Introduction

Invertebrates constitute 90% or more of all the living species in the Animal Kingdom. The class Insecta alone has more than one million recorded species. Invertebrate species can be found flourishing in virtually every ecological environment from the deepest of oceans in the vicinity of hydrothermal vents to the highest of mountains. This process of speciation during the course of evolution has not only generated a bewildering range of morphological form, size and shape but has also led to an exploitation of every conceivable physico-chemical principle in the physiological adaptation. The celebrated Danish zoophysiologist August Krogh once said that there is always some organism which has maximally expressed a particular biological phenomenon and therefore would be ideally suited for investigating that principle. Similarly, while promoting the use of insects in the study of general physiology, Wigglesworth (1948) has said: ‘Insects … are so varied in form, so rich in species, and adapted to such diverse conditions of life that they afford unrivalled opportunities for physiological study’. Furthermore, numerous invertebrate species can often be obtained in large number and maintained in the laboratory at a relatively low cost.

A perusal of any modern text-book of cell and developmental biology, comparative physiology etc. would show that over the past century most of the fundamental biological concepts were founded by investigating material from some invertebrate species. The diffusible ions Na, K, Cl, Mg, Ca, Zn and also H, OH are now implicated in virtually all the cell functions.