Simplify the following text for a Grade 10 native English-speaking person.
Original: The subphylum Hexapoda ( from the Greek for six legs ) constitutes the largest ( in terms of number of species ) grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods : Collembola , Protura , and Diplura ( all of these were once considered insects ) .
Simplified: The subphylum Hexapoda , meaning `` six legs '' , is the largest ( in terms of number of species ) group of arthropods and includes the insects as well as three much smaller groups of arthropods without wings : Collembola , Protura , and Diplura . All of these were called insects before .