AN INTRODUCTION TO ENTOMOLOGY 



many other insects. 



obscured, as in spiders and in mites (Fig. 3) ; or the jointed append- 

 ages may be absent, as in the larvae of flies (Fig. 4), of bees, and of 

 In all of these cases, however, a careful study 

 of the structure of the animal, or 

 of its complete life-history, or of 

 other animals that are evidently 

 closely allied to it removes any 

 doubt regarding its being an 

 arthropod. 



The phylum Arthropoda is 



the largest of the phyla of the 



animal kingdom, including many 



more known species than all the 



other phyla taken together. This 



Fig. 3.— A mite, an arth- vast assemblage of animals in- 



ropod in which the ^^^^^g f^^^g differing widely in 



segmentation or the " -^ 



body is obscured. The structure, all agreemg, however, 



in the possession of the essential 

 characteristics of the Arthropoda. 

 Several distinct types of arthropods are recognized; 

 and those of each type are grouped together as a class. 

 The number of distinct classes that should be recog- 

 nized, and the relation of these classes to each other are 

 matters regarding which there are still differences of 

 opinion ; we must have much more knowledge than we 

 now possess before we can speak with any degree of 

 certainty regarding them. 



Each of the classes enumerated below is regarded by 

 all as a distinct group of animals ; but in some cases there 

 may be a question whether the group should be given 

 the rank of a distinct class or not. The order in which the classes 

 are discussed in this chapter is indicated in the following list. 



southern cattle-tick, 

 Boophilus annulatus. 



LIST OF THE CLASSES OF THE ARTHROPODA 

 THE MOST PRIMITIVE ARTHROPODS 



Class Onychophora, page 4 



THE AQUATIC SERIES 



Class Crustacea, page 6 

 Class Palaeostracha, page 8 



AN OFFSHOOT OF THE AQUATIC SERIES, SECONDARILY AERIAL 



Class Arachnida, page 9 



