Pachybrachis Chevrolat, 1836

Barney, Robert J., LeSage, Laurent & Savard, Karine, 2013, Pachybrachis (Coleoptera, Chrysomelidae, Cryptocephalinae) of Eastern Canada, ZooKeys 332, pp. 95-175 : 103-109

Pachybrachis Chevrolat, 1836

Pachybrachis Chevrolat in Dejean, 1836: 420. Type species: Cryptocephalus hieroglyphicus Laicharting, 1781, by subsequent designation of Jacoby 1908: 265.

Pachybrachys : Mannerheim 1843: 311. [incorrect subsequent spelling].

Remarks.

There has been some debate as to the correct spelling of the genus Pachybrachis . Fall’s (1915) monumental work used Pachybrachys Chevrolat and cited its general American usage by J. L. LeConte. However, this emendation was unjustified under the rules of the International Code of Zoological Nomenclature ( ICZN 1999, Article 32).

Pachybrachis is a member of the subfamily Cryptocephalinae Gyllenhall, 1813, commonly known as the case bearers due to the fact that all known larval stages live in a case constructed of their fecal matter and often plant debris ( LeSage 1985). Their cylindrical, compact body characterizes the adults, which usually have the head retracted into the pronotum to the level of the eyes.

In the recent revision of family-group names in Coleoptera ( Bouchard et al. 2011), the former tribe Pachybrachini Chapuis, 1874 was relegated to subtribe under the tribe Cryptocephalini Gyllenhal, 1813. Pachybrachina Chapuis, 1874 contains only two genera north of Mexico, Griburius and Pachybrachis , and is characterized by long filiform antennae, with a marginal bead at the base of pronotum which is not crenulate. Riley et al. (2002) separated the two genera by prosternal charateristics (prosternum broad, as wide as long in Griburius , narrower, longer than wide in Pachybrachis ). Additional generic keys can be found in Blatchley (1910), Chagnon and Robert (1962), Downie and Arnett (1996), and Ciegler (2007).

Useful morphological characters. Fall (1915) provided a very detailed "Review of Structural Characters Useful in Taxonomy", which we will not repeat here. However, there are a few key characters that will be useful to separate the seventeen Canadian species. These features will be described, detailed and illustrated, most of them being used in the identification key.

Size. The seventeen species can generally be divided into four size classes by average length: very small, <1.75 mm; small,>1.75 mm to 2.35 mm; medium,>2.35 mm to 2.85 mm; and large,>2.85 mm to 3.30 mm. Pachybrachis hepaticus is the only species in the very small category, with a mean length of 1.68 mm. Pachybrachis m-nigrum (2.59 mm), Pachybrachis othonus othonus (2.63 mm), and Pachybrachis luridus (2.65 mm) are in the medium category. Pachybrachis trinotatus (3.09 mm) and Pachybrachis bivittatus (3.12 mm) are the only species with males averaging over 3 mm in length. Small is the largest category, with the remaining eleven species. Mean length and width of males are reported for each species. Females are generally larger, thus accounting for the larger overall sizes reported by Fall (1915).

Antennae. In most species (e.g. Pachybrachis atomarius , Habitus 1 ; Pachybrachis bivittatus , Habitus 2 ), the length of antennae equals about 2/3 to 3/4 the length of the body. There are two noticeable exceptions. In Pachybrachis hepaticus ( Habitus 5 ) the antennae do not exceed half of the body length, whereas in Pachybrachis trinotatus ( Habitus 17 ) the antennae equal or exceed the body length.

Eyes. The eyes of Pachybrachis pectoralis are close to each other and separated by less than their width ( Figure 1a ). In most species the distance between the eyes roughly corresponds to their width (e.g. Pachybrachis peccans , Figure 1b ). A normal distance between eyes, coupled with the head coloration, can be diagnostic, as in Pachybrachis atomarius that has a largely yellow face ( Figure 1c ). In Pachybrachis hepaticus , the eyes are very small and markedly remote, separated by much more than their diameter ( Figure 1d ).

