Over 20 million insects, possibly. What about other arthropods?

Nature Talk
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A recent paper came out estimating there may be upward of 20 million species of insects on Earth, extrapolated from barcodes of Microgastrine Bracronids collected in Malaise traps of Costa Rica.

https://www.pnas.org/doi/full/10.1073/pnas.2524283123

Estimating the species richness of insects seems to be its very own subgenre of entomology. Estimates have been all over the place, from as low as 5 million to as high as 30 million. But something I have noticed is that, at least from my general impression, people tend to say “insect” and “arthropod” interchangeably at times.

Like when it is said that there is this or that many million species of “tropical arthropods”, is this including spiders? Mites? Myriapods? Freshwater crustaceans? If this is based on extrapolations from this malaise traps which overwhelmingly collect flying insects, to say this is the species richness of “arthropods” would be erroneous. This recent study doesn’t do this thankfully, it does specify insects.

But with that said, has anyone ever tried estimating the global species richness of non insect arthropods with methods other than rate of description? I have only seen similar estimates for mites, which go just over one million.

Model based projections in Li et al. (2025) (https://www.science.org/doi/10.1126/sciadv.adz3071) for all non insect arthropods combined barely exceed 1 million species, about the number of currently known insect species. The combined estimated total for all non insect arthropods in Chapman (2009) (https://www.dcceew.gov.au/sites/default/files/env/pages/2ee3f4a1-f130-465b-9c7a-79373680a067/files/nlsaw-2nd-complete.pdf0) is just short of 900 thousand. As far as I have seen, no study has investigated the question of non insect arthropod species richness, or total arthropod species richness with significant attention to non insects, with anywhere near the same rigor and detail as for insects.

So this begs the question. Why is the global species richness of arachnids, malacostracans, copepods, etc not investigated nearly as much as insects? And in the few times estimates are given, the predicted richness of all non insect arthropods combined is far lower than insects?

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Well, as to the question why insects are more intensively studied then other arthropods, I don’t find it surprising at all. Insects are so manyfold and touch our (human) life in so many ways, that an early and never ending interest is quite understandable. They can be easily observed at any time of day and basically any human-inhabited habitat, might be very flashy and beautyful (beetles, butterflies..), might be very annoying and persistent and even deadly ( flies, mosquitoes..), might be relevant to our food sources (bees, “pests”..) or even be a relevant food source.. no wonder interest is sparked early in individual human live and even in historical terms.

It is much harder to observe many (nocturnal) spiders, mites (small!) or mainly limnological/marine organisms. And often they are not as penetrantly beautyful or related to our lives as insects as a taxon as a whole (of course, there are enough individual examples that proof otherwise).

As to why there might be a much lower diversity in other arthropods… part of it is already mentioned in the text above: insects managed to conquer a variety of habitats, it’s amazing. So they generally need to be very adaptable. Many co-evolved with the vegetation, giving rise to so many specialized species interacting with only certain kinds of plants, filling their niche. On top of that you have those specialists interacting with other animals as parasites or even parasitoids or hyper-parasites and so on.. insect world really displays the creativity of nature.

On the other hand you have such taxa like spiders, scorpions or harvestman, which are not that creative with their diet (mainly carnivorous or rather generalist decaying plant and fungal matter) nor their habitat (terrestrial). You have examples of that effect in insect world as well actually.. while e.g. beetles (seem to fill all the niches :sweat_smile:) and butterflies ( host specificity!) are crazy in species numbers, dragonflies did not go that crazy, as pure but generalistic carnivores with similar lifestyles… it’s a bit comparable to spiders as a whole.

Or you have arthropod taxa that live mainly in marine environments, which is remarkably a habitat insects did not conquer. The variety is crazy in those as well actually, but in the end they do miss out on terrestrial opportunities.

(again i want to point out that I oversimplified here and I do know that of course Isopods exist on land.. but in the end they still depend a lot on water and did not have that much time yet for the crazy evolution game as insects had evolutionarily speaking)

(and yes, I know there are even exceptions of that with isopods inhabiting deserts… one of the first things I try to get across to my studends is that there is an exception to basically every rule I name in biology and they love to challenge me on that​:sweat_smile:)

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and there is that ONE insect that lives on the surface of the open ocean – which seems incredible.

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Well Pontomyia exists. Theres also lice of elephant seals