CHARACTERISTICS OF INSECTS AND THEIR RELATIVES 15 



Like many of the parasitic worms, these animals, in some cases at 

 least, pass their larval life in one host, and complete their development 

 in another of a different species ; some larvae being found in the bodie , 

 of herbivorous animals and the adults in predacious animals that feed 

 on these herbivorous hosts. 



The systematic position of the pentastomids is very uncertain. 

 They have been considered by some writers to be allied to the mites. 

 But it seems better to merely place them in this appendix to the 

 arthropod series until more is known of their relationships. 



Class DIPLOPODA 

 The Millipedes or Diplopods 



The members of this class are air-breathing arthropods in which the 

 head is distinct, and the remaining segments of the body form a continuous 

 region. The greater number of the body-segments are so grouped that 

 each apparent segment bears two pairs of legs. The antenncB are short 

 and very similar to the legs. The openings of the reproductive organs are 

 paired, and situated behind the second pair of legs. 



Pig. 1 8. — A millipede, Spirobolus marginatus. 



The Diplopoda and the three following classes were formerly 

 grouped together as a single class, the Myridpoda. But this grouping 

 has been abandoned, because it has been found that the Chilopoda are 

 more closely allied to the insects than they are to the Diplopoda; and 

 the Pauropoda and Symphyla are both very distinct from the Diplo- 

 poda on the one hand and the Chilopoda on the other. Owing to the 

 very general and long continued use of the term Myriapoda, the 

 student who wishes to look up the literature on these four classes 

 should consult the references under this older name. 



The most distinctive feature of the millipedes is that which sug- 

 gested the name Diplopoda for the class, the fact that throughout the 

 greater part of the length of the body there appears to be two pairs of 

 legs borne by each segment (Fig. i8). 



This apparent doubling of the appendages is due to a grouping of 

 the segments in pairs and either a consolidation of the two terga of 