Ocular lines. Many Pachybrachis species have an impressed line, called the ocular line, around the margin of the eyes, and in some species the line diverges from each eye as lines of darker colored punctures between the eyes (e.g. Pachybrachis peccans , Figure 2a ). This character is very consistent within each species, and it is easy to see provided the specimens are properly oriented and lighted. In Pachybrachis hepaticus the ocular lines are very short but distinct above the eyes ( Figure 2b ). In other species, however, such ocular lines are absent (e.g. Pachybrachis spumarius , Figure 2c ).

Femora. Except for Pachybrachis hepaticus ( Figure 3a ), the femora on the forelegs of all species ( Figure 3b ) are incrassate or thickened in relation to the other femora. This character is difficult to see because in most cases legs are folded and pressed tightly against the body. Consequently, it might be necessary to relax the legs and spread them out to compare the front femora with those of the middle and hind legs. When such preparation is achieved, the larger size of the femora becomes evident (e.g. Pachybrachis calcaratus , Habitus 3 ).

Claws. In Pachybrachis , the tarsal claws are all simple ( Figures 4a-d ), but claws on the forelegs ( Figures 4a, b ) of several species are distinctly enlarged relative to the claws on the other legs ( Figures 4c, d ), as in Pachybrachis peccans ( Habitus 12 ) or Pachybrachis pectoralis ( Habitus 13 ). Due to the position of the legs in dead specimens, this character is often easier to see in lateral view ( Figures 4c, d ) than in front view ( Figures 4a, b ).

Tibial spurs. In Pachybrachis atomarius ( Habitus 1 ), Pachybrachis m-nigrum ( Habitus 8 ), and Pachybrachis trinotatus ( Habitus 17 ), there is no apical spur on front tibia ( Figure 5a ), but a tuft of large apical setae grouped together may superficially look like a spur. In Pachybrachis spumarius ( Figure 5b , Habitus 14 ) the front tibial spur is very small, hidden and difficult to see, but the very large and exposed front tibial spur is unique and distinctive of Pachybrachis calcaratus ( Figure 5c , Habitus 3 ). In all species, except Pachybrachis hepaticus , the middle tibiae are armed with small slender apical spur ( Figure 5d ). In all species studied here, the hind tibiae are unarmed.

Pronotum. In Pachybrachis , the pronotum is margined at base, the margin usually ornamented with a row of large punctures ( Figure 6a , close up). This character is very useful to separate Pachybrachis Chevrolat from Cryptocephalus Geoffroy or Bassareus Haldeman. The last two genera superficially look like Pachybrachis but are not margined at the base of the pronotum.

The density and pattern of pronotal punctures can be a useful character. Punctures usually dissipate near the side margins, and are generally a darker color than the background.

The pronotal coloration varies from a common mottled pattern (e.g. Pachybrachis spumarius , Figure 6b ), to a black M-mark on a light background (e.g. Pachybrachis m-nigrum , Figure 6c ), to an almost entirely black pronotum with only yellow basal and lateral markings (e.g. Pachybrachis nigricornis carbonarius , Figure 6d ).

Elytra. As on the pronotum, the density and pattern of punctures on the elytra are easily seen and useful characters. The elytral punctures generally form fairly regular deep striae, consisting of one sutural, one marginal and eight discal striae on each elytron, although the first may be somewhat irregular in the basal third (e.g. Pachybrachis luctuosus , Figure 7a ). Punctures may be confused in the basal half but with a tendency towards regular rows in the apical half, as in Pachybrachis calcaratus ( Figure 7b ). Finally, punctures may be completely confused and not aligned at all in rows (e.g. Pachybrachis hepaticus , Figure 7c ).

The elytral color pattern is, of course, a very useful character for the identification of many species. The mottled pattern is common (e.g. Pachybrachis spumarius , Figure 7d ). Some species are vittate (= with longitudinal black stripes), sometimes with a lateral vitta interrupted as in Pachybrachis bivittatus ( Figure 7e ). In some species, the elytra are largely black with only a few yellow markings or with narrow apical and lateral margins (e.g. Pachybrachis nigricornis carbonarius , Figure 7f ), or the elytra can be entirely black (e.g. Pachybrachis luridus , Figure 7g ).

