516 The Animal Kingdom 



fever or scrub typhus in the Orient. Among human pests can be 

 counted the itch mite, Sarcoptes scabei, which Hves in the hair folHcles. 

 A related species causes mange in dogs, hogs, and other mammals. Many 

 mites attack plants and do much damage. The commonest of these is the 

 so-called red spider. 



One group of this order, the ticks, are small rounded forms that 

 are capable of engorging blood from their victims. Many species attack 

 man and his domesticated animals. The common dog tick, Dermacentor 

 variabilis or D. occidentalis, is very often seen ; these are vectors of Rocky 

 Mountain spotted fever. Other members of this genus such as D. 

 andersoni also transmit this disease. Texas cattle fever is transmitted 

 by the tick Margaropus annulatus. 



In the life history of these animals, the young pass through a six- 

 legged stage unique among the arachnids. In this stage, the chigger, 

 the larva of a trombiculid mite, burrows into the skin of man and causes 

 much local irritation. 



THE CLASS DIPLOPODA 



The members of this class are the familiar millipedes which have 

 many segmented, often cylindrical bodies. The body consists of the 

 head, short thorax, and many-segmented abdomen. Most of the seg- 

 ments of the body bear two pairs of legs. The head has paired antennae, 

 jaws, maxillae, and two groups of simple eyes. The genital openings 

 are near the anterior portion of the body, and the male possesses special 

 gonopods for transference of the sperm. Respiration is by tracheae, and 

 excretion is by Malpighian tubules. Some possess a series of scent 

 glands which secret an irritating substance ; a few large tropical forms 

 are able to eject this for a considerable distance. Although they have 

 many legs, they are in general slow-moving forms which curl into tight 

 balls when irritated. Most are inhabitants of the leaf mold and perhaps 

 play an important role in changing leaves and other organic debris into 

 soil. A few, however, become pests in greenhouses. 



The eggs are laid in the soil and hatch into a six-segmented larva 

 with three pairs of legs. By successive molts, new segments and legs 

 are added. 



THE CLASS CHILOPODA 



In contrast to the millipedes, the members of this class, the centi- 

 pedes, are rapidly moving forms. They have elongate, flattened, many- 



