Hydroglyphus pusillus, the Tiny Tiger

Hydroglyphus pusillus, copyright Udo Schmidt.

Let's take another visit to the world of diving beetles. Above is Hydroglyphus pusillus, one of the few representatives in northern Europe of a genus that otherwise includes close to ninety species spread through the Old World, primarily in the tropics. Hydroglyphus species are tiny diving beetles, only about two or three millimetres in length, with an elongate oval body shape. Characteristic features of the genus include basal striae on the pronotum and elytra, sutural striae on the elytra, and no transverse stria on the top of the head (Watts 1978, as Guignotus, a subsequently synonymised name). Species are often marked with distinctive colour patterns of streaks and blotches.

Hydroglyphus pusillus attacking larva of mosquito Culex pipiens, from Bellini et al. (2000).

Despite their small size, Hydroglyphus species are (like other diving beetles) voracious predators of other aquatic insects. Bellini et al. (2000) investigated the possible role of H. pusillus in controlling mosquito larvae in flooded rice fields in Italy. The larvae of H. pusillus mostly kept to the bottom sediment (so might be expected to be hunting prey other than mosquitoes) but adults were the most abundant diving beetle in the water column at the surveyed locations. One might expect that H. pusillus would not be effective predators of mosquito larvae that greatly outsized them. One would be wrong: not only are they indeed capable of taking down mosquitoes, Bellini et al. went so far as to describe their effects as "a real slaughter". A diving beetle latching onto a mosquito larva would soon find itself joined by others seemingly scenting haemolymph in the water. Between them, this mob of beetles could destroy a larva in a matter of seconds. Tiny, but terrifying.

REFERENCES

Bellini, R., F. Pederzani, R. Pilani, R. Veronesi & S. Maini. 2000. Hydroglyphus pusillus (Fabricius) (Coleoptera Dytiscidae): its role as a mosquito larvae predator in rice fields. Boll. Ist. Ent. "G. Grandi" Univ. Bologna 54: 155–163.

Watts, C. H. S. 1978. A revision of the Australian Dytiscidae. Australian Journal of Zoology, Supplementary Series 57: 1-166.

Dealing with a Clingy Male

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.

Sunburst diving beetle Thermonectus marmoratus, from Insectarium de Montréal, René Limoges.

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.

Alternate morphs of female Graphoderus zonatus with granular (left) and smooth elytra, from Holmgren et al. (2016).

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.

REFERENCES

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

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.

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.

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

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.

Morion Revisited

In an earlier post, I introduced you to the carabid beetle genus Morion, currently recognised as including about forty species from tropical and subtropical regions around the world. In that post, I mentioned how Will (2003) had questioned the monophyly of this genus, owing to its lack of derived features in comparison with closely related genera. In this post, I'll take the opportunity to dive a little further into ways the genus may be divided.

Just to remind you what we're looking at: Morion monilicornis, copyright Robert Webster.

As noted by Will (2003), many authors have recognised two subgenera within Morion, Morion sensu stricto and Neomorion. These subgenera were first established by Jeannel (1948) who identified a distinction between species he examined from the Old World (Africa and Asia) and the Americas. The Old World species, to which Jeannel gave the name Neomorion, had a number of setae along the rear margin of the last ventrite of the abdomen. In males, the basal segment of the fore tarsus had the inner apex drawn out into a tooth. The aedeagus bears a large dorsoapical orifice; in Old World Morion examined by Jeannel, this orifice was covered over by a large bilobed lamella. In the New World species of Morion sensu stricto, in contrast, the rear margin of the last ventrite bore only two setae, and males lacked a medioapical tooth on the basal fore tarsomere. The opening of the aedeagus lacked a covering lamella. In his key to Morion and related genera, Will (2003) referred only to the state of the male fore tarsus as distinguishing the subgenera. It may be that this indicates that the significance of the other characters described by Jeannel had been subject to question, but I suspect that they may have omitted by Will because their state in a number of Morion species remains unknown.

