Froghopper

Now is the time of cuckoo spit. 

It is caused by the nymphs of a froghopper.  The adults are brown and jump about, but the young nymphs are yellow or green and stay put.  Here's one on a piece of fabric.  I found it tickling on my elbow; must have brushed it off a plant as I passed.

You can usually make out the little black eyes.

They suck the juice out of the plant but as it is under pressure it is propelled through the nymph who blows a few anal bubbles into it to create the cuckoo spit - a sort of very wet fart.  This provides a hiding place; as a predator, you know it's in there somewhere but not exactly where.