Today’s giants? (Life and death of giant Insects, part 2/2)

In the first part (that you can find here), we looked at extinct giant insects and assessed their size. It is time we adopt an objective point of view on these, and to do so, we’ll review modern insect sizes.

What about modern insects?

Now let’s look at the wingspan, length and weight of modern insects. All information that I will provide is correctly verified, we are not going into mad speculations as can be seen in the domain, and sources can easily be found on the Wikipedia pages of all mentioned species. For further information, there is a beautiful book called Biggest Bugs Life-Size from George Beccaloni, which is now outdated, but stays a reliable source of information (for the anecdote, this book has been edited by the London NHM, and most of the specimens mentioned come from their collection, including what was the longest known insect. It has since been dethroned -completely blown even- by a chinese specimen, and I found easily larger specimens than those mentioned in the book in the collections of Paris MNHN, so we can say that the major achievement of the London collection is to have been measured!).

The longest known insect is a stick insect[1] discovered in China in 2016 (apparently belonging to the genus Phryganistria Stål, 1875) and that has still not been described at this time (which is rather strange as this kind of « incredible discovery » seem to be a common tool for chinese propaganda). Its body length exceeds 38 cm and it had the pleasure to be the mother of an even longer specimen whose body length neared 40 cm. In the longest insect leaderboard, this stick insect is followed by an other one, then an other one, and finally… an other stick insect. All are wingless females originating from South-East Asia. Apart from stick insects, there is no modern insect reaching more than 20 cm. Some challengers could be some elongated grasshoppers from the Proscopiidae family. After that we can have a certain number of insects reaching around 16 to 18 cm including praying mantises and beetles.

If we look at wingspan, the current record belongs to a moth, Thysania agrippina (Cramer, 1776), reaching 30 cm. This moth is subject to a certain number of speculations, and many are those who attest to have seen ones measuring between 35 and 40 cm, but surprisingly none have been measured. There are then many butterflies, beetles and katydids measuring between 25 and 30 cm, but none of them reach the symbolic wingspan of 30 cm. It can be noted that the longest wings of a modern insect is probably an other moth’s wing, the famous comet moth, Argema mittrei (Guérin-Méneville, 1847), which hindwing measures around 20 cm. I’ll let you be the judge of whether or not it deserves the title for the widest modern insect.

Last but not least, weight. It has the major disadvantage that you can’t measure it post-mortem, which implies that it is way less documented than lengths. We have two winners in this category: one for the heaviest adult insect, and one for the heaviest insect “in general”. For the first category, the record belongs to a weta (a group of insects close to katydids but not belonging to them), Deinacrida heteracantha White, 1842. A fat female full of eggs reached 71 g. She outweighs all adult beetles ever weighed, but there is a bias here, as the biggest beetle ever discovered (a titan beetle, Titanus giganteus (Linnaeus, 1771) of almost 17 cm) didn’t have the chance to be weighed. The same goes for several other orthopterans of similar size to this weta, and even for the long stick insects that have the length record. The heaviest insect “in general” is a larva from the Actaeon beetle Megasoma actaeon (Linnaeus, 1758), which reached 228 g, the weight of a big steak. Again, the biggest beetle’s larva has never been weighed, it is still a myth among entomologists, as this larva has never been observed yet.

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To put it in a nutshell, here are the biggest modern insect measurements: lengthwise, we have a chinese stick insect of almost 40 cm, in matter of wingspan we have a 30 cm south american moth, and the weight record belongs to a beetle larva of 228 g.

Extant vs. extinct

Now that we have the sizes of both extinct and extant insects, we can compare them. And the result may not be the one you expected: Meganeura and peers were neither longer nor heavier than modern largest insects. To be more precise, with 25 cm long, griffenflies were way shorter than our longest stick insects exceeding 35 cm. The same can be said for their weight: 40 g max for Meganeuropsis, versus 70 for some extant insects, and even more than 200 g for some larvae. Titanoptera could still be among the largest, but their fossils are too poorly preserved for now. So, if there is still no answer to the question “Why did giant insects disappear?” [2], maybe it’s because they merely changed form (or never existed, if you are on the cynical side of things).

