Ogata, N.; Matsuda, T.

There are several explanations for the extinction of ancient giant insects. Here, we present a new hypothesis suggesting that the innate immune system limits the body sizes of insects. In this study, we co-cultured bacteria, fungi, and insect blood cells, and performed single-cell RNA sequencing analyses of insect blood cells to determine their division of labor. In the innate immune system, prohibited molecules are listed as signals of invasion. The increasing diversity of organisms makes this list extensive. The burden of managing such an extensive list leads to a division of labor among blood cells and reduces the effective number of blood cells. Our simulation indicates that a reduced number of effective blood cells cannot protect a giant body from invaders.