Question: Are arachnids and insects so distantly related that they were already diverging before their (independent) emergence(s) on land?
I am curious to know what the fossil record and molecular evidence has had to say so far about such questions.
To the extent that arachnids and insects might not be any more closely related to one another than they are to crustaceans, and crustaceans are mostly aquatic, it does not seem impossible/implausible to me that the terrestrial transitions of insects and arachnids could have been independent of one another. In fact, considering that wood lice are crustaceans, one can conclude that there have already been at least two independent terrestrial transitions for arthropods, so postulating a third seems less of a stretch to me.
I suppose the thought process behind this is related to this question. Specifically, to the best of my knowledge, flight emerged several times among distinct insect lineages after appearing on land, in cases of convergent evolution. The fact that no lineage of arachnids ever evolved winged flight while many lineages of insect evolved flight independently of one another suggests that the "fundamental recipe" for insects and arachnids is substantially different, i.e. that they are not as closely related as one might have otherwise thought.