I stumbled across a paper by Jess Awad, an entomologist specializing in parasitoid wasps (Untangling host specialization in a "double dark taxa" system. (2025) Annals of the Entomological Society of America, 118(3), 206-219. https://doi.org/10.1093/aesa/saaf003). I've been frustrated by my inability to identify Platygastrine wasps to species level, so this struck a chord. (I know I'm not alone in this!) Platygastridae are parasitoids of gall midges (Diptera: Cecidomyiidae). Both they and their hosts are exceptionally abundant and speciose, and both are also "dark taxa, whose species identification and ecological associations are obscured by a history of taxonomic confusion and neglect". To paraphrase Awad et al: Although some Platygastridae exploit other hosts, the majority of species attack gall midges. The gall midge parasitoids, all classified in the subfamily Platygastrinae, include more than 1,800 described species. Taxonomic progress is stymied by a few genera which are both remarkably species-rich and morphologically difficult to distinguish. Chief among these is the genus Platygaster. With nearly 700 described species, it is the largest genus in the subfamily and even in the whole superfamily Platygastroidea. Platygaster has no morphological gotcha and is always placed at the end of keys, being defined by a lack of distinguishing features. Gall midges are the most diverse group of flying insects, with worldwide estimates in excess of one million species. It is therefore unsurprising that their parasitoids are similarly dominant. Hence "double dark taxa".
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. Charles Darwin, On the Origin of Species (1859)
This came a day after I'd discovered a new metaphor - the bush of life (Cruaud et al (2024) The Chalcidoidea bush of life: evolutionary history of a massive radiation of minute wasps. Cladistics, 40(1), 34-63. https://doi.org/10.1111/cla.12561). Chalcidoidea are an enormously diverse group of parasitoid wasps with an estimated 500,000 species, one of nature's most species-rich groups. The Chalcidoidea originated in the Jurassic period approximately 174 million years ago. This coincides with an important fossil record signal: major diversifications within Chalcidoidea aligned with increases in insect family diversity overall. A shift toward plant-feeding hosts corresponds with the "Angiosperm Terrestrial Revolution" - the rise of flowering plants, which dramatically reshaped terrestrial ecosystems around 130 million years ago. All of this places conventional taxonomic approaches under unbearable strain, and hence gives rise to dark taxa. The "tree of life" analogy breaks down and the explosive radiation of the Chalcidoidea to match their hosts is better represented by the idea of "a bush of life". However, I would suggest that considering that many of the hosts also represent dark taxa, what we're actually talking about here is "the shrubbery of life". The only practical way out if this mess is going to be via DNA, like it or not.
I've already dabbled in DNA barcoding but it becomes clear very quickly that much of the existing data is blighted by misidentification of the material examined. The approach of Awad et al is to remove some of the uncertainty by rearing parasitoids from hosts and then barcoding - good old-fashioned Victorian entomology! The snag with Cecidomyiidae is that they are difficult to rear as many species require living host tissue to complete development. I'm very much not an expert in rearing insects - all the parasitoids I've attempted to rear turn out to be unparasitised hosts and all the other species I've attempted turn into Ichneumons! I'm clearly going to have to do more dabbling with rearing parasitoids from plant galls, probably leaning heavily on my friends who are much more knowledgeable about plant galls than me.