Aedeagi of Morion in lateral and dorsal view from Jeannel (1948). On the left is an Old World species (Morion orientale), on the right a South American species (M. georgiae).

A particular notable lacuna in Jeannel's brief survey of Morion was that he didn't look at any Australian species. In his description of M. crassipes*, a species found in the vicinity of Cairns in Queensland, Sloane (1904) noted that the male fore tarsi differed from those of other Australian species in having the "basal joints rounded and not produced at inner apical angle", implying that most Australian Morion have tarsi resembling those of the New World species. Moore (1965), in a review of Australian genera of Pterostichinae, describes Morion as having an aedeagus with an orifice on the dorsum, with no mention of a covering lamella, again also suggesting a resemblance to New World rather than Old World species (unfortunately, Moore did not specify exactly which species his description of the genitalia was based on). Moore (1965) also noted that Australian species were distinctive among Morion in having a pronotum with more than the two setae along each lateral margin found in species from elsewhere. One species which is found in New Guinea and northern Australia, M. longipennis, does have only two pairs of marginal setae on the pronotum, and Darlington (1962) suggested that it was probably more closely related to Asian species than to other Australian Morion. The aforementioned M. crassipes differs from other Australian species in a number of significant features, including large size (it grows to a full inch in length) and modified legs, and Sloane (1904) did briefly wonder whether it should even be regarded as a Morion, but it does share the plurisetose pronotal margins. I should note that I've found no reference to the pilosity of the last ventrite in any Australian species.

*Under the name 'Morio crassipes'; confusion about whether this genus should be called Morion or Morio lingered for a long time in the early 20^th century.

So overall, there is the suggestion of three distinct groups within Morion, for the Old World, American, and Australian species, with the last two more similar to each other than to the first. The New World species of Morion are more diverse in South America than in North America, and one might be tempted to line up the relationship between the three groups with the division of Gondwana. The Old World species group may have diverged first with the separation of Africa and/or India, followed by the South American and Australian lineages diverging as their own continents became isolated. The South American species group may have spread into North America with the formation of the Central American land bridge, and the increased proximity of Australasia to Asia may have allowed members of the Old World group such as M. longipennis to invade from the northwest. However, I've based this scenario on some pretty weak assumptions based on very incomplete data, and it would require a more detailed investigation before we could say if there's any merit to it.

REFERENCES

Darlington, P. J., Jr. 1962. The carabid beetles of New Guinea. Part I. Cicindelinae, Carabinae, Harpalinae through Pterostichini. Bulletin of the Museum of Comparative Zoology 126 (3): 319–564, 4 pls.

Jeannel, R. 1948. Faune de l'Empire Français. X. Coléoptères Carabiques de la Région Malgache (deuxième partie). Office de la Recherche Scientifique Coloniale: Paris.

Moore, B. P. 1965. Studies on Australian Carabidae (Coleoptera). 4.—The Pterostichinae. Transactions of the Royal Entomological Society of London 117 (1): 1–32.

Sloane, T. G. 1904. Studies in Australian entomology. No. XIV. New species of geodephagous Coleoptera from tropical Australia. Cicindelidae (3), and Carabidae (5) [Platysmatini, Morioni, Perigonini, Masoreini, and Physocrotaphini]. Proceedings of the Linnean Society of New South Wales 29 (3): 527–538.

Will, K. W. 2003. Review and cladistic analysis of the generic-level taxa of Morionini Brullé (Coleoptera: Carabidae). Pan-Pacific Entomologist 79 (3–4): 212–229.

The Trechodini

The above figure, from Uéno (1990), shows Trechodes satoi, a fairly typical representative of the carabid ground beetle tribe Trechodini. Members of this tribe are found in many parts of the world, though they are absent from the Nearctic region and were unknown from northern Asia prior to the description of Eotrechodes larisae from the Russian Far East by Uéno et al. (1995). The greatest diversity of Trechodini is on the southern continents and most authors have accordingly assumed a Gondwanan origin for the lineage.

