Final answer:
The two mostly herbivorous arthropod groups are millipedes of the Myriapoda subphylum and various insects of the Hexapoda subphylum, both of which play important ecological roles.
Step-by-step explanation:
Millipedes, part of the Myriapoda subphylum, are known for their numerous legs and are predominantly detritivores, feeding on decaying plant matter but also consuming fresh vegetation. They play a significant role in decomposition and nutrient cycling in ecosystems.
On the other hand, many species within the insects group, which form the Hexapoda subphylum, are herbivorous. This diverse group exceeds all other animal groups in the number of species and includes numerous herbivores such as beetles, butterflies, grasshoppers, and aphids, among others. Some insects are even specialized to feed exclusively on certain types of plants or plant parts, demonstrating adaptations such as specialized mouthparts for chewing leaves, sucking sap, or boring into plant tissues.
Collectively, these herbivorous arthropods play a critical role in ecosystems, influencing plant populations and providing ecological services such as pollination and soil aeration.