Excretory System

The Malpighian tubules are the principal excretory organs of the cockroach. They are thin, yellow-coloured, thread-like structures of ectodermal origin that hang freely within the haemocoel, attached at one end to the alimentary canal at the junction between the mid-gut and hind-gut. A cockroach has roughly 150 of these tubules.

Each Malpighian tubule is lined internally by a single layer of glandular epithelial cells bearing microvilli (tiny finger-like projections that increase surface area). Functionally, the tubule is divided along its length: its distal portion (farther from the gut) is secretory, actively drawing water and nitrogenous wastes out of the surrounding haemolymph, while its proximal portion (nearer the gut) is absorptive, reclaiming useful water and salts before the waste is released. The nitrogenous waste that remains is converted into uric acid and passed into the ileum for elimination along with the undigested food. Because uric acid is the chief nitrogenous waste product, the cockroach is described as uricotelic.

Besides the Malpighian tubules, several accessory structures also contribute to excretion:

• Fat bodies, which, in addition to their storage role, can also accumulate some waste material.
• Nephrocytes (urate cells), cells found associated with the fat bodies and along the heart, which take up and store nitrogenous waste from the haemolymph; this stored waste is later released back into the haemocoel to be cleared through the normal excretory route. …