The author talks about windshields in the last paragraph to:
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Offer evidence that insect populations have declined - The mention of the decline in bug splatters on windshields serves as a metaphorical representation of the decreasing abundance of insects overall, highlighting the issue being discussed.
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Refer to an idea from paragraph 1 - The reference to windshields connects back to the author's earlier nostalgic anecdote about receiving a book related to bug splatters, reinforcing the significance of the declining insect populations in a more personal context.