Water Relations in Terrestrial Environment
Insects are the largest group of metazoans which have most successfully invaded the terrestrial environment. In addition, most arachnids, myriapods and isopod crustaceans do not depend upon the aquatic environment for their survival. Terrestrial anthropoids owe their success to life on land to the existence of an impermeable cuticle that prevents evaporation of water from the body. Their cuticle, a chitin-protein complex along with a hydrophobic wax layer on the surface is the water-proofing structure. The cuticle is an important structure which had made possible to a large extent the successful colonisation of land. Water loss through the spiracular openings is minimized by keeping the spiracles closed whenever the inspiration does not occur. The proximal region of Malpighian tubules and more importantly, the rectum play an important role in water resorption expelling only dry faecal pellets with insoluble uric acid as nitrogenous waste. Pulmonate molluscs that have taken to terrestrial habitat have a calcareous shell that avoids desiccation of the soft inner parts of the body. Physiological adaptations such as aestivation assist them to overcome adverse climatic conditions.