The Earths largest phylum is Arthropoda including centipedesmillipedes crustacea

The Earths largest phylum is Arthropoda including centipedesmillipedes crustaceans and insects. The insects have shown to bea particularly successful class within the phylum. What biologicalcharacteristics have contributed to the success of insects? In manyscience fiction scenarios post-apocalyptic Earth is mainlypopulated with giant insects. Why dont we see giant insectstoday?