The bubbles produced by spittle bugs and other organisms are spheres that are packed optimally because bubble geometry eliminates air between their surfaces.

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"Bubbles are commonly encountered in nature, across many phyla and habitats—from the slimy protective bubbles of the spittle bugs to the bubble eggs of many water-loving vertebrates, including many species of fish and amphibians, particularly frogs. Bubbles serve as insulation, moisturizer, and protection from predators. Spittle bugs use recycled sap from grass stems converted to a soapy liquid that they pump into foamy bubbles using their tails…Frog eggs and spittle bug bubbles self-cushion when packed in an array and naturally leave little air amongst them. This is because nature’s bubbles join according to three principles of soap bubble geometry. First, a compound bubble consists of flat or smoothly curved surfaces joined together. Second, the surfaces meet in only two ways: either three surfaces merge along a curve (edge), or six surfaces at a vertex. Third, when surfaces come together at a curve, or curves and surfaces at a point, they do so at equal angles. These three principles allow bubbles to eliminate air space between their flexible membranes, thereby optimally packing spheres." (Biomimicry Guild unpublished report) Edit References

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