VI INSECT PHYSIOLOGY 



which restrict the size of insects. Their small size and terrestrial habit 

 render them very prone to lose water; and the urgent need for the 

 conservation of water influences the respiratory, excretory and di- 

 gestive systems. All these systems show special changes when the 

 insect reverts to an aquatic life. 



Yet, although the interaction of these factors results in some uni- 

 formity of principle in the physiology of insects, their diversity in 

 habit, food and environment causes such endless variation in detail, 

 that, quite apart from the conspicuous gaps in our present know- 

 ledge, any limited treatment of the subject must in any case be more 

 or less arbitrary in presentation; and, were there space to insert 

 them, nearly all the generalizations that are attempted should have 

 qualifying instances. 



The history of insect physiology is peculiar. The early micro- 

 scopists, Hooke, Malpighi, Leuwenhoek, made many observations 

 on the structure of insects, and many accurate inferences about their 

 physiology. More was added by the great naturalists, Swammerdam, 

 Reaumur, de Geer. But, with such conspicuous exceptions as 

 Newport, Graber, Lubbock, Plateau, and others of more modern 

 times, the majority of entomologists, until recent years, have been so 

 fully occupied with the morphology and taxonomy of their colossal 

 group that such advances in physiology as have been made have 

 commonly been mere by-products of morphological study. From 

 time to time we find the physiologists of the last century, Dutrochet, 

 Treviranus, Marshall Hall, von Kolliker, Claude Bernard, turning to 

 the insects to illuminate their theme; but their concern was not with 

 the insect as such. 



Within recent years, interest in the physiology of insects has arisen 

 in a new quarter. The applied entomologist, confronted with the 

 ravages of insects in the spheres of agriculture and of public health, 

 has wanted to know something about their nutrition, about the laws 

 governing their responses to sensory stimuli, about their reactions to 

 parasites, about the precise way in which their bodies are adapted to 

 diverse climatic conditions, and about the action upon them of 

 toxic sprays and gases. With this demand for increased knowledge 

 has come a realization of our present ignorance. 



Since the first edition of this book was published more than thirty 