Pygidium. The coloration of the pygidium can be largely yellow (e.g. Pachybrachis bivittatus , Figure 8a ), dark with distinct yellow spots of various sizes (e.g. Pachybrachis cephalicus , Figure 8b ), or dark with faint small reddish spots (e.g. Pachybrachis spumarius , Figure 8c ). A completely black pygidium is distinctive of Pachybrachis atomarius ( Figure 8d ).

Sexes. Males are usually smaller and less robust than females, with their abdomen flat ( Figure 9a ). In females, the abdomen is convex beneath, the last visible segment having a deep, round, concave depression or fovea ( Figure 9b ).

Genitalia. In most cases, individuals of each sex can be identified to species using coloration and external morphological features alone. However, an examination of the aedeagus is essential for the determination of superficially similar and variable species, such as Pachybrachis cephalicus , Pachybrachis luctuosus and Pachybrachis spumarius .

In Pachybrachis , the basal portion of the aedeagus may appear bulbous (e.g. Pachybrachis luctuosus , Figure 10a ) or more tubular ( Figure 10b ), but we don’t know yet if this character is reliable and consistent. The apical half is usually considerably bent, sometimes at a right angle, the degree of the curvature being an important diagnostic feature. In lateral view, the tip of the aedeagus may appear straight, sinuous and curved upwards, or sinuous and curved downwards (e.g. Pachybrachis spumarius , Figure 10b ). In dorsal view, the tip offers various shapes: small, large, pointed, triangular, lanceolate, nipple-shaped (e.g. Pachybrachis spumarius , Figure 10c ), etc.

Although the genitalic features are very constant and most reliable, they have been rarely described and illustrated in Pachybrachis . In the following key to the males of the 17 species treated here, the aedeagus is reported for only three species when external morphological characters may not be sufficient. The female genitalia are still unknown for all of them.

