How many insects are there?

This deceptively simple question is part of a larger one, namely: how many distinct life forms are there on planet Earth? The late Bob May (Professor Robert May, Baron May of Oxford) famously quipped that if aliens visited the earth and asked us how many species there were here, we could – embarrassingly – only be able to come up with a best guess, which might be anywhere from three to 100 million: and that’s just eukaryotes, i.e. not including bacteria or viruses (May, 2010). Despite the huge uncertainties, May (2011) subsequently favoured a number between 2 and 10 million distinct eukaryotic species alive on earth today, and commented that “if I had to buy a ticket in a sweepstakes, I’d have chosen 5 million”.

Butterflies (and a moth?) mudpuddling at Iguazu falls, Argentina Photo by Raymond JC Cannon

In terms of species that have been classified – i.e. named and documented – by scientists, there are about 1.05 million insects (see below), and a total biota of between 1.4 million to 1.8 million (Foottit and Adler, 2017). It is worth pointing out however, that this figure of one million plus insects, merely represents the number of species names available, and may not be the number of described species, as some may have been described accidentally more than once (Stork, 1988). Nevertheless, it is an impressive total, and represents the work of many thousands of taxonomists over several centuries, i.e. since the time of Linneaus. Nevertheless, despite these impressive numbers of species known to science, it is thought that more 80% of all extant species still remain unknown!

Number of described species (2022)
(https://ourworldindata.org/grapher/number-of-described-species?country=Fishes~Insects~Reptiles~Mammals~Birds~Amphibians~Corals~Molluscs~Crustaceans~Arachnids)

One early estimate of the number of insect species was by John Ray (shown below), who estimated their number as being well over 10,000 (Ogilvie, 2012; other reports say he thought there could be 20,000). Whatever; we now know that this was a very large underestimate! But it is not a bad guess for the number of insect species in the UK, which is about 27,000.

John Ray (1627 – 1705) by unknown artist https://upload.wikimedia.org/wikipedia/commons/b/bc/John_Ray_from_NPG.jpg

At the other end of the scale, Terry Erwin, speculated that there could be as many as 30 million living arthropod species, globally (Erwin, 1982). This estimate was based on extrapolations, from estimated numbers of forest tree species, the host specificity of tropical forest canopy insects, and the actual number of beetles on one particular species of tree: Luehea seemannii (Malvaceae), from Panama. Erwin’s main point, was that the vast majority of beetles were only found in the samples of one particular type of forest. In other words, in biodiverse regions like the Amazon basin, large numbers of species are associated with certain, highly restricted forest types, in a mosaic of habitats (Erwin, 1988). There may be 150 or more different tree species per hectare in these tropical forests, which led Erwin (1988) to subsequently revise his estimate of the number of insects species, upwards, to 50 million or more!

Butterflies are generally, well known and well studied, compared to some other groups of insects. Broad-banded swallowtails in Argentina. Photo by Raymond JC Cannon

These estimates certainly caught the attention of other scientists, but some were sceptical about there being such large numbers of insect species. Nigel Stork’s (1988) research on Bornean lowland rainforest trees – where a single tree can provide a home for more than 1000 insect species – suggested that Erwin’s estimate, that beetles represent 40% of the canopy arthropod species, was almost certainly an over-estimate. Stork (1988) proposed a more conservative estimate, of 7 to 10 million species, and subsequent research narrowed this down to a figure of 5.5 million insect species (with a range 2.6–7.8 million) (Stork et al., 2015). The most recent consensus estimates by Stork (2018) are of 5.5 million insects and 7.0 million terrestrial arthropod species. However, this estimate is still very uncertain, because some of the most diverse insect orders (Diptera, Hymenoptera) have families that have “received little taxonomic attention despite signs of very high species richness” (Hebert et al., 2016).