The Trechodini are a subgroup of the subfamily Trechinae (in the restricted sense; sometimes this grouping is reduced to a tribe in which case Trechodina is treated as a subtribe thereof). Trechines are a distinctive group of relatively small ground beetles, features of which include a head with well-developed frontal furrows extending from the front of the head to behind the eye, and two pairs of supra-orbital setae. Trechodini differ from other trechines in distinctive male genitalia in which the ejaculatory duct of the aedeagus is entirely exposed dorsally, the median lobe is open above and gutter-like, and there is no basal bulb. They also usually have three obtuse teeth near the base of the mandible though the South African genus Plocamotrechus is missing one of these teeth in the left mandible (Moore 1972).

Habitus of Canarobius oromii, from Machado (1992).

Despite being widespread, the distribution of Trechodini is patchy. They are generally restricted to damp habitats such as alongside streams and rivers. Among Australian species, Moore (1972) noted that the genera Trechodes and Paratrechodes were uniformly fully flighted whereas Trechobembix and Cyphotrechodes were often brachypterous. He suggested that this was connected to the last two genera being found in more stable habitats alongside standing water. A number of species in the tribe have moved into subterranean habitats such as caves and have reduced wings and eyes. In two genera found in lava caves on the Canary Islands, Canarobius and Spelaeovulcania, no trace of the eyes remains (Machado 1992). Considering the little-studied nature of such habitats around the world, it is possible that other trechodins remain to be discovered.
REFERENCES

Machado, A. 1992. Monografía de los Carábidos de las Islas Canarias (Insecta, Coleoptera). Instituto de Estudios Canarios: La Laguna.

Moore, B. P. 1972. A revision of the Australian Trechinae (Coleoptera: Carabidae). Australian Journal of Zoology, Supplementary Series 18: 1–61.

Uéno, S. 1990. A new Trechodes (Coleoptera, Trechinae) from near the northwestern corner of Thailand. Elytra 18 (1): 31–34.

Uéno, S., G. S. Lafer & Y. N. Sundukov. 1995. Discovery of a new trechodine (Coleoptera, Trechinae) in the Russian Far East. Elytra 23 (1): 109–117.

Boreonectes: Diversity Hidden Underwater

The beetle in the photo below (copyright Joakim Pansar) may or may not be Boreonectes griseostriatus. This small diving beetle, a few millimetres in length, has been regarded in the past as widespread with a distribution spanning the Holarctic region. However, in recent years it has become apparent that this single widespread species may actually be a number of more localised species in a skin.

This possibility had been considered for a while. In 1890, a Norwegian entomologist recognised distinct montane and coastal species, noting a tendency for the former to the neatly striped whereas the latter was more blotchy. Later authors, however, rejected this distinction. In 1953, a Russian author expressed the view that B. griseostriatus "varies markedly in many characters; all attempts to establish subspecies and varieties are unjustified, because almost all varieties are connected by transitions" (Angus et al. 2015). In its overall appearance, B. griseostriatus is a a fairly undistinguished small diving beetle. Most of the body surface is densely and finely punctate both dorsally and ventrally, and it lacks some of the modifications found in other diving beetles such as lateral grooves on the pronotum or sucker-hairs on the male tarsi (Angus 2010). This latter feature, offhand, is an adaptation that assists males who have it in clinging to the backs of females during mating. Their functionality would be much reduced in punctate species such as B. griseostriatus because the the uneven surface of the female would prevent the suckers from getting a grip, and phylogenetic studies suggest that their absence in Boreonectes may represent a secondary loss. I don't know if the Boreonectes males do anything to make up for their absence; maybe they just have to grip tighter.

Variation in parameres from male genitalia of the Boreonectes griseostriatus group, from Dutton & Angas (2007).

