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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.
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.
While some authors consider tiger beetles to be subordinate to the family Carabidae, there is by no means universal or even majority agreement with such position. As a clade, they are undoubtedly monophyletic, and their exclusion from the Carabidae does not result in paraphyly for the latter, so the question of whether they represent a distinct family or a subfamily of Carabidae is largely subjective based on the rank importance given to their distinguishing characteristics. They are closely related to be sure, but in my mind their unique life history and highly derived larval morphology tip the balance in favor of familial status - a view shared by many North American workers (Terry Erwin being a notable exception).
ReplyDeletePersonally, I've seen very few recent references that maintain a separate Cicindelidae. Not, I'll admit, that I've really been looking. I will quibble with...
ReplyDeletetheir exclusion from the Carabidae does not result in paraphyly for the latter
...because there really doesn't seem to be support either way on this one. Surprisingly few studies have been done on caraboid phylogeny as a whole, it seems - most studies have focused on specific genera or tribes. The molecular phylogeny paper I linked to in the references (Maddison et al., 1999) actually found Cicindelinae in a quite derived position within Carabidae, within the sister-group to Harpalinae + Brachinini, but I'm sure you'll not be surprised to hear that even the authors of the paper didn't really buy their results in that regard, and forcing Cicindelidae outside Carabidae did not result in a significantly different level of support. A more recent study of carabid phylogeny pretty much eliminated the tiger beetles from consideration from the start because they dicked with the analysis.
Unique features of tiger beetles wouldn't actually be informative when it comes to phylogeny, because they don't say anything about the position of tiger beetles relative to anything else. You need shared characters between taxa to establish that.
Quite a lot of taxon names are technically plural adjectives used as singular nouns. Plant family names, for instance, are explicitly required to be.
ReplyDeleteI was somewhat surprised to find from some googling that "Asteraceae" indeed seems to be more commonly treated as singular ("Asteraceae is") than plural ("Asteraceae are").
The ICBN itself, however, uses plural agreement, eg. "the Papilionaceae ... are" from Art. 18.5.
Hi Chris,
ReplyDeleteI am, admittedly, not very familiar with the Old World literature; however, North American authors overwhelmingly treat tigers as a separate family. Recent examples include the current U.S. catalogue (Freitag 1999), Pearson and Vogler's 2001 book on evolution, ecology, and diversity, and the very recent U.S. field guide (Peason et al. 2006), as well as numerous smaller works. A quick perusal of recent issues of the journal CICINDELA also shows similar majority support for familial status by authors from North America as well as other parts of the world.
I'm not saying this means there is a consensus, only that support for subfamilial status is nowhere close to majority.
My last statement regarding tiger synapomorphies was intended to emhasize their grade significance rather than being supportive of carabid monophyly. Regarding the latter, it'll take much stronger molecular data than what currently exist to convince me that the phylogenetic intuitions provided by morphology are misleading.
Wonderful discussion - thanks!
regards--ted
With the risk of saying too much from our unpublished data on the BToL project, it's unambiguous from several nuclear protein-coding genes (the largest molecular dataset yet) that the Cicindelinae are nested well within the remaining Carabidae, with strong statistical support.
ReplyDeleteDrat. Next you're going to tell me beetles are nested well within the muscoid clade!
ReplyDelete