Final answer:
The valuable by-product affected by Hypoderma lineatum and Hypoderma bovis is the hide of animals.
Step-by-step explanation:
The valuable by-product affected by the Hypoderma lineatum and Hypoderma bovis is the hide of animals. These parasitic flies lay their eggs on the skin of cattle and other animals and their larvae burrow into the skin, causing damage to the hide. This can result in reduced quality and value of the hide, affecting industries that rely on it for leather production and related products.