Ruth Ballantyne, of Ardgowan School, asks :-
How does a spittle bug make its spit?
Warwick Don, a zoologist at the University of Otago, responded.
As you correctly observed Ruth, what appear to be blobs of spit on plants in our gardens are made not by people with the dirty habit of spitting but by certain insects appropriately called spittle bugs.
The foamy globules are produced by the nymphs or larvae which when newly hatched ramble actively over plants until a suitably succulent feeding site is found. There they insert their mouthparts and suck up the sugar-rich sap. A by-pass in the gut ensures that more sap is taken in than is digested and the excess flows from the anus and sticks to the plant and the larva itself. Soon enough sap gathers to cover and protect the larva entirely. The larva is able to breathe by means of a tube-like canal beneath its abdomen.
The remarkable process of bubble-making involves vigorous movements of the abdomen: the air canal is filled with air as the tip of the abdomen is thrust outside the fluid mass. The abdomen is then strongly contracted (shortened) within the blob of sap, forcing a bubble of air out the tip. Dipping or rolling motions of the abdomen together with contractions produce several bubbles before the air supply has to be renewed. Enough bubbles can be made to cover its whole body in 15-30 minutes. When wandering larvae come across other spittle masses they may enter and begin to add more bubbles of their own. In this way they establish an insect equivalent of a boarding house.