Final answer:
The main structural difference between true flies and other insects referred to as flies is in their wing structure. True flies, like houseflies, have one pair of wings and halteres for balance, while other insects have two functional pairs of wings.
Step-by-step explanation:
The defining difference between true flies (like the housefly and blowfly) and other insects that are colloquially called flies (such as dragonflies, bees, and mosquitoes) lies in their wing structure. True flies, which belong to the order Diptera, are characterized by having only one pair of wings. Other insects, including dragonflies, bees, and mosquitoes, typically have two pairs of wings.
For example, houseflies and other true flies have their second pair of wings reduced to small structures called halteres, which are used for balance during flight. In contrast, insects such as dragonflies, bees, and mosquitoes have two functional pairs of wings. This unique feature of only having one pair of functioning wings is what sets true flies apart from other insects.
Additionally, wings in different insect groups have evolved to serve specific functions. Beetles, butterflies, and katydids have two pairs of wings that they use for flight, but their wing structures have diversified to accommodate different modes of life, illustrating the variety of forms and functions in insect wings.