Dineutus angustus LeConte, 1878

Gustafson, Grey T. & Miller, Kelly B., 2015, The New World whirligig beetles of the genus Dineutus Macleay, 1825 (Coleoptera, Gyrinidae, Gyrininae, Dineutini), ZooKeys 476, pp. 1-135 : 22-26

Dineutus angustus LeConte, 1878 Figures 6, 7, 52

Dineutes angustus LeConte 1878: 378, [ Dineutes discolor : Régimbart 1882: 414 proposed synonymy, see discussion], Dineutes discolor var. angustus : Régimbart 1892: 739, Dineutus (Cyclinus) angustus : Hatch 1925b: 447, Dineutus angustus : Ciegler et al. 2003: 15.

Type locality.

U.S.A., Florida

Specimens examined.

15

Type material examined.

Dineutus angustus LeConte, 1878: syntype (1♀ pinned) "Type/ 6095 [red label, type in typed black ink, 6095 handwritten in black ink]// Fla. [beige label, typed black ink]// Dineutus angustus / Lec. [beige label, handwritten in black ink, handwriting appears to be LeConte’s]//” deposited in the MCZ.

Material examined.

U.S.A.: Florida: Alachua Co., 3.ii.1949, leg. B.W. Cooper (2 ex. FSCA); Columbia Co., O’Leno State Park, 12.ii.1966, leg. F.W. Mead (7 ex. FSCA); Florida: Hillsborough Co., "USF Riverfront", 21.v.1975, leg. G. Cowden (1 ex. FSCA); Suwannee Co., Branford, 16.vii.1934, leg. J.D. Beamer (4 ex. KSEM).

Diagnosis.

Male (Fig. 6C-D): Size: 9.4-10.8 mm. Body form very narrowly oval, laterally nearly parallel sided; elytral apices rounded with sutural angle produced into a point, rarely with point reduced and elytra appearing completely rounded, elytral apices without serrations and/or irregularities, elytral striae very faint, most evident medially, elytra laterally with strong reticulation, giving a bronzy appearance, medially replaced by fine microreticulation and fine weakly impressed punctures; profemora with small weakly produced sub-apicoventral tooth; protibia weakly club-shaped; venter lightly colored red to reddish orange; Aedeagus (Fig. 7A, B, D) with median lobe in dorsal view parallel sided basally, weakly constricted medially, apically briefly expanded then narrowed in apical 1/4, apex obtusely rounded, in lateral view median lobe narrowed in apical 1/4, in ventral view sperm-groove parallel sided for near entirety of length, apex broadly rounded, parameres very narrow, shortly constricted in basal 1/5, weakly arced in apical half, apically flatly rounded.

Female (Fig. 6A-B): Size: 9.8-10.2 mm. Body form very narrowly oval, laterally nearly parallel sided; elytral apices roundly angled towards sutural production, with sutural angle produced into a point, apical lateral sinuation often absent, to very weakly developed, elytral apices without serrations and/or irregularities, elytral striae very faint, most evident medially, elytra laterally with strong reticulation, giving a bronzy appearance, medially replaced by fine microreticulation and fine weakly impressed punctures; profemora without sub-apicoventral tooth; protibiae laterally weakly curved, apicolateral margin weakly expanded; venter lightly colored, red to reddish orange.

Differential diagnosis.

Dineutus angustus is unique among all other North American Dineutus in its small size, parallel sided, very narrowly elongate oval body form (Fig. 6A, C), light-colored venter, and somewhat in the shape of the aedeagus (Fig. 7A). This species is most similar to Dineutus discolor and it has been debated whether the two are actually distinct species ( Régimbart 1892). Dineutus angustus of both sexes can be distinguished from Dineutus discolor by smaller size, as well as the more parallel-sided and dorsoventrally convex body form. However, the aedeagus of the two species are very similar, with some minor differences. Both aedeagi have the median lobe parallel-sided and very narrow parameres. However, in Dineutus angustus the median lobe has a weak constriction medially subtending a slight expansion apically, and a narrowed apical 1/4, whereas the median lobe in Dineutus discolor is nearly parallel-sided for most of the length, with only a very weak constriction medially. The parameres of Dineutus angustus are more curved in the apical 1/3, and accompanied by a weak constriction in the basal 1/5, in comparison to Dineutus discolor which have the parameres much more parallel-sided.

The shape of the apices of the female elytra of Dineutus angustus differs from those of Dineutus discolor . In Dineutus angustus females the apices are broadly angled towards the sutural production and usually lack an apicolateral sinuation. When it is present (rarely), it is only weakly developed. In Dineutus discolor females the elytra are regularly rounded to the sutural production, with an apicolateral sinuation present.

Distribution

(Fig. 52D). U.S.A., Southern Georgia, and Eastern Alabama to northern Florida ( Roberts 1895; Wood 1962; Régimbart 1907).

Habitat.

Lotic species, seemingly restricted to highly calcareous streams with a basic pH ( Young 1954). For more on habitat preference see discussion.

Discussion.

Régimbart (1882) originally considered Dineutus angustus to be a synonym of Dineutus discolor , eventually elevating it to a variation ( Régimbart 1892) of Dineutus discolor (comparable now to a subspecies), then eventually accepting it as a species distinct from Dineutus discolor ( Régimbart 1902). We have decided to treat Dineutus angustus as a distinct species due to the differences in morphology listed above. The size of the two species in specimens examined for this study only overlap in the extremes, in that the largest specimens of Dineutus angustus only just approach the very smallest of Dineutus discolor . Dineutus angustus was much smaller than any average sized specimen of Dineutus discolor , with only a few female specimens of Dineutus discolor approaching the size of large Dineutus angustus males. Although the aedeagi of both species, which is primarily used here as a delimiter of species boundaries, are very similar, there were some notable differences, even if minor.

Young (1954) noticed that Dineutus angustus in Florida appears to be restricted to calcareous streams with a basic pH, while Dineutus discolor appears to inhabit streams with a more acidic pH. Young (1954) mentions “intergrade” forms occuring from streams of intermediate pH with both typical forms of Dineutus discolor and Dineutus angustus present. However, the evidence for the intergrade was in the number of setigerous punctures, which are known to vary among populations, especially with size. Nevertheless, this led Young (1954) to suggest that Dineutus angustus may represent the basic stream ecotype of Dineutus discolor .

Dineutus angustus appears to be very restricted in range so far found only in northern Florida, southeastern Alabama, and southern Georgia (Fig. 52D), records from Virginia are mentioned by Roberts (1895). Similar to Young (1954), we have only seen Dineutus angustus from northern Florida, and doubt the other records. Given the extensive range of Dineutus discolor and close similarity of that species, we believe it is likely that the two species have been misidentified. Dineutus angustus is rarely collected, and not well represented in collections.