Illustrated key to males

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1a	Front femora not thicker than others (Fig. 1.i); eyes small and remote (Fig. 1.iii); punctures of pronotum and elytra dense and confused ( Habitus 5 )	Pachybrachis hepaticus hepaticus (F. E. Melsheimer)
1b	Front femora thicker than others (Fig. 1.ii); eyes narrowly separated or normal (Fig. 1.iv)	2
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2a	Ocular lines between eyes present (Fig. 2.i)	3
2b	Ocular lines absent (Fig. 2.ii)	6
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3a	Front claws larger (Figs 3.i, 3.ii) than middle or hind claws	4
3b	Front claws not enlarged; size similar to those in middle and hind legs (Figs 3.iii, 3.iv)	5
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4a	Eyes very close, separated by less than their width; ocular lines fine to indistinct (Fig. 4.i; Habitus 13 )	Pachybrachis pectoralis (F. E. Melsheimer)
4b	Eyes normal, more distant, ocular lines very distinct, with darker punctures (Fig. 4.ii; Habitus 12 )	Pachybrachis peccans Suffrian
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5 a	Last antennomere brownish; pronotum with subrectangular black markings pierced with yellow spots; elytral dark markings diffuse and irregular ( Habitus 10 )	Pachybrachis obsoletus Suffrian
5b	Antennae entirely yellow; pronotum with solid black rectangular markings; elytral dark markings well-defined ( Habitus 16 )	Pachybrachis tridens (F. E. Melsheimer)
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6a	Elytra vittate (with longitudinal color stripes) (Fig. 6.i)	7
6b	Elytra mottled (Fig. 6.ii), spotted (Fig. 16.iii), or mostly to entirely black (Fig. 6.iv)	9
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7a	Elytral punctures confused, outer (marginal) vitta often interrupted, pronotum yellow with darker reddish M-shaped mark ( Habitus 2 )	Pachybrachis bivittatus (Say)
7b	Many elytral punctures arranged in rows (Fig. 7.i); pronotum not reddish, rather yellow with black markings of various sizes and shapes (Fig. 7.ii)	8
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8a	Pronotum black, margined with yellow; sutural, discal and marginal vittae of elytra distinct ( Habitus 11 )	Pachybrachis othonus othonus (Say)
8b	Pronotum yellow with large, black, M-shaped marking; marginal and discal vittae of elytra very variable, usually not distinct ( Habitus 9a )	Pachybrachis nigricornis difficilis Fall
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9 a	Body entirely black, or sides and apex narrowly margined with yellow in some females ( Habitus 9b )	Pachybrachis nigricornis carbonarius Haldeman
9b	Pronotum mottled (Fig. 9.i) or with reddish spots (Fig. 9.ii); elytra variable, black (Fig. 9.iii) to mottled (Fig. 9.iv)	10
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10a	Elytra almost or entirely black (Fig. 10.i)	11
10b	Elytra spotted (Fig. 10.ii) or mottled (Fig. 10.iii)	13
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11a	Antennae longer than body; pronotum reddish with black M-mark; elytra entirely black ( Habitus 17 )	Pachybrachis trinotatus (F. E. Melsheimer)
11b	Antennae shorter than body; pronotum black (Fig. 11.i), mottled (Fig.11.ii), or with M-shaped marking (Fig. 11.iii, H 8 below); if pronotum with M-shaped marking, then elytra mottled, not entirely black	12
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12a	Elytra entirely black to streaked with whitish-yellow in outer areas; pronotum with reddish sides and upside-down reddish Y-mark ( Habitus 7 )	Pachybrachis luridus (Fabricius)
12b	Elytra with large median reddish spots almost joining at suture, with additional smaller apical spots; pronotum entirely black ( Habitus 15 )	Pachybrachis subfasciatus (J. E. LeConte)
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13 a	Elytra yellow with black markings; pronotum with well-defined black M-mark; size larger, 3+ mm ( Habitus 8 )	Pachybrachis m-nigrum (F. E. Melsheimer)
13b	Elytra (Fig. 13.i) and pronotum mottled (Fig. 13.ii), no decernable pattern; size smaller, less than 3 mm	14
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14a	Front tibia with large, curved spur (Fig. 14.i, Habitus 3 )	Pachybrachis calcaratus Fall
14b	Front tibia with tiny spur (Fig. 14.ii), or without spur (Fig. 14.iii)	15
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15a	Face largely yellow in males (Fig. 15.i); pygidium entirely black (Fig. 15.iii); pronotum and elytra mottled ( Habitus 1 )	Pachybrachis atomarius (F. E. Melsheimer)
15b	Face largely dark (Fig. 15.ii); pygidium spotted, with spots of some specimens being smaller and fainter than illustrated (Fig. 15.iv)	16
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16a	Elytral punctures regular in deeply impressed striae; size small (2 mm) ( Habitus 6 ); aedeagus with apical diamond-shaped denticle (Figs 16.i, 16.ii)	Pachybrachis luctuosus Suffrian
16b	Elytral punctures confused, not inserted into deep striae (Fig. 16.iii); size larger (2+ mm); aedeagus different (Figs. 17.i, 17.iii)	17
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17 a	Pronotum mostly fuscous, with relatively few pale markings, densely punctate, darker than elytra ( Habitus 4 ); aedeagus slender, sinuous, and sharper at apex in lateral view (Fig. 17.i); aedeagal tip triangular in dorsal view (Fig. 17.ii)	Pachybrachis cephalicus Fall
17b	Markings of pronotum and elytra numerous; darker areas dark brown to rufous ( Habitus 14 ); aedeagus sinuous and thicker at apex in lateral view (Fig. 17.iii); aedeagal tip nipple-shaped in dorsal view (Figs. 17.iv)	Pachybrachis spumarius Suffrian

Figures

Habitus 1. Dorsal habitus of Pachybrachis atomarius. Scale bar, 1 mm.

