v] 



THE ALIMENTARY AND EXCRETORY SYSTEMS 



117 



the other three. There is also a tendency for each pad to become 

 constricted into two parts, mid-longitudinally, so that the rectum 

 may shew, apparently, in transverse section, twelve pads closely 

 pressed together. 



The Anal Region. The anus is closed exteriorly by three small 

 valves, representing the reduced parts of a twelfth segment, and 

 known as the laminae anales (p. 32). Internally, the anal end of 

 the hind-gut is furnished with six radiating dilatator muscles, inserted 

 between the posterior ends of the rectal pads. Besides this, when 

 a separate vestibule is present, the anus may be closed off by 

 strengthening of the circular muscle tunic into a sphincter. 



THE EXCRETORY SYSTEM. 

 1. The Malpighian tubules (fig. 52). 



These are the principal organs of excretion in all insects. In the 

 imago of the Dragonfly, as well as in 



all except the first few larval instars, |_^ rm, 



they are very numerous, ranging from 

 fifty to seventy in number. The newly 

 hatched larva has only three ; but, at 

 each instar, an increase in the number 

 takes place, so that they soon become 

 numerous. 



The structure of a Malpighian 

 tubule is very simple. Each is a 

 slender, very elongated tube, of about 

 0-05 mm. diameter. It lies freely in 

 the haemocoele, and is thus bathed 

 by the blood on all sides. The free 

 end is quite closed. A small trachea 

 accompanies each tube, and supplies 

 its cells with oxygen, by ramifications 



over its outer surface. A delicate peritoneal investment (p) forms the 

 outer sheath of the tube. Next to this fine cellular layer, there 

 is an exceedingly delicate basement membrane (bm), very difficult to 

 detect. Within this are arranged the excretory cells (ex), of somewhat 

 irregular polygonal form, bounding a narrow central lumen or canal. 

 The cells are large, with conspicuous nuclei (nu-^. In transverse 



A. 



Fig. 52. Malpighian tubule from 

 Ischnura aurora Br., imago 

 ( x 167). A. Longitudinal section. 

 B Transverse section, bm base- 

 ment membrane; en canal; ex 

 excretory cells; nu^ nucleus of 

 same; p peritoneal investment; 

 nu z nucleus of same. Original. 



