Cuckoo Spit

So what exactly is cuckoo spit? I had a good idea, but I didn't know which insect secreted it. 

"Cuckoo spit is a white frothy liquid secreted by the nymphs of a sap-sucking insect known as a froghopper."

Each blob contains a creamy white insect nymph up to 4-6mm (¼in) long.