46 THE BOOK OF THE FLY 



There are two cephalic ganglions, which are regarded 

 as the brain ; these are situated in the tipper part of 

 the head close to the neck. There are also ganglions 

 in the thorax with connections extending into the 

 abdomen. 



The thorax is mainly occupied with the powerful 

 muscles which actuate the attached wings, the legs, and 

 the small appendages called halteres or balancers, which 

 are supposed to be obsolete hind wings. There are 

 three unequal segments in the thorax ; the pair of front 

 legs belong to the first segment, the wings and the pair 

 of middle legs are attached to the second larger segment, 

 whilst the third is connected with the hind legs and 

 the halteres. 



The breathing apparatus of the fly is distributed in 

 portions over the head, thorax, and abdomen ; it con- 

 sists of a number of internal air-sacks with membranous 

 ducts ramifying everywhere ; the largest air-sacks are 

 in the abdomen near the waist. There is a pair of 

 external spiracles to each segment of the body, and 

 these lead to the air-sacks. 



The lines on the wings of the house-fly called 

 ;nervures have already been alluded to in Chapter II. 

 These nervures are strengthening ribs to the trans- 

 parent tissues of the wings. The tissues are double 

 (top and bottom) enclosing the nervures, which are so 

 united to the connections called trachae of the air-sacks, 

 that the newly emerged fly helps to extend its limp and 

 crumpled wings by a process of inflating the nervures. 



The stomach is located partly in the thorax and 

 partly in the abdomen. A passage from the gullet 

 passes through the neck into the lower part of the 



