Monoxia obesula Blake, 1939

Clark, Shawn M., Rattu, Andrea & Cillo, Davide, 2014, Monoxia obesula Blake, 1939, a species native to the U. S. A. and adventive to Sardinia, Italy (Coleoptera: Chrysomelidae: Galerucinae: Galerucini), Zootaxa 3774 (1), pp. 83-89 : 84-87

Monoxia obesula Blake, 1939

Monoxia obesula Blake, 1939: 167

Diagnosis. Most species of Monoxia , including M. obesula , are unusual in that the females have simple claws. Among North American Galerucini , only Erynephala shares this claw character, but beetles in that genus are larger than those in Monoxia , being at least 6 mm long, and the pronotal punctation is much coarser, the punctures being conspicuously larger than those of the elytra. Males of Monoxia , which have bifid claws, are most likely to be confused with Ophraella Wilcox, 1965 . However, the body in Ophraella is usually more robust, the antennae usually extend beyond the elytral humeri, the terminal abdominal tergite is normally covered, and the elytral dark markings, when present, are in the form of longitudinal vittae, without isolated speckles. In Monoxia , the majority of males and some females have an exposed pygidium.

Monoxia obesula is among the smallest species of the genus, being only 2.4–3.7 mm long. However, there are several other species within this size range. The most diagnostic feature is the aedeagus ( Fig. 1-C ). In lateral view, it is rather abruptly bent just beyond the base and thereafter nearly straight, although slightly bisinuate towards the apex. In all other species of the genus, the aedeagus is more or less evenly curved from just beyond the base to the apex ( Fig. 1 -D).

Blake (1939) provided a diagnostic key to the species of Monoxia . This key was reproduced with only slight modification by Wilcox (1965). In either publication, M. obesula runs to couplet 9. At that point, this species is best recognized by the aedeagal shape mentioned above, rather than the characters provided in the key. In truth, this aedeagal character distinguishes M. obesula from all other species of the genus. It could have been inserted in any of the preceding couplets in the keys, enabling recognition of M. obesula at any step along the way.

In Europe, M. obesula is most likely to be confused with Ophraella communa LeSage, 1986, another species native to North America that has recently been discovered in Italy. The two species are distinguishable by the generic characters mentioned above.

Redescription (male specimen). Form elongate oval, slender ( Fig. 1-A ); length 3.0 mm; width across middle of pronotum 0.9 mm; width across elytral humeri 1.2 mm; width at distal third of elytra 1.3 mm. Surface densely punctate, densely covered with white pubescence. Color pale brown, with mesal line on vertex and frons, entire labrum, numerous small elytral speckles, metasternum, tarsomere 3 of each leg, distal portion of tarsomere 5 of each leg, and all tarsal claws darker brown.

Head ( Fig. 1-B ) with shallow mesal depression on vertex. Surface shallowly, rugosely punctate, densely covered with short, appressed pubescence. Interocular distance equal to 0.6 times maximum width of head across eyes; genal length nearly as great as maximum diameter of eye; antennal fossae placed at level near lower margin of eye, separated from each other by distance about equal to fossal diameter, separated from eye by distance slightly less than fossal diameter. Antennae extending slightly beyond elytral humeri; antennomeres 1–2 shiny, sparsely clothed with mostly erect setae; antennomeres 3–5 and especially 6–11 densely clothed with short appressed pubescence, also with longer, erect setae near apex of each antennomere; antennomere 1 about twice as long as broad; antennomere 2 slightly longer than broad, slightly more than half as long as antennomere 1, slightly narrower than antennomere 1; antennomeres 3–5 subequal in length and width, each distinctly longer than and about as wide as antennomere 2; antennomere 6 slightly longer than broad, subtriangular, distinctly broadened from base to apex; antennomeres 7–10 subquadrate, about as broad as antennomere 1; antennomere 11 about as broad as preceding 5, but longer, attenuate distally.

Prothorax 1.7 times as broad as long, broadest near middle; in dorsal view, anterior margin nearly straight, lateral margin strongly bisinuate, posterior margin strongly bisinuate from posterolateral corner to meson, with margin adjacent to scutellum weakly concave; anterolateral and posterolateral corners each with setose tooth; disc with large depression in anterolateral area, with large depression in posterolateral area, with large depression in mesal area from base to apex; dorsal surface densely, rugosely punctate, densely covered with short, appressed pubescence; hypomeron shiny, glabrous, impunctate. Scutellum triangular, densely covered with appressed pubescence.

Elytra together 1.8 times as long as broad, subparallel in basal three fourths. Humeri well developed, separated from basal callosity by shallow depression; postbasal depression very weak, hardly noticeable. Punctation nearly uniform throughout, with most punctures separated by a distance subequal to diameter of puncture. Pubescence dense, short, appressed. Color of each elytron pale brown, with 10–20 small, darker brown spots, these tending to be arranged in 3 or 4 longitudinal rows; some spots very pale, hardly discernible; some spots apparently coalescing to form very short longitudinal stripes.

Prosternum anterior to coxae very short, with length subequal to width of antennomere 2; prosternal process absent between coxae; front coxal cavities broadly open behind; mesosternum densely pubescent, with mesal process narrowly separating middle coxae; metepisternum and metepimeron densely pubescent; metasternum shiny, slightly less densely pubescent. Legs densely pubescent; each basal tarsomere about 0.2 times as long as tibia, slightly longer than tarsomere 2, about as long as tarsomere 5; tarsomere 3 strongly bilobed; tarsal claws bifid. Ventral areas of abdomen densely pubescent; terminal ventrite with deep mesal fovea; pygidium clearly exposed, densely punctate and pubescent.

