EXTERNAL CHARACTEES. 165 



b. The podical plates are a pair of small cuticular 



plates at the sides of the anus. (Fig. 39, p. 177.) 

 To see the podical plates, lift up the tenth tergmmvith the 

 forceps. 



c. The cerci are a pair of many -jointed styles, attached 



under the lateral edges of the tenth tergum and 

 projecting backwards. 



d. The sterna on the ventral surface resemble the terga 



on the dorsal surface. The first is rudimentary. 



In the male the ninth sternum bears a pair 

 of styles. 



In the female the seventh sternum is pro- 

 duced backwards into a large boat- shaped pro- 

 cess, which forms the lower wall of the large 

 genital pouch. The hinder sterna are modified 

 in relation with the sexual apparatus, and are 

 completely hidden by the seventh. 



B. The External Apertures. 



1. The mouth opens on the ventral surface of the head, 



behind the labrum and between the jaws. 



2. The anus is at the hinder end of the body, beneath the 



tenth abdominal tergum and between the inner 

 margins of the podical plates. 



3. The genital aperture is single, and is placed at the 



hinder end of the body, below the anus. It is 

 larger in the female than in the male, and will be 

 better seen at a later stage of the dissection. 



4. The stigmata, or spiracles, are the respiratory apertures, 



twenty in number. There are two large thoracic 

 ones on each side, one between the prothorax 

 and the mesochorax, and the other between the 

 mesothorax and the metathorax. A pair of 

 abdominal stigmata are situated between the 

 lateral margins of the tergum, and the sternum 

 at the anterior end of each of the first eight 

 abdominal segments. 



