Final answer:
The order Lepidoptera, which includes butterflies and moths, has different mouthparts in their immature (larval) and adult stages, with larvae having chewing mouthparts and adults possessing a siphoning proboscis.
Step-by-step explanation:
The order of insects that always has different mouthparts as immatures and as adults is the order Lepidoptera, which includes butterflies and moths. This order undergoes a complete metamorphosis, meaning that each stage of their life cycle (egg, larva, pupa, and adult) looks different from the others. The immature form, or larva, typically has chewing mouthparts which are suitable for consuming solid foods, whereas the adults usually have specialized siphoning mouthparts in the form of a proboscis. This proboscis is adapted for sucking nectar from flowers, illustrating a significant lifestyle change from their larval stage.
Cockroaches, on the other hand, experience an incomplete metamorphosis and maintain chewing mouthparts throughout their life stages. The varying mouthpart specialization in insects is largely a result of adaptation to their specific dietary needs and is a key indicator of their lifestyle and ecological role. For instance, aphids have piercing-sucking mouthparts which allow them to feed on plant sap, while grasshoppers, which have chewing mouthparts, can bite off bits of plants.