Malpighian tubules secrete primary urine, most of which is passed into the hindgut where water, ions and essential metabolites are reabsorbed before the fluid is excreted.
[1] These factors are produced in neurosecretory cells in the insect nervous system, and stored and released from neurohaemal sites, such as the corpora cardiaca in the brain.
It has been known for many years that insects possess diuretic and antidiuretic factors, but it has only been comparatively recently that technological advances have allowed for them to be characterised.
To date, the only insect for which both diuretic and antidiuretic hormones (acting directly on tubules) have been isolated is a beetle, the mealworm Tenebrio molitor (Tenebrionidae).
Originally known as the myokinins because of their myotropic activity, the kinins were first isolated from the Madeira cockroach, Leucophaea maderae and the cricket, Acheta domesticus.
Further studies examining the composition of the secreted fluid and electrophysiological experiments that explore ion movement, will shed more light on the actual physiological function of these factors in vivo.
For example, in vivo experiments have demonstrated that the fluid secreted by the Malpighian tubules of the desert beetle, Onymacris plana is directed to the midgut for recycling to the haemolymph.
In this way, metabolic wastes are rapidly cleared from the haemolymph without an associated loss of water, indicating that diuretic hormones may not always effect diuresis per se.
[3] The effects of diuretic factors are tested in fluid secretion experiments, usually conducted using the Ramsay assay, in which isolated Malpighian tubules are placed in droplets of saline solution under liquid paraffin.
Isolated tubules continue to secrete for many hours, and because the Ramsay assay is fairly easy to perform, many peptides are tested using this method.