The insect whose economic importance is both harmful and beneficial is the butterfly. Harmful aspects: The larval stage (caterpillars) of some butterfly species can be agricultural pests, feeding on and damaging crops. Beneficial aspects: Butterflies are significant pollinators for many plants, including various crops, contributing to food production. They also play a role in the ecosystem as part of the food chain and are indicators of environmental health. Their aesthetic value supports ecotourism. The other options are primarily harmful: Tsetse fly: Transmits trypanosomiasis (sleeping sickness) to humans and nagana to livestock, causing significant economic and health problems. Blackfly: Transmits onchocerciasis (river blindness) to humans and can be a biting nuisance to humans and livestock. The final answer is $\boxed{\text{C}}$. 3 done, 2 left today. You're making progress.