further supports that. Our own observations ( Fig. 1 )

and museum data ( Table 3) are compatible with this.

The relationship between M. bakeri and S. subterraneus is unclear. Published observations suggest that M. bakeri is an opportunistic scavenger, a kleptoparasite, and possibly also a

‘paraphage’ that feeds on the host’s dermal secretions; it was also proposed to be a commensal

(Nickel and Elzinga 1971a). The overall net impact on their host, whether detrimental

(antagonistic), neutral (commensal), or even beneficial (mutualistic), is unknown. The impact may even shift when various parameters, such as mite density on a host, change.

The relationship between M. bakeri and S. subterraneus may represent a context-dependent commensalism, as recorded in other systems, including density-dependent changes from commensalism to antagonism and commensalism to mutualism ( Mathis and Bronstein 2020).

In the S. subterraneus-M. bakeri system, mites have been observed feeding on the remains of host-killed prey (a mealworm) near the host beetle, feeding on food particles near the beetle’s mouth, feeding on debris on the surface of the beetle ( Nickel and Elzinga 1970a), and even lapping up beetle regurgitation fluid off the host ( Johnston et al. 1947). Due to the size difference between the mites and their hosts, a few mites may not steal enough nutrients from a beetle to have negative effects (i.e., they are commensals), but may negatively impact host nutrient uptake (i.e., they are antagonists) when populations are higher. Nickel and Elzinga

(1970a) observed that the mites were “in constant motion, with their chelicerae making lateral,

wiping movements” across the body of the beetle and performed a preliminary experiment where radioactive sodium acetate was injected into the beetles to test if M. bakeri would assimilate the sodium acetate and thereby indicate that the mite was feeding on the host’s secretions. Their results were positive but inconclusive since the radioactive substance could have been absorbed without actual feeding on the host’s secretions. This purported feeding on the host’s secretions may have no effect on the host, be negative if it forces the host to expend energy replacing the excretions, or perhaps even beneficial if the mites are removing fungal spores or harmful bacteria. Finally, M. bakeri eliminates its waste onto the host ( Nickel and Elzinga 1970a). During heavy mite infestations the waste accumulates as a soft mass on the anterior portion of the beetle’s mesosternum and between coxae II. If this accumulation encourages detrimental microbe growth, it could negatively impact the beetle. In short, there are many potential impacts on the beetles that may vary with mite population size, so further experiments should be conducted to determine what, if any, impact the mites have on their hosts.

The mites appear to occasionally disembark their host, presumably for seeking food, and are sometimes brushed off by the beetle. Mites have been observed remounting their host, and the adults can even jump back onto the host by leaping a distance sometimes greater than four times their body length ( Nickel and Elzinga 1970a). This suggests that M. bakeri is adapted to wander off the host, probably during the day when the host rests in its burrows. The mite could then scavenge on invertebrate remains left by its host in its burrow or possibly even prey on invertebrates inhabiting the burrows, such as nematodes, although such facultative predatory behavior has not been observed yet. A strange blattisociid mite, Krantzoseius walteri Seeman (2012) , recently described in Australia may have a somewhat analogous lifestyle to M. bakeri . Like M. bakeri , all life stages of K. walteri appear to live on the host, Trichosternus perater ( Pterostichini ), a burrowing carabid unrelated to Scarites spp. Like S. subterraneus , T. perater brings captured prey into its burrow, where K. walteri possibly feeds on microfauna associated with the prey items, such as nematodes, but may also scavenge the rotting prey itself ( Seeman 2012 ; O. Seeman pers. comm. 2024).

Phoretic histiostomatid deutonymphs, which are occasionally found sharing the same host as M. bakeri , were not attacked by M. bakeri when both were found on the same host ( Nickel and Elzinga 1970a, Nickel 1969, Riggins 2023). Indeed, the weakly, nearly edentate chelicerae of M. bakeri , with their mop-like appendages, do not appear optimal for seizing active prey, but rather for mopping up fluids, such as those oozing out of dead invertebrates, a behavior seen in other Trigynaspida (Seeman 2000). The larvae, nymphs and adult females and males all having similar chelicerae ( Nickel and Elzinga 1970a), which suggests similar feeding habits throughout ontogeny.

Micromegistus bakeri probably colonize new individuals of S. subterraneus during mating, if beetles rest near each other during the day (e.g., under a rock or log), or if new hosts enter the burrow as mites were seen to transfer between adult beetles when reared together (Nickel 1969, Riggins 2023).

The life history of other Micromegistus species is unknown beyond basic collecting information. Immatures of M. viduus have been found on ground beetle hosts, as have immatures of undescribed Micromegistus species from Brazil (on leafcutter ants) and Australia (on carabids) ( Kim and Castagnoli 2010 ; Seeman and Nahrung 2000 ; O. Seeman pers. comm. 2024). This suggests that the immatures of all species in the genus live on the host, indicating an intimate association with the host beyond phoresy, like for M. bakeri .

Figures

Figure 1 Micromegistus bakerion Scarites subterraneus.a – Dorsal and ventral view of infested S. subterraneus. b – The anterior ventral side of M. bakeriinfestedS. subterraneus. c – Adult and larvalM. bakeri.