Figure 1. Eyes: a close eyes, Pachybrachis pectoralis b normal eyes, Pachybrachis peccans c normal eyes and yellow face, male Pachybrachis atomarius d remote eyes, Pachybrachis hepaticus.

Habitus 2. Dorsal habitus of Pachybrachis bivittatus. Scale bar, 1 mm.

Figure 2. Ocular lines: a present, Pachybrachis peccans b small, Pachybrachis hepaticus c absent, Pachybrachis spumarius.

Figure 3. Front femora: a not enlarged, Pachybrachis hepaticus b thickened, Pachybrachis peccans.

Habitus 3. Dorsal habitus of Pachybrachis calcaratus. Scale bar, 1 mm.

Figure 4. Claws: a, b front claws enlarged c, d normal.

Habitus 4. Dorsal habitus of Pachybrachis cephalicus. Scale bar, 1 mm.

Habitus 5. Dorsal habitus of Pachybrachis hepaticus hepaticus. Scale bar, 1 mm.

Figure 5. Tibial spurs: a absent from front leg, Pachybrachis atomarius b minute on front leg, Pachybrachis spumarius c large on front leg, Pachybrachis calcaratus d small and pointed on middle legs in most species.

Figure 6. Pronotum: a reddish, with close-up of marginal bead, Pachybrachis bivittatus b mottled, Pachybrachis spumarius c with black M-mark, Pachybrachis m-nigrum d almost black, Pachybrachis nigricornis carbonarius.

Habitus 6. Dorsal habitus of Pachybrachis luctuosus. Scale bar, 1 mm.

Figure 7. Elytral punctures and coloration: a in rows in deep striae, Pachybrachis luctuosus b confused in basal half, in rows in apical half, Pachybrachis calcaratus c all confused, Pachybrachis hepaticus d confused and mottled, Pachybrachis spumarius e vittate with marginal vitta interrupted, Pachybrachis bivattatus f black, margined with yellow, Pachybrachis nigriconis carbonarius g entirely black, Pachybrachis luridus.

Habitus 7. Dorsal habitus of Pachybrachis luridus. Scale bar, 1 mm.

Habitus 8. Dorsal habitus of Pachybrachis m-nigrum. Scale bar, 1 mm.

Figure 8. Pygidium: a largely yellow, Pachybrachis bivittatus b with well-defined yellow spots, Pachybrachis cephalicus c with faint reddish spots, Pachybrachis spumarius d black, Pachybrachis atomarius.

Figure 9. Sexes: a male abdomen, ventral view, Pachybrachis bivittatus b female abdomen, ventral view, Pachybrachis bivittatus.

Figure 10. Aedeagus: a lateral view, Pachybrachis luctuosus b lateral view, Pachybrachis spumarius c apex, dorsal view, Pachybrachis spumarius.

Habitus 10. Dorsal habitus of Pachybrachis obsoletus. Scale bar, 1 mm.

Habitus 11. Dorsal habitus of Pachybrachis othonus othonus. Scale bar, 1 mm.

Habitus 12. Dorsal habitus of Pachybrachis peccans. Scale bar, 1 mn.

Habitus 13. Dorsal habitus of Pachybrachis pectoralis. Scale bar, 1 mm.

Habitus 14. Dorsal habitus of Pachybrachis spumarius. Scale bar, 1 mm.

Habitus 15. Dorsal habitus of Pachybrachis subfasciatus. Scale bar, 1 mm.

Habitus 16. Dorsal habitus of Pachybrachis tridens. Scale bar, 1 mm.

Habitus 17. Dorsal habitus of Pachybrachis trinotatus. Scale bar, 1 mm.

Habitus 9 a. Dorsal habitus of Pachybrachis nigricornis difficilis. Scale bar, 1 mrn.

Habitus 9 b. Dorsal habitus of Pachybrachis nigricornis carbonarius. Almost black. Scale bar, 1 mm.