Megaphragma

The smallest insects evolve anucleate neurons. Arthropod Struct Dev. 41: 29-34.
The smallest insects are comparable in size to unicellular organisms. Thus, their size affects their structure not only at the organ level, but also at the cellular level. Here we report the first finding of animals with an almost entirely anucleate nervous system. Adults of the smallest flying insects of the parasitic wasp genus Megaphragma (Hymenoptera: Trichogrammatidae) have only 339–372 nuclei in the central nervous system, i.e., their ganglia, including the brain, consist almost exclusively of processes of neurons. In contrast, their pupae have ganglia more typical of other insects, with about 7400 nuclei in the central nervous system. During the final phases of pupal development, most neuronal cell bodies lyse. As adults, these insects have many fewer nucleated neurons, a small number of cell bodies in different stages of lysis, and about 7000 anucleate cells. Although most neurons lack nuclei, these insects exhibit many important behaviors, including flight and searching for hosts.

世界最小の昆虫といわれる寄生バチの一種（Megaphragma）では、成虫の大部分の神経細胞は無核なんだそうな。それはもちろん驚きなのだが、その前にこの小さな虫のことを知らなかった自分にとっては、この体の小ささに驚かされた。こんな小さいのあり得るんやなぁ。ちなみに、下の写真はゾウリムシとアメーバとの同倍率での比較。