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The Mymaridae are the smallest insects. This video explains their numerous adaptations to being as small as 140 microns yet still complex, such as smaller cells with as little cytoplasm as possible, denucleated neurons, loss of several body parts (including eyes and hearts, depending on the species), and parasitism as an alternative to insects' usual level of nutrition in eggs. My question is why natural selection would favour their being so small in the first place.

Since few extant species have been extensively observed, this question may require some "just so story" speculation, hopefully informed by other examples of organisms becoming unusually, almost prohibitively small for their taxa. On the other hand, their fossil record covers about 100 million years, so perhaps the time and place of shrinking intermediates would suggest specific explanations.

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Fairy flies can be so small because they mooch off other insects' eggs, this is one of the reasons why they need to be small, then after that, they don't need to eat as they will die in only a few days. Information can be found on the "how do they manage to be so small" section of this PDF: https://www.cell.com/current-biology/pdf/S0960-9822(18)31343-5.pdf

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    $\begingroup$ This poses the chicken-egg problem (no pun intended) of whether they became small due to such parasitism or need such parasitism to be small. It wouldn't surprise me if the parasitism is older, and the host eggs shrank c. 100 Mya for some reason, as is likely if the host species also shrank or adapted to reduced food supplies. But do we know any of that? $\endgroup$
    – J.G.
    Apr 2 at 11:58

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