I decided to make a map of the number of arthropod species observed on Inaturalist partitioned by biogeographic realms.
The neotropical realm is the region with the largest number of observed arthropod species. This makes sense, as the tropical americas are a biodiversity hotspot. But the next most diverse realms are the palearctic and nearctic, which seem to have more species than the afrotropical or oriental realms. Based on what is known from plants and vertebrates, this should not be the case, since tropical regions are more diverse. The truth of the matter is that temperate regions of the world are much better sampled and studied than tropical regions, since the science of natural history originated in temperate regions and thus has the longest history then. Africa in particular is very under sampled, with it having less arthropod species than australia and oceania, and there is no way this is actually the case in reality.
Aside from the general sampling issue, there is also the fact that tropical regions in general and africa in particular has way less people using inaturalist. Since sub saharan africa is the least developed part of the world, people are struggling with aspects of day to day life too much to bother posting pictures of bugs on inaturalist. China makes a great case study for this, in that China’s massive output in biodiversity research corresponds with its enormous economic growth.
Other than regional differences, there is also differences among the 4 major groups of arthropods. Hexapods (insects and co.) are the most specious group, followed by chelicerates (arachnids and co.), then crustaceans, and finally myriapods at last place. Of course, taxa with more total species will have a higher number of observed species, but there is more reasons than this to explain the high prevalence of insects.
Many insects are easy to find and photograph, and (depending on the group) not always difficult to identify either. Insects like bees, butterflies, beetles, praying mantids, dragonflies, and ants are iconic and generate a lot of interest, so there are more people actively looking for them, and more expertise to identify them. They are also very easy to accidentally stumble upon. Arachnids are little worse off in this regard, for other than a few large or conspicuous spiders like jumpers or orb weavers, or large scorpions, most of them are very cryptic in their habits, and thus difficult to find and not likely to run into by accident. You easily notice a moth sitting on the wall, but you aren’t seeing schizomids unless you really know where to look. Doesn’t help that a large portion of arachnids are mites which are hard to find, hard to see, hard to photograph, hard to identify, and don’t generate much public interest. Crustaceans are yet worse off, for they have the issues of arachnids but with the added drawback that most of them live in water, which makes them harder to find and harder to photograph in situ. Many crustaceans are either microscopic or live in nigh-inaccessible habitats like the deep sea or caves. They too can be a pain to identify, especially small ones like copepods. Then coming in last are myriapods. Myriapods are essentially all creatures of the soil, living under leaf litter, rocks, logs or bark. As for arachnids and crustaceans, many myriapods are quite cryptic and hard to find unless you really know where to look, and thus not likely to be encountered and noticed by a casual person. Myriapod species can also be extremely difficult to identify, with many related species often looking highly similar.
For all these reasons, hexapods don’t only have the most species on inaturalist, but also have the highest percent of their total known species observed on inat as well. Around 20% of hexapod species are observed on inaturalist, as opposed to ~12% of chelicerates, ~11% of crustaceans, and ~10% of myriapods.