Tropical rainforests are rich in insect species

Biodiversity is unevenly distributed: some groups of organisms are exceedingly diverse, others, much less so. This variation in species richness across the Tree of Life can be extreme. For example, compare the phylum Arthropoda (1.3 million plus species) with that of the Placozoa (only three named species!). How can two such lineages of multicellular animals vary so markedly?

Insects are in general, a particularly megadiverse group, or clade, and the class includes some orders which are extrodinarily diverse (e.g. Coleoptera), but also others that are extremely depauperate (e.g. Zoraptera). For example, there are only only 35 living species of zorapterans (see below), whereas there are at least 380,000 named coleopteran species, and possibly several million more awaiting description.

Zorotypus, a zorapteran from Los Bancos, Pichincha, Ecuador by Graham Montgomery https://commons.m.wikimedia.org/wiki/File:Zorotypus_from_Los_Bancos,_Pichincha,_Ecuador.jpg#mw-jump-to-license

Beetles are so numerous, that they comprise about 40% of insects, and an even more remarkable, c. 25% of known all known species on Earth (Mckeena et al., 2015: Stork et al., 2015). There are however, probably just as many, if not more, flies and wasps.

The largest (i.e. most numerate) orders of insects – Coleoptera, Lepidoptera, Diptera, Hymenoptera and Hemiptera – are generally referred to as being megadiverse. However, the majority of insect orders have less than 30,000 described species, and are called oligodiverse (Rafael et al., 2009). These 30 or so smaller orders contain less than 10% of ineects, as we presently understsnd their distribution.

Why are there so many beetles in the world? (Rhinoceros Stag Beetle, Sinodendron cylindricum)

in terms of named species, insects are the dominant group of arthropods, with Coleoptera, Lepidoptera, Diptera, and Hymenoptera being the most speciose insect orders” (Stork, 2018)

There are approximately 380,000 described beetle species, in 177 coleopteran families, with new species being described at the rate of about 3,000 per year; for example 2,973 new beetle species were published in 2020 (Ding et al., 2022). Why are there so many of them? The answer to this probably has to do with the radiation of phytophagous beetles: their success being intimately linked to the rise of flowering plants (Farrell, 1998).

Herbivorous beetles, like these Chrysomelidae from India, radiated in concert with flowering plants

Ultimately, the variation in the diversity of species groups comes down to differences in the rate at which species are added to (speciation) and subtracted from (extinction) a taxon, over time. Both time and ecological factors influence diversification rates. The development of wings and the ability to fly – or at least glide, at first – was a significant factor promoting the diversification of insects, enabling them to exploit new habitats and evolve into novel niches. Numerous hypotheses have been put forward to try and explain this inequality of richness, in terms of the biodiversity of different insect groups, or clades (e.g. Mayhew, 2007). For example, beetle species are very resilient, with an extremely low extinction rate.

The high rate of diversification in herbivorous insects is thought to result from their specialization to distinct host-plants, which creates conditions favorable for the build-up of reproductive isolation and speciation (Simon et al., 2015). Most phytophagous insects contain microbes, e.g. gut symbionts, that probably aided them in their exploitation of new plant hosts, e.g. by overcoming the chemical defences of the plant. So, the diversification of herbivorous insects was the result of a three-way interaction between plants, insects and their microbial symbionts. Plants and insects have coevolved over millenia (perhaps 350 Myr) and a myriad of physical and chemical relationships have developed, particularly between i) plants and herbivorous insects and ii) plants and pollinating insects (below).

White-tailed bumblebee
Photo by Raymond JC Cannon

The term dark taxa – borrowing from the physicist’s term ‘dark matter’ – has been applied to groups where less than 10% of all species have been described, and where the total estimated diversity exceeds 1,000 species (Hartop et al., 2022). In other words, species-rich taxa, usually of small body size, for whom most of the species-level diversity remains undescribed. Insect taxa with a particularly large number of such suspected, unknown species, include the Diptera (flies) and the Hymenoptera (especially parasitoid wasps). 

