30 



DIPTERA OF MINNESOTA. 



Fig. 11. Internal view of posterior 

 thoracic spiracle of Blow Fly, show- 

 ing valve, and on the left the eleva- 

 tor and depressor muscles of halteres. 

 From Lowne. Much enlarged. 



high authority, are believed to function as a combined liver and 

 pancreas, and not as a kidney ; the rectal papillae, shown at rp, Lowne 

 ])elieves to have a renal function. 



The nervous system of a fly is well shown in Fig. 13. The nerve 

 cord, which lies in life in the floor of the body cavity, shows a large 



swelling, tJig, known as the thoracic 

 ganglion, from which various 

 branches go to the wing muscles, 

 halteres, legs, muscles of skin of ab- 

 domen, etc. This collection of nerve 

 cells, the thoracic ganglion, appears 

 to be the vital nerve center of the 

 organism, for if that is injured or 

 destroyed, all motion ceases, and 

 death results, whereas a decapitated 

 fly or other insect will walk about 

 "for hours with no apparent lessening of vitality, although many 

 nerves and nerve centers have been removed with the head. The 

 ventral nerve cord forms a collar about 

 the oesophagus near the mouth, and 

 above the oesophagus we have enlarge- 

 ments, ganglia, as shown in the drawing. 

 The nerves going to the antennae are 

 marked by an a ; equals the nerve to 

 the ocelli; g is the optic ganglion; n 

 the optic nerve, and c equals the retina 

 of the compound eye. 



As might be expected, the blood sys- 

 tem of a fly is very simple ; a dorsal ves- 

 sel, the so-called heart, lying along the 

 mid line just below the body wall, by its 

 pulsations, or rather by the pulsations of 

 a thin walled sac, the pepcardial sinus 

 which surrounds it, drives the blood for- 

 ward to the anterior end of the vessel, 

 whence it finds its way over the air sacs 

 and viscera of the thoracic and abdominal 

 cavity, being taken into the dorsal vessel again through valvular 

 openings in the walls of the latter. The blood obtains a part of 

 its oxygen probably from delicate tracheal vessels lying in the walls 

 of the pericardial sinus. 



Fig. 12. Alimentary canal of fly, 

 much enlarged. From Lowne's 

 "I'.low Flv." 



