Tragidion Audinet-Serville

Swift, Ian & Ray, Ann M., 2008, A review of the genus Tragidion Audinet-Serville, 1834 (Coleoptera: Cerambycidae: Cerambycinae: Trachyderini), Zootaxa 1892, pp. 1-25 : 3-7

Genus Tragidion Audinet-Serville

( Figs 1 A–F, 2A–F, 3A–F, 4A–C, 5A–D, 6A–E, 7A–F))

Tragidion Audinet-Serville, 1834:89 ; LeConte, 1850:10; Thomson, 1860:190; Thomson, 1864:429; Lacordaire, 1869:173; LeConte, 1873:314; LeConte and Horn, 1883:299; Leng, 1886:60, 81; Casey, 1893:586; Casey, 1912:321; Linsley, 1962:113; Lingafelter & Nearns, 2006:204. Type species: Callidium lynceum Fabricius, 1775 , Cerambyx coquus Linnaeus.

Trichoscelis Dejean, 1837:355 ; Bates, 1885:224.

Description. As in Figs 1 A–F, 2A–F, 3A–F, 4A–C. Form moderately-sized, elongate, orange and/or black. Head: punctate, usually throughout, front subvertical, pubescent, large, quadrate, abruptly separated from the anteocular spaces, central sulcus usually present, genae moderate; mandibles acute at apex; antennae with 11 antennomeres (12 antennomeres in males of T. dichromaticum ), filiform, occasionally spined at apex especially in males, antennomeres 6–11(12) usually carinate; antennal tubercles of male prominent; antennae of males usually long, exceeding the elytral apices by one to five antennomeres. Pronotum: pubescent, punctate; wider than long; lateral pronotal tubercles pronounced in males, less so in females, gland pore punctation in males present on the anterior half of the pronotum, pleura, and venter, a distinct area lacking gland pores is formed at the mid-posterior region of the disk around the pronotal calli; prosternum not protuberant; procoxal process generally parallel-sided, surpassing the posterior edge of the procoxae, bilobed or emarginate, occasionally with longitudinal ridge. Elytra: elongate, parallel-sided, rarely tapering to apices; lacking acute lateral margin or ivory vittae; surface entirely pubescent; apices rounded; scutellum small, equilaterally triangular, apical angle subacute; costae elevated in some species, flattened in others, forming elytral striae not elevated above the surface. Legs: moderately elongate; posterior femora not attaining apex of elytra; metatibiae densely pubescent with erect hairs, increasing in density apically. Ventral surface: moderately pubescent, pubescence sometimes with bright metallic blue reflections; mesosternum not protuberant, parallel sided, posterior margin entire; posterior metacoxal edge even with postcoxal bridge.

Diagnosis. Among the North American Trachyderini , Tragidion is similar to Scythroleus and Purpuricenus . These genera have acute mandibles at the apex, weakly produced mesosterna, and the elytra lack vitta and a distinct margined lateral edge. Tragidion differs from Purpuricenus in having the elytra covered entirely in moderately-long pubescence (> 1 mm); the elytra are carinate or with narrow, unraised striae; the prosternal process exceeds the posterior edge of the procoxae; and the metacoxae are even with the postcoxal bridge. Purpuricenus have nearly glabrous elytra which lack costae; the prosternal process does not exceed the posterior edge of the procoxae; and the posterior metacoxal edge extends beyond the postcoxal bridge. Purpuricenus males have pheromone gland pore arrays on the pleura of the prothorax only and gland pores are absent from the dorsal surface of the pronotum ( Fig. 5 D). Skiles (1979) provides a good discussion differentiating Scythroleus from the T. armatum species group. His discussion, however, does not seem to account for T. dichromaticum in the diagnosis between the two genera. Nevertheless, Tragidion possesses the following characters that separate it from Scythroleus :, the trapezoidal shape of the female pronotum, the lack of calli lateral to the median callus, and the shining elytral integument.

Tragidion also bears a striking similarity to the South American genus Polyschisis Audinet-Serville. However , this similarity is largely superficial. Polyschisis possesses a number of distinct characters that separate it from Tragidion , including: elongate labial palps, with expanded apical segments; serrate antennae (at least in males); declivous posterior pronotal margins; a pronounced tubercle on the prosternal process; an elongate scutellum with an acute apical angle; a glabrous humeral area on the elytra; lack of raised elytral costae; and more clavate pro- and mesofemora. Based on these significant morphological differences, similarities between the two genera are likely the result of analogous character convergence rather than a homologous relationship.

Discussion. A formal phylogeny of the tribe Trachyderini or any group therein has not been proposed and is beyond the scope of this review. Therefore, no known apomorphies can be conclusively defined to separate Tragidion from other trachyderine genera. Without such characters or a phylogeny, the relationship of Tragidion to other members of the tribe is uncertain at best.

Male-specific gland pores are associated with pheromone production in a number of species within the subfamily Cerambycinae (reviewed by Ray et al. 2006). Pheromone gland pores are present on the prothoraces of males ( Fig. 5 A), absent from females in all Tragidion species ( Fig. 5 B). In males, aggregations of gland pores are located at the base of prothoracic punctures ( Fig. 5 C). Females have similar punctation, however, these punctures do not contain clusters of pores. Gland pores are distributed along the anterior portion of the pronotum, as well as the pleura, and venter of the prothorax in males. There are subtle differences in the shapes of pore “areas” among species, but these differences are not diagnostic at species level. A candidate male-produced pheromone has likewise been identified in T. armatum , the only species of Tragidion examined to date ( Hanks et al. 2007).

Figures

FIGURE 1. Dorsal habitus photos of Tragidion spp. A, Tragidion agave (male); B, T. agave (female); C, T. annulatum (male); D, T. annulatum (female); E, T. armatum (male); F, T. armatum (female).

FIGURE 5. Scanning electron micrographs of Tragidion spp. A, Tragidion agave (male) prothorax 42 ×; B, T. agave (female) prothorax 42 ×; C, T. agave (male) gland pores of prothorax 2104 ×; D, Purpuricenus humeralis (male) prothorax 42 ×.