Histeridae Gyllenhal, 1808

Caterino, Michael S. & Harden, Curt W., 2022, Unseeing and Unseen: On the Distribution, Morphology, and Larva of One of North America's Rarest Histerid Beetles, Geocolus caecus Wenzel (Coleoptera: Histeridae), The Coleopterists Bulletin 76 (2), pp. 191-205 : 193-197

Histeridae Gyllenhal, 1808 Dendrophilinae Reitter, 1909

Bacaniini Kryzhanovskij, 1976 Geocolus Wenzel, 1944 GEOCOLUS CAECUS Wenzel, 1944

Subterranean pitfall trapping has revealed G. caecus to be surprisingly widespread. In addition to the type locality in central Georgia, and a locality previously posted to BugGuide (www.bugguide.net) in north-centralAlabama (Blount County, ~ 33.8847°N, 86.4219°W), the species is now also known from northwestern South Carolina ( Pickens and Oconee counties) and southeastern Kentucky ( Monroe County ), a range spanning over 300 km of latitudinal and longitudinal distance (see Fig. 5 , and Appendix 1 for details on all specimens; all vouchers are deposited in the Clemson University Arthropod Collection ). For a blind, flightless, subterranean beetle, attaining such a broad range indicates a long residence time .

Five adult specimens were collected by hand underneath embedded rocks. One individual was found under a large flat rock on damp sand near the Little River (34.3964°N, 85.6271°W) in Cherokee Co., AL on 11 October 2021. The beetle was on a soil casting produced by either a worm or a millipede and became active as soon as it was exposed . Four specimens were found under rocks near Martin Creek Landing (34.6388°N, 82.8644°W) on Lake Hartwell in Oconee Co., SC on 22 October 2021. The rocks were all deeply embedded in damp, sandy soil in deciduous woods . Two specimens were under the same rock (one on the soil surface, the other on the underside of the rock); the other two individuals were found singly, one on soil and the other on a moist, rotten root. Other invertebrates found under the same rocks were diplurans, linyphiid spiders, anilline carabids, and Diptera larvae (possibly bibionids). No Geocolus were collected from the soil washing samples taken near the type locality in Peach Co., GA .

Subterranean pitfall trapping produced 36 adult specimens, all but one of them from a forested plot in a residential neighborhood (34.7252°N, 82.8247°W) in Pickens Co., SC. From 26 April 2020 to 14 August 2021, three traps were operated at this site. They were collected and reset on 12 July and 2 October 2020; and 16 January, 11 April, and 14 August 2021. Total numbers of captured Geocolus for each of the three traps were zero, three, and 22. After 14 August 2021, the least productive trap was removed and four new traps were installed. The six traps were collected on 24 October 2021, and produced 10 additional specimens, with all but one trap collecting at least one. Three individuals were collected from traps near Hestand, in Monroe Co. , KY: a single adult female was collected between 8 May and 3 July 2021 in one of four traps set in a small patch of deciduous forest behind a residence, with hayfields on either side (36.6579°N, 85.6259°W) . A single larva was collected in another trap at the same site from the same period. Traps set from 25 February–8 May and 3 July–4 September 2021 collected no specimens at this site. During the same 8 May–3 July 2021 period, one larval specimen was also collected from a trap set in a mature deciduous forest in a steep gorge nearby (36.6580°N, 85.6218°W). No adults were collected at this last site.

With regard to seasonality, it seems that adults may be active nearly year-round. Alabama collections have been made during January, March, May, and December. For other localities, successful traps were out over longer ranges, from April–July and July– October in South Carolina, and from May–July in Kentucky. Further cool season trapping will continue in order to document clearer patterns in activity times.

