What's the name of the insect that appears only in the monsoon in India & gets attraced to light?

This is more often found in East India. They come from April & till the end of October (in monsoon), they have no trace. From where do they originate? After it rains in evening, they come in thousands from the window and cover the light brown. The most interesting part of them is that they know that if they go near any illuminating object like the tubelight, they will die but still they come near it. Till morning, all of them die & then again in the next evening their successors come. Today I cleaned at least 40,000 of their dead bodies. Why is it that they are so numerous in summer & rainy & almost extinct in winter? I would be pleased to know because I'm collecting articles on mysterious animals like the prehistoric dinosaurs and others of that kind. What are they called & why are they so mysterious?