M. jocaste was observed laying eggs in a water-containing tree-hole in a forest of Bolivia. The insect hovered above the hole and making jerky movements with the curved abdomen, threw individual eggs at the water surface in the direction of the shaded area underneath the roof of the hole. The abdomen did not touch the water and the eggs, after drifting horizontally for a short distance, remained floating. The increased efficiency brought about by the possibility of throwing eggs into tree-holes from the outside was probably the most important selective factor involved in the appearance of this unique type of oviposition. A new interpretation for the long abdomen of pseudostigmatids is proposed based on its mechanical advantages for throwing eggs.