The complicated nature of B. griseostriatus' identity became really apparent in the 2000s when karyotypic studies on European specimens identified several different chromosomal races, distinct not only in chromosome topography but also in number, that may represent distinct species. The original B. griseostriatus of lowland Sweden possesses a karyotype of thirty pairs of autosomal chromosomes plus the X sex chromosome (sex is determined in this genus by an X0/XX system where males have one copy of the X chromosome and females have two, with no Y chromosome). Boreonectes multilineatus, the Scandinavian montane species, has 28 autosomal pairs. Other species have fewer. It appears likely that a similar thing is happening in Boreonectes to the situation I described in an earlier post for the bat genus Rhogeessa where mutations lead to chromosomes becoming fused or split. It is notable in this regard that Angus (2010) found several specimens of B. ibericus from Morocco that were heterozygous for a chromosomal fusion, so that a single fused chromosome was paired meiotically with distinct chromosomes 1 and 24.

Externally, however, these genetically distinct species remain all but indistinguishable. There may be a tendency for one species to be larger than another, or towards slightly different genital morphologies, but these differences are not distinct enough or consistent enough to provide a reliable guide to identification. Which, if you don't have access to fresh specimens allowing a karyotype spread, is a problem.

REFERENCES

Angus, R. B. 2010. Boreonectes gen. n., a new genus for the Stictotarsus griseostriatus (De Geer) group of sibling species (Coleoptera: Dytiscidae), with additional karyosystematic data on the group. Comparative Cytogenetics 4 (2): 123–131.

Angus, R. B., E. M. Angus, F. Jia, Z.-N. Chen & Y. Zhang. 2015. Further karyosystematic studies of the Boreonectes griseostriatus (De Geer) group of sibling species (Coleoptera, Dytiscidae)—characterisation of B. emmerichi (Falkenström, 1936) and additional European data. Comparative Cytogenetics 9 (1): 133–144.

Morion

Morion monilicornis, copyright Charles Schurch Lewallen.

Just a quick one today. This is a typical member of Morion, a genus currently recognised as including over forty species of carabid beetles though there may be many more yet to be described. Characteristic features of this genus include a somewhat flattened body form, moniliform antennae (that is, the antennal segments are all short and similar in form, like beads on a string), a more or less cordiform (heart-shaped) pronotum, and a bilobed median tooth on the mentum (a sclerite on the underside of the head that might be thought of as the 'lower lip' of the mouth) (Will 2003). Though currently recognised as pantropical, Will (2003) suggested that its defining features were potentially plesiomorphic relative to some closely related genera. Further studies may identify Morion in its current sense as a paraphyletic grade to those genera, possibly leading to a reclassification.

The flattened body form of Morion and its relatives (the Morionini) reflects their preferred habitat. Like other carabids, Morion species are voracious predators (both as adults and larvae). Morionins are specialised for hunting in dead wood and under back, forcing themselves through enclosed gaps in search of other insects that might have thought themselves secure in their lignified fortresses.

REFERENCE

Will, K. W. 2003. Review and cladistic analysis of the generic-level taxa of Morionini Brullé (Coleoptera: Carabidae). Pan-Pacific Entomologist 79 (3–4): 212–229.

Trichosternus

Trichosternus vigorsi, copyright Udo Schmidt.

Many of the carabid ground beetles tend to attract a lot of attention from amateur entomologists due to their size and striking appearance, but it must be admitted that they are often not the easiest of animals to work with from a taxonomic perspective. The larger species tend to fall into the category of 'big, black, massive sharp mandibles' and it can require a lot of practice to reliably identify which genus a specimen belongs to, let alone species.

Trichosternus is a genus of ground beetles found in far eastern Australia, from the base of Cape York in Queensland to a bit north of Sydney in New South Wales, in the band of land between the coast and the Great Dividing Range. There is also a single isolated species T. relictus in the southwest corner of Western Australia, and apparently another in New Caledonia (Darlington 1961). However, considering the difficulty that many authors have had in the past in providing an exact definition for Trichosternus relative to other closely related genera, it would be interesting to see if future studies corroborate the inclusion of these outlying species. By way of contrast, a reasonable number of New Zealand species assigned at one time or another to Trichosternus have all long since been moved elsewhere.