A question remains: Meganeuropsis’ wingspan is two times superior to any extant insect, butterflies included, which wings are enormous compared to their body. So, why did insects with such wingspan disappear? First, with 40 g for 70 cm of wingspan, Meganeuropsis is very light. Indeed, many beetles exceed this weight for only 25 cm of wingspan. In modern birds, the environment has a major impact on wingspan, and forest birds tend to have a shorter wingspan for the same weight (Harpy eagle vs. Steller’s sea eagle: weight difference: 3%, wingspan difference: 25%). Would it be possible that griffenflies had lived in very open forests, where such a wingspan didn’t matter? Vegetation changes have always had a huge impact on insects, and for example flower plants diversification has completely disrupted these organisms, probably even more than the Cretaceous-Paleogene extinction event. I am unfortunately not a palaeobotanist, and can’t back up this hypothesis. Griffenflies’ disappearance matches the emergence of flying vertebrates, and this is often mentioned to explain it. It is not impossible, even more when these insects weren’t very good at flying: low strength, big flappy wings very far from the jewels enabling modern dragonflies to perform their acrobatic flight. Lastly, there is still a hypothesis to consider in my opinion: if we exclude griffenflies, all other known fossil insects don’t exceed 40 cm, which is quite close to the modern ones, and can probably be explained by the mere lowering of atmospheric pressure. In truth, griffenflies are the only insects with an extravagant wingspan, which is only one taxon. Their population must have comprised only a low number of individuals, due to their size, and their last members are known from the end of the Permian, thus just before the worse extinction event ever (but we’ll see about that in 50 years), so didn’t they just have bad luck?

What about other arthropods?

While insects were apparently not larger than nowadays, what about other Arthropods? Were millipedes, spiders and scorpions larger?

Let’s begin with a warning, because there’s a lot of bullshit on this subject, we have already seen this but I have other examples: if you have already heard about fossil insects, you may note that I never mentioned any giant Carboniferous cockroach (by the way, there’s no proper cockroach in Carboniferous, only basal Dictyopterans, you can find more info in Li (2019)). And the reason is simple: they just don’t exist. Except for some obscure blogs mentioning 50 cm cockroaches, one of the main reliable sources is a -rather good- sci com book written by the famous (at least in France) Jean-S. Steyer : Earth before Dinosaurs. Giant arthropods are quickly mentioned, and a figure represents a “cockroach”, larger than a human hand, so 15 cm long, and measuring more than any extant cockroach. This one is named Spilloblatta. Informations on this genus are hard to find (I didn’t even find the descriptor’s name!), but several months ago, before the pandemic, while I still had access to my gigantic lab’s library and is now inaccessible to me (well, I could get access, but I’m not going to go to the lab for this alone). The truth is, these Dictyopterans had nothing particular, and are smaller than American cockroaches!

The spider case is also quite interesting: you probably remember the mesothelae spider from the BBC Walking with Monsters, which would, in modern days, “be hunting cats”. Mesothelae are a group of spiders considered as the sister group of all other spiders. Some are actually known from the Carboniferous, with incredible 3D fossils, but they are all tiny. The fossil that inspired the BBC was actually Megarachne servinei Hünicken, 1980, which is in fact a sea scorpion (which was known when the documentary came out, hence the “Mesothelae” instead of Megarachne). The largest known fossil spider is Mongolarachne jurassica (Selden, Shih & Ren, 2011), and is not much larger than the extant european largest ones (for the anecdote, the genus Mongolarachne has been associated with a case of scientific fraud: researchers pretended to have discovered a new species, but had actually sculpted part of the fossil, originally crustacean).

Now that we’ve put aside sketchy specimens, there still has been spectacularly sized terrestrial arthropods. First Pulmonoscorpius Jeram, 1993 which largest specimen would have measured 70 cm (Jeram, 1993). However it is rather fragmentary, and not figured, thus I can’t talk about it too much. Second example, the famous Arthropleura Meyer, 1854, a genus of Myriapoda which largest specimens would have reached around 2 m long [3]. Against those arthropods, even the largest insects we’ve seen earlier are rather small (even though Pulmonoscorpius would probably deserve to be revised). In fact, insects have probably never been the largest terrestrial arthropods, and the reason is simple: wings. Indeed, the immense majority of insects bear them, and use them to fly. But by their very anatomy, wings and their muscular system do not allow to lift immense and heavy creatures, and to abandon them is probably disadvantageous, seeing how few actually did it. By the way you might notice that both the longest extant insect and the heaviest adult insect do not fly, which corroborates rather well this hypothesis. Thus, today, the largest terrestrial arthropods are still not insects, though the latter represent 3/4 of the former, but a crab, Birgus latro (Linnaeus, 1767). This one can reach 40 cm long, 1 m legspan and weigh more than 4 kg, the dimensions of a cat!