One reason why taxonomists have been slow to describe these hyperdiverse, hidden species, is because they often occur in sites containing thousands of specimens belonging to hundreds of species, which need to be sorted via detailed microscopic investigations. However, portable DNA barcoding devices (below) can now be used to sort specimens to species, for further morphological verification of a small subset. In effect, portable laboratories which can fit inside a backpack can be easily transported to remote tropical forest sites to quickly and accurately identify species using DNA barcoding (e.g. Pomerantz  et al., 2017).

MinION is a handheld DNA-sequencing device developed by Oxford Nanopore https://www.ox.ac.uk/news/2015-10-15-mini-dna-sequencer%E2%80%99s-data-belies-its-size

Hyperdiverse phorid flies.

Phoridae sp. from Malaise trap, Aranda, ACT, Australia https://www.flickr.com/photos/dhobern/51329917028/in/photolist-2mcRf2s-2mcLpna-2mcTSKu-ChWsN1-2m1KpFP-2m1QSAV-2m1KpHx-2mdKFK1-2iMMLWF-pownZ7-2kbghWM-2kR3a2B-e8SZ89-23F7F2z-EXmGKC-2m1FJKu-2nLQQ9q-jxXgNN-4eYwjg-2o8QnMb-k1KTFH-k1KTCX-2o8Vxmx-2m1KpAo-EUhb4y-k1KTUi-QSNREP-vLPcRY-oZUMiR-23Z4zxo-ogAitW-dCbQcS-ezgrdy-nRWozg-dzUNwM-23Z4zBS-obcd9z-FHRHak-nRWoJe-o97Fx2-2igY1cQ-2ih2uaX-2928ctV-2gPGuUi-2gPGvxx-2gPHi43-2gPGvvZ-2m1KpC7-2m1QSCo-2kGAd14

An example of a specimen and species-rich taxon is the hyperdiverse fly family Phoridae (Diptera). A field site in Africa – Kibale National Park, Uganda – was found to be home to an estimated 1,000 species of phorids, or scuttle flies, using low-cost DNA sequencing (Srivathsan et al., 2019). Based on this research, the Afrotropical diversity of this family (Phoridae) could well exceed 200,000 species! Even more remarkable, was the fact that 90% of the newly discovered phorids captured in a single Malaise trap (below), belonged to the genus Megaselia.

A Malaise trap https://en.m.wikipedia.org/wiki/Malaise_trap#/media/File%3AMalaise_trap.jpg

there are thousands of vials labeled “Phoridae” shelved in all major museums worldwide” (Srivathsan et al., 2019).

If there are five or six million species of insects (or more), and only just over a million have been described, then there is clearly a great deal of work to do, which needs to be carried out with all urgency. Many undiscovered species have remained unknown to science because they are difficult to find, cryptic, small in size or have very small geographic ranges (Scheffers et al., 2012). Many of these uncatalogued or undescribed species may never have been collected, perhaps because the live in remote, difficult to access habitats, like cave systems, soils, or in the upper canopies of rainforests.

Sadly, the complete inventory of extant life on Earth, our global biodiversity, will probably never be fully known, because of the ever growing extinction crisis, which means that there is less and less time to discover and catalogue it all.

Links

http://tolweb.org/tree/

https://www.washingtonpost.com/news/speaking-of-science/wp/2015/03/18/why-are-there-so-many-freaking-beetles-science-may-finally-have-the-answer/

https://www.wired.com/story/big-fight-small-wasps-taxonomy-biodiversity/

http://www.onezoom.org/life.html/@Bilateria=117569?img=best_any&anim=flight#x158,y372,w0.5245

References

Ding, Q., Li, L., Lu, Y., Zhou, X., Dai, S., Yang, F., … & Bai, M. (2022). The world new taxa of Coleoptera in 2020. Biodiversity Science30(3), 21507.

Erwin, T. L. (1982). Tropical forests: their richness in Coleoptera and other arthropod species. The Coleopterists Bulletin.

