Living With Uric Acid

Wandering stage of the tobacco hornworm caterpillar

In the larva stage of the tobacco hornworm, nitrogen waste is removed from the hemolymph and excreted by the digestive system. However, waste processing must drastically change during metamorphosis.

A fully developed tobacco hornworm will drop from a plant to the ground and begin a wandering phase prior to pupation. At the beginning of the wandering phase, a larva will empty its gut contents and release substances inside the gut that kill the gut bacteria.  A wandering larvae digs a burrow in the soil and pupates, protected underground. Once inside the burrow the larva and pupa will excrete no waste, an adaptation that limits growth of potential pathogens.

During pupation and adult formation, the cells of a larva die and release their contents. Imaginal disk cells grow and divide to produce the adult. Metamorphosis requires extensive metabolism that generates substantial nitrogen waste. Starting with the wandering stage larva, nitrogen waste is no longer excreted but is stored in vacuoles within fat body cells. After the adult structures form, the the uric acid is transferred from fat body to the digestive system where it becomes part of the meconium, the waste fluid that will be excreted shortly after adult emergence.

The drastic change in nitrogen metabolism requires precise regulation of enzymes and cells structures. The process is controlled by hormones that synchronize the metamorphosis and the molt.