Aciliini

Belongs within: Dytiscinae.

The Aciliini are a clade of diving beetles, distinguished by the possession of bifid metatibial spurs, and extremely long gonocoxae with expanded and rounded apices and apicolateral margins concave (Miller 2003). Members of the Aciliini lay their eggs in cracks in wood, moss or other cryptic locations (Miller 2001). Within the Aciliini, members of the Holarctic genus Acilius are characterised by large, dense punctation over much of the body surface in contrast to the finer, sparser punctation found in other genera. The genus Thermonectus includes two species, T. marmoratus and T. zimmermanni, that are strikingly patterned with bright yellow markings (Bouchard 2014).

Dealing with a clingy male

Published 19 October 2020

Diving beetles of the family Dytiscidae are a distinctive component of the freshwater environment in most regions of the world. They have an oval, streamlined body form and powerful hind legs, usually with fringes of stiff setae, that are ill-suited for movement on land but make them adept swimmers. They are also almost always capable fliers, allowing them to find their way to water bodies of any size from large lakes to small, temporary pools. Both adults and larvae are active hunters, preying on other aquatic arthropods or even small vertebrates. Most diving beetles are fairly dull in coloration but exceptions are found among members of the tribe Aciliini.

Members of the Aciliini are moderately sized diving beetles, generally between one or two centimetres in length. Dorsally they have a yellow to red base coloration with contrasting dark markings. The hind legs are robust with the hind tibia short and broad. Males have the base of the tarsus of the front legs broadened into a round palette with setae on the underside modified into sucking discs, used to hang onto the females when mating; this discs may be present on the tarsus of the mid pair of legs as well. They are strong swimmers, often venturing into the open waters of lakes and pools, and contrast with other diving beetles in that they may be found in pools lacking submerged vegetation (Roughley & Larson 2001; Bergsten & Miller 2006). Larvae have a distinctive arched body shape with a small head (Bukontaite et al. 2014), kind of shrimp-like, and also tend to be more pelagic than the larvae of other diving beetles. Females have gonocoxae (the appendages at the end of the abdomen that function as the ovipositor) that are relatively long with a broadened, spoon-like ending (Miller 2001); these are used to insert eggs into damp moss or under loose bark of vegetation lying just above the waterline. There is usually just one generation per year and adults in cold regions overwinter in larger water bodies that remain unfrozen.

Perhaps the most intriguing aspect of aciliin diving beetles regards their sexual dimorphism. As noted above, males have a set of suckers on the fore legs for hanging onto females when mating. However, females of some species have sculpted elytra rather than the smooth elytra of males, such as a granular surface in Graphoderus species or long, setose sulci in female Acilius. The uneven surface produced by these features presumably functions to reduce the efficacy of the males’ suckers, allowing the females more control when selecting a mate. That such a conflict exists is supported by the observation that the more developed the males’ sucker arrays in a population, the more likely the females are to have repellent sculpturing. Males of some diving beetle species have been observed grabbing at any female they encounter, followed by the female swimming rapidly and erratically in an attempt to shake the male off or knock him off against the substrate or objects in the water (Miller 2003). Where this becomes really interesting is that some species have dimorphic females with some females in the population having sculpted elytra whereas others are smooth. What could be the reason for such variation? The presence of both forms in the population suggests that neither has a complete advantage over the other. It may be that smooth-backed females trade reduced defenses for improved swimming ability. Alternatively, a defensive female may be able to ensure that only the strongest and most resilient males can mate with her, but runs the risk of not mating at all if she never encounters a male who can overcome her defenses. A less defensive female may be more vulnerable to any male she encounters but at least she’s bound to be fertilised at some point.

