ARTHROPODA 89 



The Centipedes have been already referred 

 to. These, with the Millipedes, form the group 

 Myriapoda. In outward form, at any rate, these 

 suggest an intermediate position between Peri- 

 patus and Insects. 



The true Insects have a definite head, sepa- 

 rated from the thorax, and a constriction between 

 the thorax and the abdomen ; this is why they 

 are called insects, " cut in two." The thorax 

 bears three pairs of legs, the mouth has typically 

 three pairs of appendages, which may be altered 

 and modified in many different ways, according 

 to the nature of the animal's way of feeding. 

 While the Crustacea are typically adapted for 

 breathing in water by means of their gills, the 

 Insects are adapted for breathing air. This they 

 do by means of their air-tubes or tracheae, the 

 inlets of which open on their sides. These are 

 divided into fine branches, which diffuse air 

 through the body of the Insect. Two interest- 

 ing points must be noticed about insects. The 

 first is that they were the first group in which 

 zoologists were able to study the nature of larval 

 forms, long before the microscope had revealed 

 the larval forms of marine animals. The changes 

 undergone by insects are known as metamor- 

 phosis, or change of form ; and are typically 

 represented by the life-history of a caterpillar, 

 which assumes during the winter a resting form 

 called a Chrysalis or Pupa, and finally emerges 

 as a Butterfly. Insects have sometimes been 

 classified according to the greater or less com- 

 pleteness of the metamorphosis they undergo, 

 which in some cases is comparatively slight. It 

 has been mentioned elsewhere that larval forms 

 usually exist where the young animal is placed 



