

TEXT-BOOK OF ENTOMOLOGY 



PAKT I. -MORPHOLOGY AND PHYSIOLOGY 



POSITION OF INSECTS IN THE ANIMAL 



KINGDOM 



ALTHOUGH the insects form but a single class of the animal king- 

 dom, they are yet so numerous in orders, families, genera, and 

 species, their habits and transformations are so full of instruction 

 to the biologist, and they affect human interests in such a variety of 

 ways, that they have always attracted more attention from students 

 than any other class of animals, the number of entomologists greatly 

 surpassing that of ornithologists, ichthyologists, or the special stu- 

 dents of any other class, while the literature has assumed immense 

 proportions. 



Insects form about four-fifths of the animal kingdom. There are 

 about 250,000 species already named and contained in our museums, 

 while the number of living and fossil species in all is estimated to 

 amount to between one and two millions. 



In their structure insects are perhaps more complicated than any 

 other animals. This is partly due to the serial arrangement of the 

 segments and the consequent segmental repetition of organs, espe- 

 cially of the external appendages, and of the muscles, the tracheae, 

 and the nerves. The brain is nearly or quite as complicated as that 

 of the higher vertebrates, while the sense-organs, especially those of 

 touch, sight, and smell are, as a rule, far more numerous and only 

 less complex than those of vertebrates. Moreover, in their psychical 

 development, certain insects are equal, or even superior, to any other 

 animals, except birds and mammals. 



The animal kingdom is primarily divided into two grand divisions, 

 the one-celled (Protozoa) and many-celled animals (Metazod). In the 

 latter group the cells and tissues forming the body are arranged in 

 three fundamental cell-layers ; viz. the ectoderm or outer layer, the 

 mesoderm, and endoderm. The series of branches, or phyla, com- 



B 1 



