Heterocerus pallidus Say, 1823

King, Jonas G. & Lago, Paul K., 2012, The variegated mud-loving beetles (Coleoptera: Heteroceridae) of Mississippi and Alabama, with discussion and keys to the species occurring in the southeastern United States, Insecta Mundi 2012 (275), pp. 1-53 : 14-16

1. Heterocerus pallidus Say

( Fig. 11 , 37 , 54 )

Heterocerus pallidus Say 1823: 199 .

Neoheterocerus pallidus (Say) : Pacheco 1964: 82.

Description. Length 4.5 - 6.9 mm. Dark brown to yellowish brown. Elytra dominated by pale markings that are highly variable in shape and definition ( Fig. 11 ); striae absent. Pronotal disc usually darker than elytra, pronotal margins pale in most southeastern and midwestern specimens, while in northeastern specimens, only the anterior angles are pale normally. Post-metathoracic coxal and post-mesothoracic coxal lines absent. Male genitalia ( Fig. 37 ) of the typical Heterocerus type; dorsal plate depressed on the right anterior edge and only slightly convex on posterior edge; parameres triangular. Hypermandibulate males common.

Diagnosis. This species is similar in general appearance to Heterocerus gnatho , and the two can be difficult to differentiate. The elytra of both species vary from dark brown to sandy yellow with distinct to rather indistinct elytral markings. The pronotum in both species varies from completely dark brown to light brown with yellow margins; however, H. gnatho generally has much narrower light margins. In H. pallidus the light margins may extend over one sixth the width of the pronotum. Males of both species may be hypermandibulate. Characters of the male genitalia represent the most trustworthy means of separating the two species. Heterocerus pallidus has a slight depression on the anterior edge of the median plate ( Fig. 37 ), while the depression is much deeper in H. gnatho ( Fig. 38 ). The medial edges of the parameres are nearly parallel in H. pallidus , while they diverge towards their apices in H. gnatho . Although these species are sympatric throughout a large portion of their range, H. pallidus rarely occurs west of the Rocky Mountains, a region where H. gnatho is very common. Rarely is the latter found east of the Rocky Mountains.

Notes. In Mississippi, H. pallidus is found generally where the shoreline is a mixture of sandy and muddy areas. Specimens are commonly collected along the Mississippi River in northern Mississippi and from the banks of the Pascagoula and Leaf rivers in the south. Additional specimens were obtained from scattered ponds and small lakes in north-central Mississippi.

Hefley (1937) and Kaufmann and Stansly (1979) discussed the bionomics of H. pallidus . The species may be encountered along the shorelines of midwestern rivers through all stages of yearly floodplain succession ( Hefley 1937). The beetles are gregarious and prefer sandy areas covered with a thin layer of mud; therefore, they are constantly moving to new burrowing sites as water levels fluctuate. They overwinter in flask-shaped hibernacula 2-5 cm beneath the surface, emerging when temperatures rise above 16 o C ( Kaufmann and Stansly 1979).

Kaufmann (1987) compared the behavior and reproductive strategies of H. pallidus and Augyles auromicans . These two species belong to two distinctly different lineages of heterocerids, as recognized by most previous authors. (Molecular data gathered during this study suggests that these two species represent two of three major branches of heterocerid diversity.) Heterocerus pallidus generally forms larger, denser colonies and is more likely to fly in response to an environmental disturbance than is A. auromicans . The latter most often opted to dig deeper into the substrate rather than fly when their tunnels were flooded or mechanically compromised. A more gregarious and active lifestyle increases the frequency of mating encounters in H. pallidus and has apparently led to the development of non-priority in this species (sperm from multiple males are stored and mixed in the spermatheca before fertilization; a situation also called non-precedence). In H. pallidus , spermatogenesis and oviposition are closely linked to summer weather patterns. Spermatogenesis slows drastically in response to large summer storms that would presumably flood larval galleries or increase the flow of water drastically and potentially wash away a bank. This response is not seen in the males of A. auromicans , which produce sperm at a constant rate for most of their lives.

Great similarity in genital structure suggests that H. pallidus and H. gnatho are sister species. Molecular data collected as part of this study, however, strongly supports a phylogeny in which H. pallidus is more closely related to H. texanus than it is to H. gnatho . Heterocerus texanus belongs to the undatus species group, a group containing Heterocerus with atypical genital morphology. Based on genital morphology, the apparent close relationship between H. pallidus and the undatus group is surprising. The strikingly similar genital morphology seen in these two disparate congeners, H. pallidus and H. gnatho , along with evidence from recent molecular analysis ( King et al. 2011), suggests that the basic structure of these genitalia is likely similar to the ancestral condition for Heterocerus .

Distribution. Heterocerus pallidus is common from New England west to southern Alberta and south to eastern Arizona and the coast of Mississippi. Except for several records from Pennsylvania, it is not known to occur southeast of the Ohio River. A few records from Utah are the only evidence that this species occurs west of the Rocky Mountains.

Mississippi appears to represent the extreme southeastern edge of the range of H. pallidus , and no specimens were seen from southern states east of Mississippi ( Fig. 54 ). Considering the large amount of light trap material studied from Alabama, it appears unlikely that the beetle occurs here, but the reason for its absence from areas east of Mississippi is currently an enigma.

Specimens examined. 949 (See Appendix).

Figures

Figures 11-17. Habiti of large Heterocerus species lacking post-mesocoxal lines. 11) H. pallidus (male is hypermandibulate). 12) H.intermuralis (male is hypermandibulate). 13) H.fatuus. 14) H. gnatho. 15) H.longilobulus. 16) H. sandersoni. 17) H. angustatus.Scale line = 1mm.

Figures 32-44. Male genitalia of southeastern Heterocerus species. The ventral view is shown unless otherwise stated. 32-39) Species that lack post-mesocoxal lines. 32) H. fatuus. 33) H. glicki. 34) H. intermuralis. 35) H. angustatus; left - ventral view, right - dorsal view. 36) H. longilobulus. 37) H. pallidus; left - ventral view, right - dorsal view. 38) H. gnatho; left - ventral view, right - dorsal view. 39) H. sandersoni; left - ventral view, center - dorsal view, left - lateral view. 40-44) Species with post-mesocoxal lines. 40) H. sinuosus; left - dorsal view, right - ventral aspect. 41) H. mollinus. 42) H. insolens; left - dorsal view, right - ventral aspect. 43) H. parrotus. 44) H. brunneus.

Figures 54-57. Distribution by county in the United States. 54) Heterocerus pallidus. 55) Heterocerus gnatho. 56) Heterocerus longilobulus. 57) Heterocerus insolens.