Trichosternus species are all flightless and in most the elytra are fused and cannot open (the exception is T. relictus). Most species have a distinctive male genital morphology, with the genital opening deflected to the right and the right paramere (the parameres are two sclerotised 'arms' on either side of the genitalia) modified into a specialised falcate shape, the exact functional significance of which seems to remain unknown. Again, the outlier in this regard is T. relictus in which said paramere retains a primitive styloid shape. Similar falcate parameres are also known from members of related genera such as Megadromus and Nurus; the latter is particularly similar to Trichosternus with the only real difference between the two being that Nurus is more robust with longer mandibles. Trichosternus relictus also has a distinctive female genital morphology, in which the internal passage between the median oviduct (where emerging eggs are fertilised by sperm stored in the spermatheca) and the vagina is remarkably extended and concertina-like. Again, the function of this structure is unknown though Moore (1965) suggested that it might be related to viviparity.

Northern Trichosternus species found in tropical Queensland are all inhabitants of rainforest (hence the restriction of the genus to east of the Great Dividing Range: on the western side of the range, rainforests are absent and the arid zone begins). Southern species are found in upland temperate rainforests or in savannah woodland (Darlington 1961). Some species have very restricted ranges: T. montorum, for instance, is known from two mountains on the Spec Plateau, Mts Bartle Frere and Bellenden Ker.

REFERENCES

Darlington, P. J., Jr. 1961. Australian carabid beetles VII. Trichosternus, especially the tropical species. Psyche 68 (4): 113–130.

Moore, B. P. 1965. Studies on Australian Carabidae (Coleoptera). 4.—The Pterostichinae. Transactions of the Royal Entomological Society of London 117 (1): 1–32.

Ground Beetles for Today

European zuphiin Polystichus connexus, by Cristoph Benisch.

The subject of today's post is a group of ground beetles (Carabidae) that has been treated in the past as the subfamily Zuphiinae, but seems to now be more commonly treated as a supertribe Zuphiitae within the Harpalinae. Whatever their appropriate formal name, the zuphiites are distinguished by a relatively long and thick scape (the first major segment of the antennae) and spination on the first stylomere of the female's ovipositor (Ball 1985; Ober & Maddison 2008); the clade is also supported by molecular data (Ober & Maddison 2008). Many zuphiites also have a relatively narrow pronotum (noticeably longer than wide) and truncate elytra. Species of Zuphiitae are found around the world, primarily in the tropics, with known centres of diversity in Australia and the Neotropics (Baehr 1985). Diversity in Asia and Africa is supposedly lower but, as most of the Australian taxa appear to be ultimately derived through immigrations from Asia (Baehr 1985), it would not be at all surprising if this distribution was biased by higher levels of study in the former continents*.

*Though 'higher level of study' is definitely a relative term: the Australian fauna, for instance, seems to owe its recognised diversity overwhelmingly to the work of Martin Baehr alone.


The galeritin Trichognathus marginipennis, photographed by Guilherme Ide.

Within the Zuphiitae, current classifications recognise the tribes Zuphiini, Anthiini, Galeritini, Dryptini, Helluonini and Physocrotaphini, though the monophyly of some of these tribes is not firmly established (Ober & Maddison 2008). The Zuphiini are distinguished by a particularly long scape, while the Galeritini possess asymmetrically dilated tarsi in the males (Baehr 1985).

REFERENCES

Baehr, M. 1986. Revision of the Australian Zuphiinae. 6. The genus Planetes Macleay. Supplement to the other genera (Insecta, Coleoptera, Carabidae). Spixiana 9 (2): 151-168.

Ball, G. E. 1985. Reconstructed phylogeny and geographical history of genera of the tribe Galeritini (Coleoptera: Carabidae). In: Ball, G. E. (ed.) Taxonomy, Phylogeny and Zoogeography of Beetles and Ants: A volume dedicated to the memory of Philip Jackson Darlington, Jr. (1904-1983) pp. 276-321. D. W. Junk Publishers.

