AN INTRODUCTION TO ENTOMOLOGY 



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

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

 many other insects. 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 

 vast assemblage of animals in- 

 cludes forms differing widely in 



Fig. 3. — A mite, an arth- 

 ropod in which the 

 segmentation of the 



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

 southern cattle-tick, ^^ ^-^^ possession of the essential 

 Boopmiiis annulatiis. ^ 



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 clasoes 

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



LIST OF THE CLASSES OF THE ARTHROPODA 

 I. THE MOST PRIMITIVE ARTHROPODS 



Class Onychophora, page 4 



THE AQUATIC SERIES 



Class Crustacea, page 6 

 Class Palffiostracha, page 8 



AN OFFSHOOT OF THE AQUATIC SERIES, SECONDARILY AERIAL 



Class Arachnida, page 9 



Fig. 4. -Larva 

 of a lly, Tip- 

 nla abdonii- 

 nali s ; an 

 arthropod in 

 which the de- 

 velopment of 

 the legs is re- 

 tarded. 



II. 



III. 



