Alimentary Canal 



107 



state. Tlie month-parts, thougli of elaborate structure, are 

 never used in feeding, and tlie alimentary canal of the fly 

 is empty, except for a greenish fluid, which fills the 

 stomach of the pupa and newly emerged fly. In male 

 flies the abdomen is empty and collapsed, the under-side 

 being concave and applied to the upper, except in the 

 segments which contain the reproductive organs. The 

 pulsating dorsal vessel and the 

 tracheal system can be seen by 

 the microscope, but the sto- 

 mach, subcutaneous muscles, 

 and nerve-cord are hardly 

 visible. In males reared in 

 captivity the abdomen is com- 

 paratively plump for a day or 

 two. The beginning of the 

 stomach, which is enclosed in 

 the metathorax of the larva, 

 gets close to the head in the 

 pupa and fly, in consequence 

 of the shortening of the oeso- 

 phagus and prothorax. 



The rectum, which is unde- 

 veloped in the larva, is easil}^ 

 demonstrated in the fly. It 

 forms a short, wide chamber, 

 containing two oval papillae, which are largely supplied 

 with tracheae (fig. 77). These appear to correspond to 

 the ' rectal glands ' or ' folds ' found in many insects of 

 different orders. They vary greatly in number. Chiro- 

 nomus has two, most other Diptera four, Pulex, most 

 Hymenoptera, Neuroptera, and Orthoptera six, Lepido- 

 ptera 60-200, Coleoptera and Hemiptera none. They are 

 absent in larvae, with a few exceptions. In many cases 

 the rectal papillae are freely supplied with tracheae, 



Fig. 77. — Rectal papilla of fly, 

 with tracheae. 