Systematics of Aciliini

<==Aciliini [Thermonectini]
    |--Thermonectus Dejean 1833M03, RL01 [=ThermonetusBM06]
    |    |  i. s.: T. ornaticollisM01
    |    |         T. zimmermaniB14
    |    |--+--T. variegatusBMB14
    |    |  `--+--T. alfrediBMB14
    |    |     `--T. marmoratus Hope 1832BMB14, B14
    |    `--+--+--T. circumscriptus Latr. 1809BMB14, FS90 (see below for synonymy)
    |       |  `--T. leprieuriBMB14
    |       `--+--T. nigrofasciatusBMB14
    |          `--+--T. succinctusBMB14
    |             `--+--+--T. basilaris Harris 1829BMB14, FS90 (see below for synonymy)
    |                |  `--T. intermediusBMB14
    |                `--+--T. margineguttatus Aubé 1838BMB14, FS90
    |                   `--T. nobilisBMB14
    `--+--+--Tikoloshanes Omer-Cooper 1956BMB14
       |  |    `--T. eretiformis Omer-Cooper 1956BMB14
       |  `--Aethionectes Sharp 1882BMB14
       |       |--A. apicalisM01
       |       |--A. oberthueriBMB14
       |       `--A. optatusM01
       `--+--+--Rhantaticus Sharp 1882BMB14, LM87
          |  |    `--R. congestus (Klug 1833) (see below for synonymy)LM87
          |  `--Sandracottus Sharp 1882BMB14, LM87
          |       |  i. s.: *S. mixtus (Blanchard 1853) [=Acilius mixtus]LM87
          |       |--S. dejeaniiBMB14
          |       `--+--S. fasciatusBMB14
          |          `--+--S. bakewelli (Clark 1864)BMB14, LM87 [=Hydaticus bakewelliLM87; incl. S. guttatus Sharp 1882LM87]
          |             `--+--S. gueriniBMB14
          |                `--S. insignisBMB14
          `--+--Graphoderus Dejean 1833M03, RL01 (see below for synonymy)
             |    |--G. liberusM03
             |    `--+--G. fascicollisM03
             |       `--+--G. perplexusBMB14
             |          `--+--+--G. bilineatusBMB14
             |             |  `--+--G. austriacusBMB14
             |             |     `--G. occidentalisBMB14
             |             `--+--+--G. adamsiiBMB14
             |                |  `--G. bieneriBMB14
             |                `--+--G. cinereus (Linnaeus 1758)BMB14, NR97 [=Dytiscus cinereusNR97, *Prosciastes cinereusNR97]
             |                   `--G. zonatusBMB14
             `--Acilius Leach 1817 (nom. cons.)M03, RL01 [=Heteroelytrus Ádám 1993BM06; incl. Homoeolytrus Gobert 1874BM06]
                  |  i. s.: A. florissantensis Wickham 1909BMB14
                  |         A. oeningensis (Heer 1847) [=Dytiscus oeningensis]BM06
                  |         A. praesulcatus Łomnicki 1894BM06
                  |         A. sinensis Peschet 1915BM06
                  |--+--+--A. canaliculatus (Nicolai 1822)BMB14, BM06 (see below for synonymy)
                  |  |  `--A. duvergeri Gobert 1874BMB14, BM06 [=A. (*Homoeolytrus) duvergeriBM06]
                  |  `--+--*A. sulcatus (Linnaeus 1758)BM06, BMB14, NR97 (see below for synonymy)
                  |     `--+--A. japonicus Brinck 1939BMB14, BM06
                  |        `--A. kishii Nakane 1963BMB14, I92 [=A. japonicus kishiiBM06]
                  `--+--A. athabascae Larson 1975BMB14, BM06
                     `--+--+--A. fraternus (Harrsi 1828)BMB14, BM06 (see below for synonymy)
                        |  `--A. mediatus (Say 1823)BMB14, BM06 (see below for synonymy)
                        `--+--+--A. abbreviatus Aubé 1838BMB14, BM06 (see below for synonymy)
                           |  `--A. semisulcatus Aubé 1838BMB14
                           `--+--A. confusus Bergsten & Miller 2006BMB14, BM06
                              `--A. sylvanus Hilsenhoff 1975BMB14, BM06

Acilius abbreviatus Aubé 1838^{BMB14, BM06} [=A. semisulcatus var. abbreviatus^BM06; incl. A. latiusculus LeConte 1857^BM06, A. semisulcatus latiusculus^BM06, A. semisulcatus var. oregonensis Crotch 1873^BM06, A. simplex LeConte 1852^BM06, A. semisulcatus var. simplex^BM06]

Acilius canaliculatus (Nicolai 1822)^{BMB14, BM06} [=Dytiscus canaliculatus^BM06, Heteroelytrus canaliculatus^BM06, Hydaticus canaliculatus^BM06, Dytiscus sulcipennis Sahlberg 1824^BM06; incl. Acilius caliginosus Curtis 1825^BM06, A. dispar Lacordaire 1835^BM06, A. kotulae von Ulanowsky 1884^BM06, A. laevisulcatus de Motschulsky 1845^BM06, A. subimpressus de Motschulsky 1845^BM06, Dytiscus sulcatus var. sulcatoaffinis Hummel 1822^BM06, A. canaliculatus var. xantischius des Gozis 1911^BM06]

Acilius fraternus (Harrsi 1828)^{BMB14, BM06} [=Dytiscus fraternus^BM06; incl. A. fraternus dismalus Matta & Michael 1976^BM06]

