DORSAL VESSEL OF THE IMAGO OF THE BLOW -FLY. 639 



2. THE DORSAL VESSEL OF THE IMAGO OF THE 



BLOW-FLY. 



The dorsal vessel commences near the posterior border of 

 the fifth abdominal segment, arches forward immediately 

 beneath the integument, and terminates in the aorta at the 

 inferior margin of the mesophragma. 



It can be most readily seen in situ by making a vertical 

 longitudinal section, a little on one side of the median line, 



FIG. 86. A section through the dorsal part of the abdomen of a Blow-fly, showing 

 the dorsal vessel, a, aorta ; c/i, chyle stomach ; z', intestine ; m, mesophragma ; 

 /, pericardial septum ; sc, scutellum ; z/, v", v'", 2nd, 3rd, and 4th ventricular 

 enlargements of the dorsal vessel. The longitudinal fan-like alar muscle is seen 

 covering the two anterior enlargements. 



through the body of a Blow-fly which has been previously 

 hardened in a i per cent, solution of potassium bichromate, 

 and afterwards in strong alcohol (Fig. 86). 



The anterior part of the dorsal vessel is then seen between 

 the intestine and the abdominal wall ; the posterior part is 

 more difficult to trace, and can only be studied in serial 

 sections and when the whole pericardium and dorsal vessel have 

 been removed from the body of an insect. 



This can be done by taking away all the structures which 

 lie beneath it, so as to leave the pericardium on the inner 

 surface of the dorsal wall of the abdomen. The posterior 



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