Things about arthropods I was most surprised to learn were not always common knowledge:
That caterpillars and other larvae can’t lay eggs or make babies
That termites are not related to ants
That insects even have distinct sexes and need to mate
That ticks and fleas are different things
That crustaceans count (as arthropods) just as much as insects or arachnids do and are not in fact “their own thing”
That scorpions are arachnids
That there are arachnids other than spiders
That “venomous” isn’t synonymous with “dangerous”
That bed bugs are a specific kind of insect (some people assume the old rhyme is broadly referring to biting pests in general)
That not every introduced species is necessarily invasive and needs to be killed on sight
That not everything in your house is a pest
That winged ants aren’t a species, but a special caste of a typical ant colony
That fleas, lice and ticks are all totally wingless
That there are no winged arachnids
That there are virtually no insects that live in the ocean
That mosquitoes breed in water
That spiders do not lay their eggs in or on other living things
That they have any organs at all and are not just walking shells “full of juice”
That they breathe oxygen like we do
That they have muscle tissues
The biggest one that never ends: that they’re animals at all