Acilius mediatus (Say 1823)^{BMB14, BM06} [=Dytiscus mediatus^BM06, A. (Homoeolytrus) mediatus^BM06, A. (Homoeodytes [l.c.]) mediatus^M01, Thermonectus mediatus^BM06; incl. Colymbetes (Hydaticus) maccullochii Kirby 1837^BM06]

*Acilius sulcatus (Linnaeus 1758)^{BM06, BMB14, NR97} [=Dytiscus sulcatus^NR97, *Heteroelytrus sulcatus^NR97, Hydaticus sulcatus^BM06; incl. Acilius sulcatus blancki de Peyerimhoff 1927^BM06, A. brevis Aubé 1837^BM06, A. sulcatus brevis^BM06, Dytiscus fasciatus De Geer 1774^BM06, A. fasciatus^BM06, D. punctatus Scopoli 1763^BM06, D. scopolii Gmelin 1790^BM06, Acilius sulcatus var. scoticus Stephens 1828^BM06, A. tomentosus de Motschulsky 1845^BM06, A. sulcatus var. varipes Stephens 1828^BM06]

Graphoderus Dejean 1833^{M03, RL01} [incl. Derographus Portevin 1929^RL01, Graphothorax Motschulsky 1853^RL01, Prosciastes Gistel 1856^RL01]

Rhantaticus congestus (Klug 1833) [=Hydaticus congestus; incl. H. signatipennis Castelnau 1834, *Rhantaticus signatipennis]^LM87

Thermonectus basilaris Harris 1829^{BMB14, FS90} [incl. T. forstroemi^FS90, T. incisus^FS90, T. nimbatus^FS90, T. sculpturatus^FS90]

Thermonectus circumscriptus Latr. 1809^{BMB14, FS90} [incl. T. brasiliensis^FS90, T. havaniensis^FS90, T. insculptus^FS90, T. insularis^FS90, T. subfasciatus^FS90]

*Type species of generic name indicated

References

[BM06] Bergsten, J., & K. B. Miller. 2006. Taxonomic revision of the Holarctic diving beetle genus Acilius Leach (Coleoptera: Dytiscidae). Systematic Entomology 31: 145–197.

[B14] Bouchard, P. (ed.) 2014. The Book of Beetles: A lifesize guide to six hundred of nature’s gems. Ivy Press: Lewes (United Kingdom).

[BMB14] Bukontaite, R., K. B. Miller & J. Bergsten. 2014. The utility of CAD in recovering Gondwanan vicariance events and the evolutionary history of Aciliini (Coleoptera: Dytiscidae). BMC Evolutionary Biology 14: 5.

[FS90] Fleutiaux, E., & A. Sallé. 1890. Liste des coléoptères de la Guadeloupe et descriptions d’espèces nouvelles. Annales de la Société Entomologique de France, 6^e série 9: 351–484.

Holmgren, S., R. Angus, F. Jia, Z. Chen & J. Bergsten. 2016. Resolving the taxonomic conundrum in Graphoderus of the east Palearctic with a key to all species (Coleoptera, Dytiscidae). ZooKeys 574: 113–142.

[I92] Iwahashi, J. (ed.) 1992. Reddo Deeta Animaruzu: a pictorial of Japanese fauna facing extinction. JICC: Tokyo.

[LM87] Lawrence, J. F., B. P. Moore, J. E. Pyke & T. A. Weir. 1987. Zoological Catalogue of Australia vol. 4. Coleoptera: Archostemata, Myxophaga and Adephaga. Australian Government Publishing Service: Canberra.

[M01] Miller, K. B. 2001. On the phylogeny of the Dytiscidae (Insecta: Coleoptera) with emphasis on the morphology of the female reproductive system. Insect Systematics and Evolution 32: 45–92.

[M03] Miller, K. B. 2003. The phylogeny of diving beetles (Coleoptera: Dytiscidae) and the evolution of sexual conflict. Biological Journal of the Linnaean Society 79: 359–388.

[NR97] Nilsson, A. N., & R. E. Roughley. 1997. The genus- and family-group names of the Dytiscidae—additions and corrections. Beiträge zur Entomologie 47 (2): 359–364.

[RL01] Roughley, R. E., & D. J. Larson. 2001. Dytiscidae Leach, 1815. In: Arnett, R. H., Jr & M. C. Thomas (eds) American Beetles vol. 1. Archostemata, Myxophaga, Adephaga, Polyphaga: Staphyliniformia pp. 156–186. CRC Press: Boca Raton.