Fly have carved up, special place in ecology, and sometimes their no.varies with specific biotic and abiotic change in the environment....so how can a fly be pinned as an EHI and possible ways to measure it ??
Whats your question if you want to address a fly as indicator species for environmental health then you have to plan a study which support that presence of particular species of fly or threshold density of that particular species increases as their is a destruction in environment of health of environment is hampered in addition to this you need to describe the health of environment (parameters) on which you can say the change is seen
and I am sure you may find flies as indicator of particular habitat
Some fly groups are known to contain species that are environmental indicators, especially in aquatic environments, like Simuliidae and Chironomidae. You could focus on the species or groups already known to be indicators and then design an experiment (with proper control) in which you focus on those dipterans. A literature search will return papers that should guide you.
I think you want to say, used insect as Bio monitoring agent , Chironomidae is a good monitoring agent in Indian rice agro ecosystem even as industrial effluents.
you should learn about their biology through paper and article of Proff. Abhijit Mazumdar, Dept. of Zoology, University of Burdwan, India.