Aedeagus dorsoventrally flattened for most of its length, in lateral view very narrow, except near base, in lateral view rather abruptly bent near basal fourth, thereafter nearly straight, except slightly bisinuate towards apex ( Fig. 1- C ).

Variation. Male 2.4–3.3 mm long; female 3.1–3.7 mm long. Elytral spots reduced in number or entirely missing in some specimens. Tarsal claws of female simple.

Material examined (adults). ITALY, Sardegna, Cagliari, St. Molentargius, 2-VIII-2013, A. Rattu (5 males, 6 females, AJGC; 96 males, 116 females, BYUC; 7 males, 8 females, EGRC; 10 males, 9 females, USNM); ITALY, Sardegna, Cagliari, St. Molentargius, 2-VIII-2013, A. Rattu, sweeping Atriplex halimus (40 males, 26 females, ARC; 35 males, 15 females, DCC; 2 males, LFC; 2 males, ZFMK; 2 males, ZMHB); ITALY, Sardegna, Cagliari, St. Molentargius, 2-VIII-2013, A. Rattu, sweeping Atriplex portulacoides (2 males, MCSN).

U.S.A., Maryland, Baltimore Co., Dundalk, 21-VIII-1991, J. Cavey, feeding with larvae on Chenopodium sp. (10 males, 4 females, BYUC); U.S.A., Nebraska, Lancaster Co., Arbor Lake, 40°54'04.1"N, 96°40'52.9"W, 348 m, 2-IX-2013, K. Miwa, sweeping Atriplex dioica Rafinesque (5 males, 13 females, BYUC); U.S.A., Nebraska, Lancaster Co., Arbor Lake, 40°54'04.1"N, 96°40'52.9"W, 348 m, 7-IX-2013, K. Miwa, on Atriplex dioica (3 males, 3 females, AJGC; 20 males, 26 females, BYUC; 3 males, 3 females, EGRC; 3 males, 3 females, USNM); U.S.A., Nebraska, Lancaster Co., Arbor Lake, 40°54'04.1"N, 96°40'52.9"W, 348 m, 7-IX-2013, K. Miwa, on Chenopodium sp. (2 males, 3 females, BYUC); U.S.A., Nebraska, Lancaster Co., Waverly, 13-XI-1923, O. Bryant (2 males, BYUC); U.S.A., Texas, Kleberg Co., Kingsville, 12-V-1909, McMillan, feeding on Chenopodium (2 male paratypes, USNM).

Hosts and habitats. Clark et al. (2004) summarized published plant associations, stating that M. obesula had been reported from Atriplex and Chenopodium (both Amaranthaceae sensu lato, including Chenopodiaceae ). However, no species were mentioned within these plant genera.

Based on our current investigations in Nebraska, we report that this species is associated with Atriplex dioica Rafinesque , a plant that occurs in saline habitats ( Fig. 2 ). This plant is quite widespread in North America ( Welsh 2003). However, it is rather unusual in Nebraska, being found commonly only in western areas, where it was collected in the late 1800’s but not again for a full century, and in Lancaster County, in the eastern part of the state, where it has been collected repeatedly since the late 1800’s ( Kaul et al. 2011). This is the same county where M. obesula has been collected. Such limited distribution of A. dioica in Nebraska and other states may partly explain the rarity of M. obesula in collections. However, the beetles have been found feeding also on Chenopodium sp., in Nebraska and elsewhere.

In Sardinia ( Fig. 3 ), M. obesula is associated with Atriplex halimus L. and A. portulacoides L., as identified with the aid of Pignatti (1982). Both of these plants occur in brackish environments. The insects were collected in The Regional Nature Park of Molentargius – Saline. This park represents one of the most important wetlands in Europe. It is spread over an area of about 1600 hectares bordered by the urban expansions of Cagliari, Quartu Sant'Elena, and Selargius, and by the promenade Poetto. There are basins of saltwater and freshwater, separated by a characteristic arid plain called "Is Arenas." The vegetation consists of four types: a freshwater hygrophytic community formed by Phragmites , Typha , and Scirpus ; halophytic or xerophytic vegetation consisting mainly of Suaeda , Arthrocnemum , Salicornia , Atriplex , and other Amaranthaceae s. l.; a xerophytic vegetation composed of herbaceous or shrubby species ( Arthrocnemum , Suaeda , Atriplex , Salsola , etc.), this interspersed with either fresh or brackish water vegetation ( Tamarix , Arundo , Phragmites ); and finally a hydrophytic habitat with algae and aquatic plants, such as Ruppia maritima L., these varying according to the salinity of the water.

Figures

FIGURE 2. Habitat of Monoxia obesula Blake; Arbor Lake, Lancaster Co., Nebraska, U. S. A.

FIGURE 3. Habitat of Monoxia obesula Blake in Sardinia.

FIGURE 1. A – C, Monoxia obesula Blake; A, dorsal habitus; B, head, frontal aspect; C, aedeagus, dorsal and lateral aspects. D, Monoxia minuta Blake, aedeagus, dorsal and lateral aspects.