PART I. 



GENERAL OBSERVATIONS ON THE ANATOMY AND PHYSIOLOGY 

 OP THE FLY, 



Section I. Introductory Remarks. 



rPHE typo, or plan, upon which insects are organised is so 



L very different from the type of the vertebrata, that if we 



wish to understand the Anatomy of the Fly, it is necessary 



to bear in mind the chief characteristics of the class to which 



DESCRIPTION OF PLATE I. FBONTISPIECE. 



This plate represents a dorsal view of the whole of the internal organs 

 as they appear in situ, when the integument of the back is removed, 

 except on the right side of the thorax, where the great dorsal muscles 

 have also been removed to show the deep layers. Commencing at the 

 head, the following organs are displayed : the frontal sac occupies the 

 frontal region and is tinted yellow : on each side of this sac are the 

 antennal nerves, and the sides of the head are occupied by the great eyes, 

 which are seen in section : between these the cephalic nerve centre with 

 its main tracheal trunks occupies the greater part of the head ; it is cut 

 through horizontally near the eyes to exhibit the arrangement of the deep 

 tracheae. The left side of the thorax is occupied by a mass of muscles, 

 longitudinal near the centre, and vertical at the side ; the central muscles 

 are more especially used to depress the wings in flight, and the lateral are 

 partly elevators of the wings and partly flexors and extensors of the legs 

 being inserted into their basal joints. The tracheal vessel, which occupies 

 the posterior portion of this side is connected with the posterior thoracic 

 spiracle. On the right side, the chyle stomach and convoluted salivary 

 gland occupy the portion nearest the mesial line. Immediately external 

 to the latter is the main lateral trachea collapsed, except its posterior 

 portion which remains filled with air ; it communicates with the posterior 

 thoracic spiracle. The nerve to the halter is seen crossing this portion of 

 the tracheal trunk. The anterior tracheal vessel exhibited on this side is 



