HEXAPODA 23 



single species is beyond computation: who can count the aphid s or the 

 scale-insects in a single orchard, or the bees in a single meadow? 



Not only are insects numerous when we regard individuals, but the 

 number of species is far greater than that of all other animals taken 

 together. The number of species in a single family is greater in several 

 cases than the number of stars visible in a clear night. 



The word insect is often applied incorrectly to any minute animal; 

 but the term should be restricted to those forms possessing six legs and 

 belonging to the class, Hexapoda. Thus spiders, which have eight legs, 

 are not insects. 



The name Hexapoda is from two Greek words: hex, six; and pous, 

 foot. It refers to the fact that the members of this order differ from 

 other arthropods in the possession of only six feet. 



Insects breathe by means of a system of air-tubes (tracheae) which 

 extends through the body. This is true even in the case of those that 

 live in water and are supplied with gill-like organs (the tracheal gills; 

 see p. 39). The head is distinct from the thorax, and bears a single pair 

 of antennas; in these respects they are allied to the millipedes and centi- 

 pedes although they are apparently more closely related to a small group 

 of animals known as symphylids of the class, Sym phyla (see "An Introduc- 

 tion to Entomology" by J. H. Comstock). 



They can be easily distinguished by the number of their feet, and, 

 usually, also by the presence of wings. 



The Metamorphoses of Insects 



Nearly all insects in the course of their lives undergo remarkable 

 changes in form. Thus the butterfly, which delights us with its airy 

 flight, was at one time a caterpillar; the bee, which goes so busily from 

 flower to flower, lived first the life of a clumsy, footless grub; and the 

 graceful fly was developed from a maggot. 



In the following pages considerable attention will be given to descrip- 

 tions of the changes through which various insects pass. It is our wish 

 in this place merely to define certain terms which are used in describing 

 these changes. 



Development without metamorphosis. — In two of the orders of insects, 

 the Thysanura and Collembola, the young insect just hatched from the 

 egg is of the same form as the adult insect. These insects. merely grow 

 larger, without any more marked change in form than takes place in our 

 own bodies during our life. They are said, therefore, to develop without 

 metamorphosis. 



Incomplete or gradual metamorphosis. — There are many insects which 

 undergo a striking change of form during their life, although the young 

 greatly resembles the adult. Thus a young locust just out from the egg 

 can be easily recognized as a locust. It is of course much smaller than 

 the adult, and is not furnished with wings. Still the form of the body is 

 essentially the same as that of the adult (Fig. 42). (The hair-line above 

 the figure indicates the natural size of the insect.) After a time rudi- 

 mentary wings appear; and these increase in size from time to time till 

 the adult state is reached (Figs. 43 to 47). During this development 

 there is no point at which the insect passes into a quiescent state corre- 

 sponding to the chrysalis state of a butterfly. Those insects which, like 



