Rhagomicrus bonvouloiri (Horn, 1886)

Otto, Robert L., 2012, Eucnemid Larvae of the Nearctic Region. Part I: Description of the Larva ofRhagomicrus Bonvouloiri (Horn, 1886) (Coleoptera: Eucnemidae: Melasinae: Dirrhagini), With Notes on Its Biology, The Coleopterists Bulletin 66 (3), pp. 219-223 : 220-222

Rhagomicrus bonvouloiri (Horn, 1886) , mature larva ( Figs. 1–5 )

Diagnosis. Rod-shaped dorsal and ventral prothoracic scleromes and five lateral projections on each side of the head capsule should be diagnostic to separate this species from any known larval Eucnemidae in the Nearctic region.

Description. Length 12.0–13.0 mm, width 1.00– 1.25 mm. Orthosomatic. Body ( Fig. 1 ): Subcylindrical, sides parallel, cream-yellow with head, prothoracic sclerome patches, and caudal end of the 9 th abdominal segment dark brown. Setae reduced or absent. Pair of small legs reduced to very short setae near posterolateral areas of each thoracic segment. Dorsal and ventral microtrichial patches slightly darker in color compared to their surrounding areas. Head ( Fig. 2 ): Strongly flattened, prognathous and inserted into the prothorax. Dorsal and ventral cephalic discs subcircular and unmodified. Anterior portion of the head capsule heavily sclerotized. Lateral sides of the head capsule unsclerotized. Each lateral side of head capsule with five projections. First lateral projections directed anteriorly. Second lateral projections blunt. Third, 4 th, and 5 th lateral projections each directed anterolaterally. Three-segmented antennae minute, arising between 4 th and 5 th lateral projections. Basal antennal segment short and rounded. Second and 3 rd antennomeres nearly equal in length. Sensory papillae indistinct. Mandibles minute, resting in the mesal acumination of the head capsule. Each mandible heavily sclerotized, distinct, oval, longer than wide with 2 outwardly projecting teeth. Labial and maxillary palpi extremely small and segmented. Ligula, mala, lacinia and galea not visible. Hypostomal rods absent. Prothorax ( Figs. 3 and 4 ): Subequal to subsequent 2 thoracic segments. Dorsoanterior aspects consisting of 2 sclerotized bars extending down the length of the segment, converging toward the posterior end. Trapezoidal microtrichial patch between sclerotized bars. Oval areole present near posterior end, below microtrichial patch. Microtrichial patch and scleromes on the sternum similar to the tergum. Mesothorax: Tergum with oval microtrichial patch. Two short, diverging sclerotized lines present beneath the patch. Oval areole present near base of thoracic segment. Ventral microtrichial patch slightly rounded. Short, sclerotized lines more divergent posteriolaterally on the sternum. Oval areole present near base of segment. Metathorax: Tergum with small suboval microtrichial patch near anterior end. Oval areole present near base of segment. Sternum very similar to metathoracic tergum, except sclerotized bars much shorter. Abdomen: Abdominal segments I-IX subequal in length and width. Abdominal terga I-VIII each with large, subtriangular microtrichial patch near anterior end and small, oval areole at posterior end of segment. Sterna similar in structure to terga, except areole absent near posterior end of abdominal segment VIII. Abdominal tergum IX lacking patches, but sparsely punctate near caudal end; heavily sclerotized at caudal half ( Fig. 5 ), with prominent semicircular ring of asperities around the anal region. Urogomphi absent on abdominal segment IX. Spiracles annular-biforous.

Distribution. Rhagomicrus bonvouloiri is known from CANADA: Ontario; USA: Alabama, Arkansas, District of Columbia, Florida, Georgia, Indiana, Iowa, Kentucky, Louisiana, Maryland, Missouri, New Jersey, North Carolina, Ohio, Pennsylvania, South Carolina, Tennessee, Virginia, West Virginia, and Wisconsin ( Muona 2000). All specimens used in this study came from Wisconsin.

Biology. Although R. bonvouloiri is a widespread species, the collecting records are rather old across eastern North America ( Muona 2000). Very little is known about the natural history of this species. In Wisconsin, R. bonvouloiri was found thriving in a variety of wet forest systems (floodplain forests, northern hardwood swamps, northern wet-mesic forests, southern hardwood swamps) as well as upland, drier forest systems (northern mesic forests, oak barrens, southern mesic forests). Searching in conifers yielded no larvae, which may indicate this eucnemid might be a deciduous specialist. In southern Wisconsin, R. bonvouloiri was observed burrowing in a wet, fermenting oak log. The specimens collected on both properties in Oconto Co. were exclusively found either in white-rot-affected maples or wet, rotten aspens. I found larvae of five eucnemid species during one of my trips to the second property, a small deciduous forest roughly several hectares in area. One log supported three eucnemid species in close proximity to each other in one section. Nematodes penetrans (LeConte, 1852) was found in a drier, hard area and both R. bonvouloiri and Deltometopus amoenicornis (Say, 1836) were found in the moist, white-rotted areas of the same log. Larvae of R. bonvouloiri were positioned parallel with the grain of the sapwood and burrowing between layers of wood fibers, leaving no galleries behind them. Their wedge-shaped head capsule is useful as a wedge, allowing the larva to move through the sapwood. Mature larvae were observed constructing pupal chambers at about 2.5 cm beneath the surface. The larva assumes a U-shape position as it molts into the pupa in the chamber. I observed no physical differences between mature larval and prepupal stages. Pupation requires about two weeks.

Twenty-nine adults emerged from the wood pieces from 13 May to 25 May 2007. All were placed in plastic vials with wood pieces to observe their behavior and to attempt to breed these beetles. No eggs or first instars were present in these vials following completion of adult observations. Like other eucnemid species, adults of R. bonvouloiri are capable of snapping into the air when placed on their back. They quivered their extended antennae while in captivity in these vials. Adult beetles are short-lived, lasting about a week in captivity before dying. It is likely R. bonvouloiri completes its development in about one year under optimal conditions, but may develop slower under less optimal conditions.

Figures

Figs. 1–5. Rhagomicrus bonvouloiri, mature larva. 1) Dorsal habitus; 2) Head, dorsal view; 3) Head and thoracic regions, dorsal view; 4) Head and thoracic regions, ventral view; 5) Abdominal segments VII-IX, ventral view.