Giant and dwarf axons in a miniature insect, Encarsia formosa, (Hymenoptera, Calcididae)

Miniaturization effects in the central nervous system (CNS) of a very small calchicid wasp, Encarsia formosa (0.6 mm long), are obvious for the overall morphology and at the level of axon sizes. Parasagittal sections show that most ganglia are fused and leave connectives only in the neck and the petiole. The thoracic complex is partly squeezed between muscles, enwraps cuticular apodemes and protrudes laterally into the coxae of legs. Somata of neurons are similar in size and form a multiple layer around large neuropile regions of the CNS. In TEM sections of connectives the range of axon diameters lies between 0.045 and 3.8 μm. Extremely small axon diameters below 0.1 μm are supposed to present spatial restrictions for ion channels and internal organelles. In theory, that can cause frequent spontaneous releases of action potentials (AP) which impede regular information transfer by normal APs. Therefore, axon sizes were studied in connectives between ganglia where longer distance information transfer requires action potentials even in the smallest axons. The diameters of many interganglionic axons below 0.08 μm contradict the theory. The luxury of large axon diameters exceeding 2–3 μm is reserved for several “giant” interneurons in the thoracic and in the abdominal ganglion complex. They should belong to rapid sensory alerting systems. The largest, a bilateral pair in the abdominal CNS, could integrate afferents from long wind sensitive hairs on the abdomen.

Graphical abstractCompares head and body/CNS size of Drosophila and Encarsia formosa.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The CNS takes relatively more space in miniature wasps than in larger insects. ► Thoracic ganglia bulge out into the coxae of legs. ► All neuron somata are equally small (1.5–3.0 μm across). ► Many axons with diameters below 0.08 μm between ganglia contradict theories of AP conduction. ► Large (“giant”) axons may serve rapid escape responses to wind-stimuli and to visual input.