In 2016, Chapman University in California, USA found in their survey on American Fears that 25% of their respondents were afraid of insects and/or spiders. It is a common, and for some people severe, fear, but thankfully for them, they aren’t living in the Carboniferous, when giant insects swarmed the Earth.
Some of the biggest arthropods to have ever lived on land were alive in the Carboniferous. These bugs include such characters as Arthropleura, a giant relative of today’s millipedes that may have grown to be more than 1.5 metres in length and half a metre wide. Whilst this sounds terrifying, it is thought that they lived on a diet of dead plant matter from the swamps they inhabited, a diet similar to millipedes today.
Another giant of this age was the griffinfly Meganeura, a giant version of today’s dragonfly. It had a wingspan of 65 to 71 cm, about the size of a pigeon. It has been hypothesised that one of the reasons that it could get this big was because it had little competition, due to the absence of other aerial predators. It was a predator, likely using the sharp, terminal incisors on its mandibles for cutting prey, such as other insects.
Don’t worry though, bees and wasps hadn’t yet evolved in the Carboniferous, in fact they didn’t evolve until the Mesozoic era. Moreover, what was thought to be the largest spider ever, Megarachne servinei, at almost 40 cm long, has now been reinterpreted as an odd-looking Eurypterid, or sea scorpion.
So how did all of these creepy-crawlies get so big? The Carboniferous was dominated by huge expanses of swampland, and all of the vegetation released so much oxygen that atmospheric oxygen levels were almost 50% higher than they are today. Arthropods responded to this huge increase in oxygen by getting huge themselves. In large quantities, oxygen can be deadly, and insect larvae and similar organisms are unable to regulate the amount of oxygen they take in, unlike adult arthropods. Therefore, to survive, they evolved so their larvae were bigger when firstborn, so they are less affected by the high oxygen concentrations. This then produced larger adults.
So arthropods got big because they had to survive, not because they could, and luckily for all of us, most of the giant insects died out at the end of the Carboniferous when the Earth began to get drier and the swamps they made their homes in disappeared.
 A reconstruction of an Eryops stalking an Arthropleura along the desiccating margins of a Late Pennsylvanian ephemeral lake. Available at: https://www.researchgate.net/figure/Reconstruction-of-an-Eryops-stalking-an-Arthropleura-along-the-dessicating-margins-of-a_fig1_250071013.
 A fossilised Meganeura monyi. Available at: https://en.wikipedia.org/wiki/Meganeura.
Information References and Further Sources
 Labandeira, C. C. (1997). Insect Mouthparts: Ascertaining the Paleobiology of Insect Feeding Strategies, Annual Review of Ecology and Systematics, 28, pp.153-193.
doi: https://doi.org/10.1146/annurev.ecolsys.28.1.153. Available at: https://repository.si.edu/bitstream/handle/10088/5969/AR_Ecology_Sys_1997.pdf?sequence=1&isAllowed=y. Accessed 8th August 2019.
 M, Uri. (Unknown). ‘Wingspan of Meganeura spp., extinct flying insects ~3 times the size of the largest flying insects today’, B10NUMB3R5. Available at: https://bionumbers.hms.harvard.edu/bionumber.aspx?id=113319. Accessed: 08th August 2019.
 Peters, R. S., Krogmann, L., Mayer, C., Donath, A., Gunkel, S., Meusemann, K., Kozlov, A., Podsiadlowski, L., Petersen, M., Lanfear, R., Diez, P. A., Heraty, J., Kjer, K. M., Klopfstein, S., Meier, R., Polidori, C., Schmitt, T., Liu, S., Zhou, X., Wappler, T., Rust, J., Misof, B., and Niehuis, O. (2017). Evolutionary History of the Hymenoptera, Current Biology, 27 (7), pp. 1013-1018. doi: https://doi.org/10.1016/j.cub.2017.01.027. Available at: https://www.cell.com/current-biology/fulltext/S0960-9822(17)30059-3. Accessed 8th August 2019.
 ‘Pigeons – Everything there is to know about the pigeon’, Pigeon Control Resource Centre. Available at: https://www.pigeoncontrolresourcecentre.org/html/about-pigeons.html#topofpage. Accessed: 08th August 2019.
 Romm, C. (2016) ‘Insects Are Scary Because Your Brain Confuses Disgust With Fear‘, The Cut. Available at: https://www.thecut.com/2016/10/why-are-so-many-people-scared-of-bugs.html. Accessed: 08 August 2019.
 Selden, P. A., Corronca, J. A., and Hünicken, M. A. (2005). The true identity of the supposed giant spider Megarachne, Biology Letters, 1 (1), pp. 44-48. doi: https://doi.org/10.1098/rsbl.2004.0272. Available at: https://royalsocietypublishing.org/doi/abs/10.1098/rsbl.2004.0272. Accessed 8th August 2019.
 Sues, H-D. (2011). ‘Largest Land-Dwelling “Bug” of All Time’ , National Geographic. Available at: https://blog.nationalgeographic.org/2011/01/15/largest-land-dwelling-bug-of-all-time/. Accessed: 8th August 2019.
 Than, K. (2011). ‘Why Giant Bugs Once Roamed the Earth’, National Geographic. Available at: https://www.nationalgeographic.com/news/2011/8/110808-ancient-insects-bugs-giants-oxygen-animals-science/ Accessed: 8th August 2019.