Water is necessary for . . . . . . . . life but . . . . . . . . life of these insects does not depend on water.

"The" is placed before the latter part of "life" in the sentence as we are referring to a specific life i.e. life of insects is being referred to in the given sentence hence we use a definite noun . On contrary the former "life" doesn't require an article as definite article is not used ahead of a general thing/noun. The former "life" does not require an article to modify it whereas the latter requires an article as we are modifying the noun.