Cuckoo spit (or is it from a witch?)

Blobs of frothy liquid on young stems and leaves can be seen as you walk around the Wetlands. You may wonder what is in these blobs and their name does not give any more detail. They are commonly known as “Cuckoo spits” but apart from appearing about the same time as cuckoos are heard, the froth has no connection with any bird. Certainly cuckoo-spit was an important ingredient in witches' brew (as in Macbeth) and in Scandinavian countries the froth is known as 'witches' spit'! Instead it is secreted by the immature stages of sap-sucking insects known as froghoppers. The adult insects live openly on the plant and do not produce any cuckoo spit.

The froghopper is an insect, and a member of the order Homoptera. They are called froghoppers because they are able to hop significant distances when disturbed. They feed on plant sap which they extract from the leaves and stems of plants. The life-cycle of the Froghopper is in three stages: egg, nymph, and adult. In this respect it is similar to such insects as grasshoppers and dragonflies. Up to 100 eggs are laid by the adult females in the late summer into an incision made into the tissues of the host plant. The eggs hatch into nymphs the following spring.

The nymph is the sexually immature stage. It resembles the adult in shape but unlike the adult has no wings and only rudimentary legs and eyes. Crucially, it has but a thin outer body layer, or exoskeleton, which unlike the adult's is not covered by a waxy layer. For this reason it needs to protect itself from desiccation. This is achieved by surrounding itself in a frothy mass of soapy bubbles known colloquially as cuckoo-spit. The froth also helps to protect the developing nymph from predators by disguising the nymph. If determined to seek out the insect, the predator will be dissuaded by the unpalatable taste of the froth. Enclosed within this froth, the nymph moults several times before emerging as an adult in early summer. The froth is created by the insect excreting a fluid, the result of excess undigested plant sap, exuding through the anus. This sap, as it is excreted, mixes with a secretion from the abdominal glands. Air bubbles are introduced through a special valve on the abdomen which acts like a bellows, and contact with the air causes the liquid to ferment, forming the froth.