biology zoology blog benno meyer rochow bubbles, froths, and foams

Bubbles, Froths, and Foams

The spittlebug is but one example

For years I had been seeking an opportunity to test my idea that the foam spittlebug nymphs surround themselves with, serves these insects not just as a deterrent to predators or to avoid desiccation, but has an additional important function as well. And finally during a research stay at the Chinese Academy’s Institute of Zoology in Beijing I got my chance.

I teamed up with Dr Xu Chen and our project was in 2016 joined by Dr A. Fereres and M. Morente of Spain. Froths and foams (like those of the spittlebug) basically consist of air-filled bubbles that are held together by very slightly sticky aggregations or clusters of ‘membranes’ that contain protein and mucopolysaccharides and adhere to one another. Spittlebugs produce their exudates known as cuckoo spit (but actually resembling human spit) via their anal openings, but different animal species may produce their frothing secretions by other ways. Snails, bubble nests used by certain species of frogs and fish, and even foams of insects like those secreted anteriorly (e.g. Stauronematus compressicornis) may all appear equally chaotic superficially, but they are likely to possess their own unique compositions and properties.

As to the functions of such foams it has often been suggested that they serve as a protection against desiccation, as a physical defence against predators (parasitoids, birds and spiders in case of the spittlebug), as irritants, as an anti-feeding agent to suggest unpalatability of the frothing animal itself or the plant it is sitting on. However, one possible function had not been considered earlier and that is the possibility that the bubbly foam, at least that of the spittlebug, protects the insect against harmful radiation, especially the shorter blue and ultraviolet rays. This idea came to me when a doctoral student of mine by the name of Esi Keskinen found that eyes of spittlebug nymphs reacted sensitively to bright lights and the integument of these larval insects was thin and green. A small green larva would be far less conspicuous than the white blotches of ‘spit’ on a plant, so why surround itself with white froth? With their sucking apparatus firmly anchored in the xylem (a plant’s water-transmitting system) desiccation did not seem a big problem (tiny aphids also sit on plants and feed in bright sunlight although some psyllid larvae do secrete white waxy flakes known as ‘lerp’).

To answer whether the spittlebug’s foam protected the insect against radiation, we reared nymphs under different conditions of brightness with and without foams. The result was that all nymphs showed a considerable degree of photo-avoidance and that nymphs without covers reared under the brightest lights (but identical rearing conditions in terms of environmental temperature and humidity) exhibited the highest fatalities. It thus seemed highly likely that the foam had a function not simply as a physical barrier to deter predators, but also as an optical one to filter out, i.e. absorb short wavelengths like blue and ultraviolet, known to possess the potential to alter the genome and to negatively affect the function of cells and tissues. In order to show that the foam itself did function as a filter we measured its reflectivity qualities and confirmed that ultraviolet and blue transmittance was reduced.

It was satisfying to see an idea confirmed by experiment and we concluded that nymphs, depending on developmental stage and intensity of the external light, can shield themselves against harmful radiation. What needs to be done now is to investigate whether the exudates of other organisms also have photoprotective roles. And this goes in particular for foams containing or surrounding eggs and larvae like those that float on freshwater bodies or seawater where there is no protection from solar radiation. Who’s up to the job?

© Dr V.B. Meyer-Rochow and, 2019.
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