The primary excretory system of an insect consists of a group of hollow tubes, the Malpighian tubules, which arise as evaginations at the anterior end of the hindgut (Figure 1, mt). These tubules vary in number from one to more than several hundred, and their distal, free ends are closed. These tubules function in removing nitrogenous wastes and in regulating, together with the hindgut, the balance of water and various salts in the hemolymph. Ions apparently are actively transported across the outer membrane of the tubule, generating an osmotic flow of water into the lumen. Along with this water a number of small solutes-amino acids, sugars, and nitrogenous wastes-enter the tubule passively. This primary urine is therefore an isosmotic solution containing the small molecules present in the hemolymph. Some of these solutes and the water may be actively resorbed into the hemolymph in the basal portions of the Malpighian tubules or in the hindgut. The principal nitrogenous waste is usually uric acid (Figure 2), a chemical that is relatively nontoxic (and can therefore be tolerated in higher concentrations) and insoluble in water (again, recall the water balance problems inherent to a small terrestrial organism).
Fig1. Internal organs of a grasshopper, shown in longitudinal section (somewhat diagrammatic). ans, anus; ao, dorsal aorta; be, bursa copulatrix; ca, corpus allatum; cec, circumesophageal connective; cg, cerebral ganglion (part of the brain); em, gastric caeca; en, colon; cp, crop; eg, eggs; eso, esophagus; gn, ganglia of ventral nerve cord; hr, heart; hyp, hypopharynx; il, ileum; lbm, labium; lbr, labrum; mg, midgut or mesenteron; mt, Malpighian tubules; nc, ventral nerve cord; og, optic ganglion (part of the brain); ovd, oviduct; ovp, ovipositor; ovt, ovarian tubules; phx, pharynx; rec, rectum; segn, subesopha geal ganglion; slg, salivary gland; sld, salivary duct; spth, spermatheca; vag, vagina. (Redrawn from Robert Matheson: Entomology for Introductory Courses, Second Edition. Comstock Publishing Company, Inc.)
Fig2. Structure of uric acid.
In some insects, most notably many beetle and moth larvae, the Malpighian tubules are bound very closely to the hindgut; these are called cryptonephridia. In species such as the mealworm, Tenebrio molitm· L., that live in conditions of high drought stress, this arrangement of the tubules is apparently in volved in extracting water from the fecal pellets. The mealworm, in fact, can extract water vapor from the air when the relative humidity exceeds 90%.
In addition to the Malpighian tubules, various in sects have a range of methods of removing wastes or toxic substances from the hemolymph. One method is to store chemicals, such as uric acid, more or less permanently within individual cells or tissues. This process is known as storage excretion. Cockroaches store uric acid in their fat body, and the white pigment in the scales of pierid butterflies derives from uric acid stored within them. At the anterior end of the dorsal blood vessel may be a group of cells, the pericardia! cells, that are important in absorbing and breaking down colloidal particles in the hemolymph. In other cases, similar cells may be widely distributed throughout the hemocoel.
