Im just curious if invertibrates, namely insects and/or arachnids were ever apex predators in a terrestrial evironment?

yes they were when reptiles first evolved giant scorpions attacked the lizard things maybe someone else can give more detail

Just look up "Carboniferous Period" on Google.

Well, I don't know if this would qualify but I'd say that mosquitoes and fleas are doing a pretty good job at staying at the top.
After all, the diseases they transmit sicken or kill millions of humans each year.
Yellow fever, malaria, dengue fever, typhoid --- they spread a great deal of disease and remain a formidable enemy.

Probably the Ant