Peculiarities of the brain organization and fine structure in small insects related to miniaturization. 2. The smallest Hymenoptera (Mymaridae, Trichogrammatidae)

The organization and fine structure of the brain in the smallest parasitic wasps, Anaphes flavipes and Trichogramma evanescens, were studied using serial histological sections as well as TEM and computer-assisted 3D reconstructions. The data on the number and size of neurons in the brain of Mymaridae and Trichogrammatidae were obtained. They confirm and supplement the hypothesis about the factors limiting miniaturization of insects. The organization of the brain of the smallest parasitic wasps is compared with that of Hemiptarsenus sp. (Eulophidae), a large representative of Chalcidoidea. Some brain areas reveal strong allometry. The relative volume of the antennal lobes, lateral protocerebrum, and protocerebral bridge increases significantly as the body size decreases. Miniaturization is accompanied by an increase in the relative brain volume and changes in the spatial orientation of some brain structures. The number and size of neurons of A. flavipes and T. evanescens are significantly different from those of large representatives of Chalcidoidea, but similar to those of other tiny insects. Miniaturization of the nervous system is limited by the size of neurons and the diameter of axons.