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Why do insects need immune defenses? They are so small and multiply so rapidly that it is understandable to think that, although infected individuals might die, the species could survive by sheer reproductive capacity. The fallacy of this reasoning is in the numbers. Under optimal conditions, bacteria double every 20 min or so, whereas even the rapidly maturing (and favorite model) insect, the fruit fly Drosophila melanogaster, requires close to 2 wk to reproduce. If insects were unable to fight off bacterial infections, they would never survive to reproductive maturity.
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Dushay et al. (1998) studied this question.
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