Ober, K. A., & D. R. Maddison. 2008. Phylogenetic relationships of tribes within Harpalinae (Coleoptera: Carabidae) as inferred from 28S ribosomal DNA and the wingless gene. Journal of Insect Science 8 (63): 1-32.

The Diversity of Ground Beetles

Mating pair of the ground beetle Leptoferonia hatchi (Harpalinae, Pterostichini). Photo by Kipling Will.

It is well-known that beetles are one of the most diverse groups of animals in existence, and include more described species than any other "order" of organisms (though as we make further inroads into the remaining reservoir of undescribed species, I fully expect mites and Hymenoptera to give beetles a good run for their money). Within the beetles, one of the largest and best-known families, with over 30,000 described species*, are the ground beetles of the Carabidae. By way of comparison, this is a similar number of species to the entire collection of terrestrial vertebrates.

*Tree of Life says 30,000; Wikipedia says 40,000. In the absence of any other authority, I'll go with Tree of Life.

Carabids have been one of the best-studied group of beetles because they are relatively large, often very colourful, and some species (notably the tiger beetles of the Cicindelinae) are often highly visible. Carabids are mostly active predators, generally feeding on any small invertebrate unfortunate enough to cross their path. The phylogeny of carabids is poorly resolved (Beutel et al., 2008), but one large clade that is generally recognised on morphological grounds (if not necessarily on molecular grounds - Maddison et al., 1999) is the Carabidae Conjunctae which, because I've got a thing against taxon names with more than one word in them, I'm just going to call Conjunctae from here on in*. The Conjunctae include three subfamilies (in the broad sense) of carabids, the Broscinae, Harpalinae and Psydrinae (Roig-Juñent & Cicchino, 2001), though Roig-Juñent & Cicchino (2001) indicated that "Psydrinae" was para/polyphyletic with regard to the other two subfamilies, and other sources such as the Carabidae of the World database divide each group among a number of smaller subfamilies. Conjunctae are united by (and get their name from) their conjunct mesocoxae - the plates of the sternum (the underside of the thorax) are expanded to enclose the mesocoxae (the basalmost segments of the second pair of legs). Roig-Juñent & Cicchino (2001) identified a few other features that might unite the Conjunctae, such as the presence of an elytral plica (a distinct indentation at the back end of the elytra), but as those authors didn't include a great many non-Conjunctae carabids in their analysis I'm not sure how certain those features are to be apomorphic.

*I think I can argue that this is fairly standard taxonomic practice. Quite a lot of taxon names are technically plural adjectives used as singular nouns. Plant family names, for instance, are explicitly required to be.


Mormolyce phyllodes, a somewhat different-looking ground beetle (Harpalinae, Lebiini) from South Asia. Photo by Sarefo.

The Conjunctae are one of the largest clades of carabids, but that is primarily due to the inclusion of the Harpalinae, which alone account for more than half of carabid species. Many Conjunctae (particularly many Harpalinae sensu lato) produce noxious defensive secretions when threatened, and members of the harpaline or near-harpaline tribe Brachinini are the infamous bombadier beetles, which are able to mix their defensive secretions to form an explosive mixture. The Harpalinae also include the Harpalini, which are unusual in having abandoned the carnivorous habits of their ancestors and turned to a life of seed-eating.

And just as an example of some of the unexpected things that sometimes turn up when I search online for stuff to use in these posts - it appears that Rita Skeeter might have belonged to the Conjunctae.

REFERENCES

Beutel, R. G., I. Ribera & O. R. P. Bininda-Emonds. 2008. A genus-level supertree of Adephaga (Coleoptera). Organisms Diversity & Evolution 7 (4): 255-269.

Maddison, D. R., M. D. Baker & K. A. Ober. 1999. Phylogeny of carabid beetles as inferred from 18S ribosomal DNA (Coleoptera: Carabidae). Systematic Entomology 24: 103-138.

Roig-Juñent, S., & A. C. Cicchino. 2001. Chaltenia patagonica, new genus and species belonging to Chalteniina, a new subtribe of Zolini (Coleoptera: Carabidae). Canadian Entomologist 133: 651-670.