Little blobs of spindrift or spume spatter the countryside, snagged on everything from the base of grasses to the spines of gorse. Horseflies and even the stars have been blamed, and so – as its British common name of ‘cuckoo spit’ suggests – has the expectorated sputum of a once often-seen spring bird. It’s actually none of these things, as a little exploration with a moistened artist’s paintbrush will prove.


What is cuckoo spit?

Get up close and you’ll see that these lathery dollops comprise hundreds of tiny bubbles. If you slowly tease them away from the plant on which they are slung, you’ll uncover a rather lovely little yellow-green insect hidden away inside. Once you have uncovered it, a game of hide-and-seek often ensues, as the now vulnerable creature takes cover behind the stem or leaf.

This insect is the nymph of the common froghopper, Philaenus spumarius. The adults are a familiar feature of a summer meadow or hedge bank – just walk slowly through long vegetation and you can turn up dozens of the things. However, they’re equally shy in adult form, and they’ll quickly recover from any disorientation, pinging off with all the energy of a fully stretched elastic band.

But the juvenile stages don’t possess this superpower and instead have to rely on a unique fortress – one made of what biologists call ‘bio-foam’, which is known as cuckoo spit or Spittlebugs.

To make this, a nymph plunges its mouthparts intoa plant’s plumbing, and then fills up until it is turgid with the pressure of sap. After that, it makes one or two priming pumps of its abdomen, before starting to secrete the first of many clear liquid droplets from its anus. This carries on until it is, in effect, sitting in a puddle.

Just like making bubbles between your forefinger and thumb in a dish of soapy water, the nymph then opens up two finger-like projections at its tip. I imagine these to be like double doors, letting air in, before closing and plunging back into the liquid, where the air is allowed out to form the first of hundreds of bubbles.

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What is the purpose of cuckoo spit?

The purpose of this bio-foam is not only to hide the edible insect life within; it’s a physical barrier against small predators, as effective as any spines or bristles. And for birds, just having to sort through the froth is more effort than it’s worth. Other potential advantages for the nymph are avoiding dehydration and exposure to temperature fluctuations.

This bubbly shroud is pretty economical to make. These insects literally tap into the moist and verdant essence of spring itself: the sap that is pumping its way through the micro-plumbing of everything that is green and juicy. Our froghopper – being a member of the order of true bugs, or Hemiptera – simply jams its super-sharp drinking straw mouthparts into the plant’s tissue, and plumbs itself directly into the flow. Imagine if you’d perforated the water mains with a drinking straw!

The froghopper nymph is then able to take in such large quantities of this juice that it consumes over 1,000 times its own body weight in a day – and it barely has to suck or pump. All it has to do is remove enough proteins and sugars from the liquid to live and grow. The rest? Well, that pops out the other end as bubbles.

How do the spittlebugs breathe?

The protective cuckoo-spit nests whipped up by sap-sucking froghopper nymphs present an obvious problem to an air-breathing insect – it’s very wet in there. Even if they don’t drown, you’d think the accumulation of exhaled CO2 would be fatal. It was only recently discovered that the nymphs use their telescopic abdomens as snorkels, pushing the tip out of the nest to fill their spiracles (respiratory openings) with fresh air. This is not possible, though, during their final moult, when their abdomens lose their elasticity. Instead, they burst many of the tiny bubbles within the nest to create an air-filled chamber, which sees them through until their adult cuticle has hardened and they can leave the nest


Main image © Getty Images


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Nick BakerTV presenter and naturalist