We obtained COI sequences from adult specimens from Alabama (three) and South Carolina (two), and a first-instar larva from Kentucky (deposited in GenBank under accession numbers ON072257–ON072262; see Supplemental File 1 for data matrix). Phylogenetic analyses of these sequences along with various available Histeridae barcode sequences, including representatives of five other Dendrophilinae ( Dendrophilus Leach [two spp.], Bacanius LeConte [two spp.], and Paromalus Erichson ) joined all putative Geocolus sequences into one clade with> 90% parsimony bootstrap support ( Fig. 4 ), supporting unambiguously the identity of the first-instar larval specimen from Kentucky with Geocolus . Sequences of Geocolus from the three localities differed by over 8% (pairwise uncorrected p-distance), ranging from 8.3% KY-AL to 12.7% SC-AL. Under some common assumptions of mitochondrial divergence rates (2.3–3.5%/lineage/million years; Brower 1994; Papadopoulou et al. 2010), this suggests somewhere between 1.8 and 2.8 million years of separation for populations across this range. Other prospective relationships were poorly supported, as expected for COI alone at such deep divergences.

Such divergence levels point to the possibility that these populations could represent distinct species. However, close examination of available specimens does not reveal clear morphological differences to support such a hypothesis. The genitalia of males from the South Carolina and Alabama localities do show slight differences in size proportions of the aedeagus (see Figs. 6–9 ), with that from Alabama being slightly shorter in length, and thicker in depth. We have not compared these directly to the male type specimen (from Georgia), but Wenzel’s figures suggest a thicker aedeagus similar to that of the Alabama male. No males from Kentucky are yet available to compare. Future exploration of detailed differences from additional populations, and sequences of additional genes, may eventually support distinct species status for some of these locations .

Description of the First-Instar Larva. Length: 1.8 mm. Body elongate, slightly narrowed posteriad, conspicuously setose, creamy white, head and prothorax light brown, with smaller and more weakly pigmented sclerites on other thoracic and abdominal segments, intersegmental membranes thin, weakly secondarily annulate, wrinkled. Head capsule, dorsal ( Figs. 10 , 12 ): Depressed, nearly parallel-sided, slightly longer than wide, lacking stemmata; epicranial sutures completely dividing base of vertex, united at base, divergent anteriad to near antennal insertions; nasale with 2 obliquely truncate teeth, subacute apicolaterally, each with fine median emargination; nasale anterolaterally with 2 medium-length frontal marginal setae (gFR); setae PA2 and PA8 prominent on nasale, PA3–6 small and interspersed; seta PA22 longest of those on head, extending laterad about one-half head capsule width, inserted about one-third from head base; PA21 slightly shorter, inserted close to antennal insertion; most other dorsal parietal setae and pores identified by K&P present and similar in position, except PA 13 in a more medial position, between the large PA14 and PA22; head ventrally ( Fig. 13 ) with conspicuous tentorial fossa, anterior margin of head capsule with narrow, rounded lobe produced below base of labium; setae PA23–26 conspicuous along lateral margin, pores PAn–p, r–s present as in K&P, PAq absent. Mandible with single, mesal tooth, inner edges smooth, dense penicillus restricted to small area near mesal base; with MN1 long, one-third from base, MN2 minute, pore MNa present, close to MN1, MNb near lateral margin two-thirds from base, MNc distad mesal tooth near mesal edge. Maxilla ( Figs. 14–15 ): Stipes with gMX1 linear, conspicuous, comprising ~10 long setae, weakly plumose; gMX2 absent; MX 2–4, a–b present on ventral surface, MX 3 longest; inner apical membrane of stipes lacking setae; basal palpomere with MX 7 long, MXe (ventral) and MXf (dorsal) present, its apical membrane with 1 fine seta near base of appendage; digitiform appendage of basal palpomere with 1 long seta, MX 8; palpomere 2 lacking setae, with pore MXg; palpomere 3 with MXh on ventral surface, and single series of circumferential pores (figured but not individually identified by K&P); apical palpomere with numerous pores, elongate sensorium near apex of dorsal surface, and about 5 small terminal setae. Labium ( Figs. 15, 18 ): Prementum about 1.5× as long as wide, lateral margin with blunt expansions just basad middle; ventral surface of prementum with tiny LA1 seta near base, LA2 close to apex of sclerotized portion, LAa near midline; dorsum of prementum with numerous small teeth uniformly distributed to near apex; LAb large, toward apex, MX 3 in its apical membrane, with unnumbered seta basolaterally; basal labial palpomere short, without setae or pores; apical palpomere with single conspicuous ventral pore two-thirds from base (not identified among those figured by K&P), inner, dorsal surface with elongate sensorium (“SD” of K&P) near apex, apex with ~6 short setae. Antenna ( Fig. 14 ): Basal antennomere with dorsal ANa and ANc near inner edge, ANb and ANd near outer edge; second antennomere with ANe about one-third from base, apex with large sensoria SE1 and SE2, with 4 smaller sensoria, 2 basad of SE2, 2 along outer edge; terminal antennomere with 6 apical sensilla; all antennal sensilla with distinctly broad bases and narrow apices.

