Assigning a gender shows you have a certain sympathy for the fly, or that you're interested in what the fly is doing and how it is feeling. We use gendered pronouns for animals to show affection, concern or interest. The fly's unfortunate situation seems more real to us if we use 'he'. Why 'he'? Well, English pronouns are based purely on biological gender, rather than grammatical gender. If we say 'he', we're presuming that it's a male fly rather than a female one. Not that it matters, really. I don't think many people can tell the sex of a fly just by looking at it. But even in these days of supposed gender equality, living creatures are assumed to be male unless proved otherwise. If an animal isn't showing any particular feminine qualities - such as laying eggs or looking fluffy and cute - we use the default masculine pronoun.

There's not a hard rule that you need to use a gender. You can use it if you like.

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