MUSCLES AND VISCERA OF THE IMAGO. 217 



rhythmic regularity. The blood from the head is forced into the 

 frontal sac, and back again into the head capsule. These 

 movements result from the action of a pair of large fan- shaped 

 muscles, which arise from the lower edge of the occipital fora- 

 men. Their fibres diverge and are inserted into the gense and 

 sides of the face ; some of these fibres are also inserted into the 

 frontal sac (retractors of the sac), the inner surface of which 

 is covered by numerous muscle-fibres (compressors). Most 

 of these are afterwards absorbed, and in the adult imago 

 they are often entirely absent. The fan-shaped muscles are 

 very conspicuous for a week or more after the insect emerges, 

 but the retraction of the frontal sac takes place within an 

 hour of the escape from the pupa. 



The Cavity of the Thorax is chiefly occupied by muscles, but 

 there are large air-sacs in the scutum and scutellum, the alar 

 regions, and tympanic bullae. Beneath the great dorsales, in 

 the axis of the cavity, there is a narrow blood sinus, which con- 

 tains the thoracic viscera. The following parts are found from 

 above downwards in the middle line in the sinus ; the aortic 

 portion of the dorsal vessel, the chyle stomach and proventri- 

 culus, the oesophagus, and the thoracic nerve-centre. On 

 either side of the chyle stomach lies the convoluted salivary 

 gland (lingual gland), and externally to this a membranous 

 tracheal vessel, which passes from the head to the abdomen 

 the paragastric tracheal trunk (Fig. 37). 



The sinus is bounded below by the horizontal plates of the 

 entothoracic apophyses, which give origin to numerous 

 muscles. 



The anterior process of the scutellum contains a group of 

 chordotonal organs, and the tympanic bulla is filled by a large 

 air-sac connected with the production of the humming sound 

 heard in flight. The halteres also possess complex ganglia 

 and nerve-end organs, and a remarkable group of nerve- 

 end organs lies on the prosternum, the function of which is 

 unknown. 



The Abdominal Cavity contains two very large air-sacs, 

 the aerostats of Leon Dufour [19] , I term them the abdominal 



152 