Thorax ( Figs. 19–20 ): Pronotum with broad sclerotized shield, with separate sclerotization along posterolateral edge, desclerotized along midline, with “brick-wall-like” pattern of microsclerites anteriad; anterior margin of shield with (from midline to lateral corner) TEa, small TE1, TEb, medium TE2, TEc, TE13, a small unrecognized pore, then TE12 laterad; lateral TE4 short, TE5 long, nearly half pronotal width; middle of disc with TE8–11, TEd–i; posterior margin with 2 small pores TEa–b with 2 small setae TE1–2 between them; small pleural sclerite bearing 1 seta TE17; large lateral TE19 present at side; ventrally, presternal plates lightly sclerotized, very finely asperate, anterolateral sclerites more lightly sclerotized; median presternal sclerite with PR7, 9; lateral presternal sclerite with PR4–5; PR23–24 short, present on lower edge of hypomeral sclerite, ST40 present on hypomeron laterad procoxa; prosternite with SE44, SE46; ST30 absent; precoxite present, small, bearing setae ST32, ST36. Mesothorax: Dorsally with broad median and small lateral sclerites present, median mesotergite desclerotized along median ecdysial line and more weakly at sides, seta TE5 very long, inserted at posterolateral corner, TE4 medium, in anterolateral corner, small TE8 at middle of anterior margin, with TEf–g slightly behind, equally spaced to midline; TE7, TE9 (longest), and TE11 present near posterior margin, TE10 not evident, TEi present anterolaterad TE7; lateral sclerite bearing large TE19, medium TE16 anteriad, and 2 small setae (TE17 posteriorly, other unidentified) along inner edge; TE2 and TEb present in small sclerite between major tergites; mesosternum with large quadrate median sclerite with ST30 anteriad midline and ST44 and ST46 along lateral margin, membrane anteriad and laterad mesosternite with numerous minute denticles; small anterolateral sclerite (precoxite of K&P) with ST31, 32; ST35 and ST36 present in membrane anteriad mesocoxa; triangular laterosternite present behind coxa, lacking setae; small sclerite laterad coxa with ST23, 24, and 40; ST28 free in pleuron. Metathorax: Dorsal sclerites similar to those of mesothorax, though shorter and narrower, with medially divided mesotergite and small, elongate lateral sclerites, chaetotaxy same; metasternite shorter, wider than mesosternite, otherwise metaventral sclerites and chaetotaxy similar to mesothorax.

Legs: All legs similar in form and chaetotaxy, homologies with K&P not determined; coxa with approximately 8 medium setae around apical margin; trochanter with 2 medium and 4 small setae; femur with several ventral setae; tibia with 2 small ventrobasal setae, numerous tiny apical setae; 2 setae in membrane near base of claw, and 2 setae on ventral base of claw; tarsungulus not divided into basal ring and apical claw (as K&P show for Onthophilus ).

