Mycotrupes LeConte, 1866

KrantzK, Gerald W., 2023, Notes on the New World allopatric beetle genus Mycotrupes (Scarabaeoidea: Geotrupidae) and on Geotrupacarus mycotrupetes (Krantz & Mellott) (Acari: Macrochelidae), an early derivative phoretic mite associate of Mycotrupes gaigei Olson & Hubbell, Acarologia 63, pp. 29-38 : 29-31

Mycotrupes LeConte

The geotrupid beetle genus Mycotrupes ( Fig. 1 ) comprises five flightless, relict, allopatric species that occupy deep, well-drained sand ridges overlying porous limestone in peninsular Florida ( Woodruff 1973) and a narrow, more northerly elevated sand hill zone separating the Atlantic Coastal Plain from the adjacent Piedmont in South Carolina and Georgia, a zone referred to as the Fall Line ( Fig. 2 ). Mycotrupes species are efficient burrowers, often tunneling down through sand to depths exceeding six feet ( Howden 1954, 1955). Their sandy habitat is mostly a xeric, open woodland in which the most abundant trees are turkey and bluejack oaks that originally served as the understory for a great pine forest that thrived there in earlier times. Clumps of seedling oaks and low shrubs surrounded by quantities of dead leaves, along

with a scattering of grasses and various herbs, also are common there ( Olson & Hubbell 1954).

Mycotrupes species are generalist feeders that take advantage of virtually any available organic

food substrate in their heterogenous environment: dung, decaying fungi, plant litter, and even

exposed soft tissue of acorns. As noted by Beucke (2009), reliance on a particular food source

where its availability from year to year or season to season often is unpredictable would be a

risky strategy for a genus whose members are flightless and consequently restricted in their

searching capacity. According to Howden (1955, 1963), flightlessness in Mycotrupes arose

early in the development of the genus and is a consequence of metathoracic wing degradation,

coupled with median fusion of the prothoracic elytrae.

The deep sand ridges that shelter Mycotrupes species are believed to be remnants of ancient

shorelines laid down during the alternating flooding/emergence episodes of the Pleistocene

Epoch ( Cooke 1945, Fig. 3 ). These sand ridges are isolated from one another by areas of poor

drainage, shallow sand, swamps, marshes, rivers, low-lying dry timberland, or dense vegetative

hammocks. Barriers such as these apparently were enough to produce the vicariance events

that led to allopatry of Mycotrupes populations and to their subsequent speciation.

Hubbell (1954) presented a compelling evolutionary scenario for Mycotrupes in which he

considered Mycotrupes gaigei Olson & Hubbell ( Fig. 1 ) to be the most aberrant member of

the genus, and its paleogeographic niche in western peninsular Florida ( Fig. 2 ) to be the site

where the genus made its first appearance during the late Tertiary. Hubbell’s conclusions were

later evaluated during the course of a molecular phylogenetic study Mycotrupes of by Beucke

(2009), whose findings were based on nucleotide sequence data derived from a 481 base pair

fragment of the Cytochrome Oxidase I gene taken from all five Mycotrupes species. Beucke’s

parsimony analysis revealed that the two Fall Line species, Myco. retusus (LeConte) and Myco.

lethroides (Westwood) , are basal to the other three described members of the genus Myco (.

gaigei , cartwrighti O. & H. and pedester Howden ), from which Beucke concluded that the

genus originated on or close to the Fall Line, possibly during the Tertiary. He also concluded

that, at some point after the sea level had fallen well below the level of the elevated Fall Line for

the last time in the late Pleistocene ( Fig. 3 ), a southward dispersal Myco of. retusus gave rise to Myco. gaigei , cartwrighti , and pedester , along with a possible cryptic species in the cartwrighti lineage. Based on Beucke’s findings, sea level changes generated by intermittent glaciation and melting events during the Pleistocene evidently played a major role in the evolution of Mycotrupes .

Figures

Figure 1 Mycotrupes gaigei Olson & Hubbell (from Woodruff 1973).

Figure 2 Mycotrupes species distribution in peninsular Florida and along the more northerly Fall Line (from Hubbell 1954, after Geol. Soc. Amer. 1952).

Figure 3 Diagram showing Pleistocene terraces (interglacial periods) and major sea level depressions (glacial periods), of the southeastern Coastal Plain and their presumed chronology (from Cooke 1945).