Erwin, T. L. (1988). The tropical forest canopy. Biodiversity, 123-129.

Farrell, B. D. (1998). ” Inordinate Fondness” explained: why are there So many beetles?. Science281(5376), 555-559.

Foottit, R. G., & Adler, P. H. (Eds.). (2009). Insect biodiversity: science and society. John Wiley & Sons.

Hartop, E., Srivathsan, A., Ronquist, F., & Meier, R. (2021). Large-scale Integrative Taxonomy (LIT): resolving the data conundrum for dark taxa. BioRxiv.

Hebert, P. D., Ratnasingham, S., Zakharov, E. V., Telfer, A. C., Levesque-Beaudin, V., Milton, M. A., … & DeWaard, J. R. (2016). Counting animal species with DNA barcodes: Canadian insects. Philosophical Transactions of the Royal Society B: Biological Sciences371(1702), 20150333.

Larsen, B. B., Miller, E. C., Rhodes, M. K., & Wiens, J. J. (2017). Inordinate fondness multiplied and redistributed: the number of species on earth and the new pie of life. The Quarterly Review of Biology92(3), 229-265.

May, R. M. (2010). Tropical arthropod species, more or less?. Science329(5987), 41-42.

May, R. M. (2011). Why worry about how many species and their loss?. PLoS Biology9(8), e1001130.

Mayhew, P. J. (2007). Why are there so many insect species? Perspectives from fossils and phylogenies. Biological Reviews82(3), 425-454.

Ogilvie, B. W. (2012). Attending to insects: Francis Willughby and John Ray. Notes and records of the Royal Society66(4), 357-372.

Pomerantz, A., Peñafiel, N., Arteaga, A., Bustamante, L., Pichardo, F., Coloma, L. A., … & Prost, S. (2017). Real-time DNA barcoding in a remote rainforest using nanopore sequencing. bioRxiv, 189159.

Rafael, J. A., Aguiar, A. P., & Amorim, D. D. S. (2009). Knowledge of insect diversity in Brazil: challenges and advances. Neotropical Entomology38, 565-570.

Scheffers, B. R., Joppa, L. N., Pimm, S. L., & Laurance, W. F. (2012). What we know and don’t know about Earth’s missing biodiversity. Trends in ecology & evolution27(9), 501-510.

Scholl, J. P., & Wiens, J. J. (2016). Diversification rates and species richness across the Tree of Life. Proceedings of the Royal Society B: Biological Sciences283(1838), 20161334.

Simon, J. C., d’Alencon, E., Guy, E., Jacquin-Joly, E., Jaquiery, J., Nouhaud, P., … & Streiff, R. (2015). Genomics of adaptation to host-plants in herbivorous insects. Briefings in functional genomics14(6), 413-423.

Stork, N. E. (1988). Insect diversity: facts, fiction and speculation. Biological journal of the Linnean Society35(4), 321-337.

Stork, N. E. (2018). How many species of insects and other terrestrial arthropods are there on Earth. Annual review of entomology63(1), 31-45.

Stork, N. E., McBroom, J., Gely, C., & Hamilton, A. J. (2015). New approaches narrow global species estimates for beetles, insects, and terrestrial arthropods. Proceedings of the National Academy of Sciences112(24), 7519-7523.

Srivathsan, A., Hartop, E., Puniamoorthy, J., Lee, W. T., Kutty, S. N., Kurina, O., & Meier, R. (2019). Rapid, large-scale species discovery in hyperdiverse taxa using 1D MinION sequencing. BMC biology17(1), 1-20.

6 comments

  1. I like the term ‘dark taxa’! Outside the arthropods, the nematodes would seem to be another example of a group where the number of described species is only a small fraction of the likely total number.

  2. I like the term ‘dark taxa’! Outside the arthropods, the nematodes would seem to be another group where the number of described species is only a small fraction of the total number of species.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s