Deltometopus amoenicornis (Say, 1836)

Otto, Robert L., 2012, Eucnemid Larvae of the Nearctic Region. Part II: Description of the Mature Larva ofDeltometopus amoenicornis (Say, 1836) (Coleoptera: Eucnemidae: Macraulacinae: Macraulacini), with Notes on Its Biology, The Coleopterists Bulletin 66 (3), pp. 285-288 : 286-288

Deltometopus amoenicornis (Say, 1836) , mature larva ( Figs. 1–5 )

Description. Length 9.0–10.0 mm, width 1.0 mm. Orthosomatic. Body ( Fig. 1 ): Subcylindrical, sides parallel, yellow with head, prothoracic sclerome patches, and caudal end of abdominal segment IX dark brown. Setae reduced or absent. Pair of small legs reduced to dome-like structures present near posteriorlateral areas of each thoracic segment. Dorsal and ventral microtrichial patches slightly darker in color compared to its surrounding areas. Head ( Fig. 2 ): Strongly flattened, prognathous, disproportionally small and inserted into the prothorax. Head capsule heavily sclerotized, disc unmodified. Venter of head capsule similar to dorsum, except for pair of yellow lines extending from first lateral projection as it sinuates toward posterior end of the head capsule. Each lateral side of head capsule with four projections. First lateral projection directed laterally to posteriolaterally. Lateral side of first projection darkly pigmented brown to black. Second through 4 th lateral projections each directed laterally. Third lateral projection distinctly longer than other projections. Antennae nearly indistinct, arising between 3 rd and 4 th lateral projections. Mandibles minute, resting in the mesal acumination of the head capsule between the 4 th lateral projections. Each mandible heavily sclerotized, oval, longer than wide with 2 outwardly projecting teeth. Labial and maxillary palpi extremely small. Hypostomal rods absent. Ligula, mala, lacinia, and galea not visible. Prothorax ( Figs. 3 and 4 ): Subequal to preceding 2 thoracic segments. A pair of subtrapezoidal scleromes present, widest at the lateral side, narrowing towards the median on the tergum. Elongated, tear drop-shaped microtrichial patch present between the scleromes. Small circular areole present beneath it, near middle of the segment. Sternum similar to tergum, except sclerome more triangular and internally incised near posterior end. Meso– and metathorax: Each tergum and sternum with oval microtrichial patches. Small, oval areole present near middle of each segment, beneath the patch. Abdomen: Abdominal segments I-IX subequal in length and width. Abdominal terga I-VII each with relatively large, subtriangular microtrichial patch. Small, oval areole present beneath the patch, near middle of the segment. Smaller microtrichial patch and circular areole present on abdominal tergum VIII. Abdominal sterna I-VIII similar to each terga. Ninth abdominal tergum lacking patches, but sparsely punctate near caudal end. Ninth abdominal sternum ( Fig. 5 ) heavily sclerotized at caudal half, with prominent oblong, semicircular ring of asperities around anal region. Urogomphi absent on abdominal segment IX. Spiracles annular-biforous.

Distribution. Deltometopus amoenicornis is known from CANADA: New Brunswick, Nova Scotia, Ontario, Prince Edward Island, Québec; USA: Alabama, Arkansas, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, Oklahoma, Ohio, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, West Virginia, and Wisconsin ( Muona 2000; Majka 2007). All specimens used in this study came from Wisconsin.

Biology. Deltometopus amoenicornis , found largely east of the Rocky Mountains, with records in Colorado, is one of the most commonly collected species of false click beetle. The species thrives in a variety of forest systems. In Wisconsin, I collected adult specimens in northern dry-mesic forest, northern hardwood swamp, northern mesic forest, northern wet-mesic forest, floodplain forest, southern dry-mesic forest, southern hardwood swamp, southern mesic forest, and oak barrens. Blatchley (1910) wrote that the beetles were often beaten from vegetation and found in damp localities. Van Horn (1909) found this species under the bark of Pinus virginiana Miller (Pinaceae) , and mentions that no other biological information is known. Knull (1947) noted that he reared the species from badly decayed beech and that it overwintered as a larva.

From my rearings, 13 adults emerged from 17 May to 22 May 2007. All were placed in a vial with pieces of rotten wood to observe their behavior and to attempt to breed them. Adult beetles were observed on the wood pieces for a period of two weeks in the vial. During that time, they chewed numerous holes in the wood pieces, sometimes reducing it to dust. They snapped into the air when placed on their back. Like other eucnemid species, D. amoenicornis quivers its extended antennae. Eggs were laid on the wood pieces and on the side of the vial. First instars were first seen on 6 June 2007 when larval cadavers were found on the side of the vial.

Once the larva ecloses, it can be postulated that it immediately finds a suitable opening to burrow into the sap wood. Larvae were positioned parallel with the wood grain and burrowing between layers of wood fibers, leaving no galleries behind. The head capsule is useful as a wedge while the body undergoes peristalsis to allow the larva to move through the sapwood. I found this species does not undergo a hypermetamorphic lifecycle, as I observed in other genera like Nematodes Berthold, 1827 or Dirrhagofarsus Fleutiaux, 1935 . With the exception of the first instar, all other larval stages are similar in their physical appearances. I observed third, fourth, and fifth instars of different lengths found together in one area of the white rotten log.

All larvae have been extracted exclusively from white rotten wood. In Wisconsin, I have found these larvae mainly in rotten maples ( Acer sp. ) and once in a rotten log of shagbark hickory ( Carya ovata (Miller) K. Koch ; Juglandaceae ). The rotten logs were quite moist in the parts from which the larvae were extracted. Larvae are usually found at about 2.5 cm beneath the surface.

I observed two or more eucnemid species utilizing the same areas of the log. In addition to extracting larvae of D. amoenicornis , I also found larvae of Rhagomicrus bonvouloiri (Horn, 1886) . As I observed for that species ( Otto 2012), larvae of D. amoenicornis will construct a pupal chamber near the surface of the wood. They assume a U-shaped position inside the chamber before their final molt into a pupa. There are no physical differences between a prepupal stage and other larval instars. Pupation requires about two weeks. Deltometopus amoenicornis usually complete its development in about 1–2 years.

Figures

Figs. 1–5. Deltometopus amoenicornis, 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.