Abdomen ( Figs. 19–20 ): Segments separated by strong intersegmental constriction; each segment further subdivided all the way around the body by weaker constriction into major anterior (bearing all sclerites) and minor posterior subsegments; dorsum with transverse band of ampullar asperities, ventrally these become increasingly concentrated into paired bilateral ampullae toward end of abdomen. Segment I: Dorsum of abdominal segment I with wide tergites, narrowly divided transversely by a dense band of asperities; AB1 with small egg-bursting teeth (eb) present on posterior edge of anterior tergite; anterior tergite weakly subdivided at sides, with long lateral seta TE3 and closely associated pore TEe; posterior tergite partially subdivided by anterolateral indentations, bearing minute TE10–11 along anterior margin near midline, TE9 large, TE8 minute, and pore TEg closely associated with base of TE9; TE 4 in anterior corner, and TE5 posterolaterad separation; triangular anterolateral sclerite bears TE16 medially and TE19 laterally; small lateral sclerite bears small TE26 and long TE27; posterior abdominal sterna with more distinctly defined bilateral ampullae, with setae ST44–45 immediately behind them; else similar through sternum VIII, small ST46 setae becoming obsolete; tergum IX with single wide, short median sclerite, bearing only TE16 at its side; TE21 on small lateral swelling and TE26–27 on small lateral sclerite; sternum IX with narrow, transverse sclerite bearing small ST38 near middle and large ST40 at side, ST27 and ST28 present in lateral membrane; segment X (pygopod) dorsally with weakly subdivided oval median sclerite with PP9 medially, larger PP5 laterally, PP19 on round lateral sclerite; fine PP 18 in membrane behind median sclerite; thick, distinctly peglike UG3 present in membrane near base of urogomphus, with fine UG2, UG4 on either side; ventrally with anterolateral PP7, laterally with tiny PP6, posterior surface of pygopod with PP2 and PP4 large, PP5 small and nestled between them; base of urogomphus with strong UG6 ventrally, penultimate segment of urogomphus with 2 setae, UG8–9, along inner apical margin with 2 pores UGc–d interspersed; terminal segment of urogomphus with UGe on dorsum and long UG12–13 at apex.

Differences in Putative Second Instar (from SC, No Sequence Obtained): Marginal teeth of nasale blunter; appendages of head (antennae, labium, maxilla) narrower, more elongate; lateral processes of prementum less prominent, and teeth of dorsal surface of labium restricted to basal third of surface. Posterolateral margins of the pronotal shield thickened, darker. Cervical PR1–3 setae not evident anteriad median prosternal plate. PR4 and PR5 obscured among asperities of anterolateral prosternal plates. Large posterior TE9 seta missing, possibly broken off (though also not visible in pre-extraction photos); TE1, 2, and 13 absent from anterior pronotal margin (possibly missing due to damage). Meso- and metatergites not weakly subdivided at sides, thoracic chaetotaxy otherwise similar. Egg bursters absent from abdominal tergum I. Median sclerites of abdominal ventrites largely coalesced into wide plate, transversely sulcate, the sulcus weakly connecting lateral ampullar fields.

Figures

Fig. 5. Map of known localities for Geocolus caecus.

Fig. 4. Parsimony cladogram showing monophyly of adult and larval Geocolus caecus, with bootstrap values under branches. Outgroup sequences have been pruned.

Figs. 6–9. Aedeagus of male Geocolus caecus. 6) From Blount Co., AL, dorsal view; 7) From Blount Co., AL, lateral view (tegmen only); 8) From Pickens Co., SC, dorsal view (tegmen and median lobe); 9) From Pickens Co., SC, lateral view (tegmen only).

Figs. 10–11. Larval habitus of Geocolus caecus. 10) Cleared second-instar larva, dorsal view; 11) Uncleared second-instar larva, lateral view.

Figs. 12–18. Larva of Geocolus caecus, first instar. 12) Head, dorsal view (in this and the next figure, setae are numbered on the left side of the figure, pores are lettered on the right side); 13) Head, ventral view; 14) Antenna, dorsal view; 15) Labium, dorsal (inside) view; 16) Maxilla, dorsal (inside) view; 17) Maxilla, ventral view; 18) Labium, ventral view.

Fig. 19. Larva of Geocolus caecus, first instar, dorsal view, thoracic (I–III) and abdominal (VIII–X) terga (setae numbered on the left side, pores lettered on the right side).

Fig. 20. Larva of Geocolus caecus, first instar, ventral view, thoracic (I–III) and abdominal (VIII–X) sterna (setae numbered on the left side, pores lettered on the right side).