A word of conclusion

I have warned you, this article proposes an original point of view on giant insects, and my opinion on this subject is clear, they are more akin to a scientific legend than reality. Is it a problem? Well, no, because if we have effectively lost a part of the phylogenetic diversity of insects with the course of time, which is normal, we may not have lost as much of their ecological diversity, as all sizes of insects that have ever appeared are still present nowadays. And does size even matter? Do we really need giant insects for them to be interesting? In my opinion no, and the strength of insects would rather be in their small size than in their larger sizes. Flies alone are more diversified than vertebrates will ever be, and this can be explained by their size. Moreover it is true that studying gigantism is interesting, but it is also true for miniaturization, and insects are a perfect study model for this.

For example, did you know that crickets are too small to localize the origin of a sound in space? Yet they manage perfectly! How? Now is your turn to search for the answer!

Notes

[1] The author disengages themselves from any modification made to their text by the blog owner Djigr, such as the addition of this link.

[2] The most commonly admitted hypothesis to explain the extinction of giant insects is the fall of oxygen levels in the atmosphere (35 % at the end of Carboniferous and the beginning of Permian, 21 % today). It is more and more questioned: indeed, the correlation between maximum insect wingspan and oxygen level is far from perfect, and the more we discover new griffenflies, the more it withers. It also doesn’t explain Titanopterans. Many patches have been proposed to save this hypothesis, but I won’t go into details as it’s not the point of this article, and I simply didn’t do enough research on it. You can still read the articles from Clapham & Karr (2012) and Dorrington (2016) to know more.

[3] No reference here, I have to admit that I simply don’t want to search for one, and as I almost abandoned writing this article because of it, I figured it was better to have an article without reference rather than no article at all, I hope you will understand.

EDIT: A friend of mine Benjamin Carbuccia had touched the subject of extinct giant spiders on his blog, Chroniques de Nopeland in the following article (in french): https://nopelandchronicles.blogspot.com/2019/01/spider-tales-1-les-geants-du-passe.html

References

Clapham, M. E. & Karr, J. A. (2012). Environmental and biotic controls on the evolutionary history of insect body size. Proceedings of the National Academy of Sciences, 109(27): 10927-10930.

Dorrington, G. E. (2016). Heavily loaded flight and limits to the maximum size of dragonflies (Anisoptera) and griffenflies (Meganisoptera). Lethaia, 49: 261–274.

Jeram, A. J. (1993). Scorpions from the Viséan of East Kirkton, West Lothian, Scotland, with a revision of the infraorder Mesoscorpionina. Transactions of the Royal Society of Edinburgh: Earth Sciences, 84: 283-299.

Li, X. (2019). Disambiguating the scientific names of cockroaches. Palaeoentomology, 2 (4): 390–402.

May, M. (1982). Heat Exchange and Endothermy in Protodonata. Evolution, 36(5): 1051-1058. doi:10.2307/2408082.

Nel, A., Prokop, J., Pecharová, M., Engel, M. S. & Garrouste, R. (2018). Palaeozoic giant dragonflies were hawker predators. Scientific reports, 8(1): 1-5.

Ren, D., Nel, A. & Prokop, J. (2008). New early griffenfly, Sinomeganeura huangheensis from the Late Carboniferous of northern China (Meganisoptera: Meganeuridae). Insect Systematics & Evolution, 39(2): 223-229. doi:10.1163/187631208788784075.

Sharov, A. G. Filogeniya ortopteroidnykh nasekomykh. Trudy Paleontologicheskogo Instituta, Akademiya Nauk S.S.S.R. 118, 1–216, Moskva. [in Russian, Translated in English in 1971: Phylogeny of the Orthopteroidea. Israel program for scientific translations, Keter Press, Jerusalem, 1–251.] (1968).