DORSAL VESSEL OF THE IMAGO OF THE BLOW-FLY. 641 



The Anterior Ventricle (PI. XLV., Fig. 86, v'} is pyriform, 

 about i'o mm. in length, with a pair of large obliquely-placed 

 ostia ; its broadest part is in front, where its diameter is about 

 "25 mm. Its posterior extremity is narrowed, and communi- 

 cates with the third chamber of the heart. 



The third chamber of the heart is fusiform and not more 

 than half the diameter of the anterior ventricle. It has a pair 

 of obliquely-placed ostia midway between its extremities. 



The two remaining chambers are very long and narrow, 

 not more than *o6 mm. in breadth, and are more intimately 

 connected with the pericardium than the anterior chambers. 



The Valves of the Ostia. Each lateral opening is protected 

 by a pair of semilunar valve flaps, which project forwards 

 into the cavity of the ventricle. These valve flaps are partly 

 membranous and partly muscular, and their free edges are 

 attached by fine fibres to both the dorsal and ventral median 

 line in front of the ostia. 



Owing to the small size of the parts in the Blow-fly, it is 

 exceedingly difficult to make out the precise disposition of 

 the valves, as sections are not favourable to their investigation. 

 So far as I can make out they seem to be very similar to those 

 described by Graber in the Cockchafer, and by Verloren in the 

 larva of Rhyncophora (see p. 638). 



Inter-ventricular Valves. No inter-ventricular valves exist 

 in the dorsal vessel of the imago of the Blow-fly, but there 

 are, apparently, a pair of valves opening forwards in the 

 aortic bulb, which serve to prevent the regurgitation of the 

 blood from the aorta. Neither Verloren nor Graber describe 

 any true inter-ventricular valves in the imago stage of the 

 insects which they examined. If such valves ever exist they 

 must be exceptional. Inter-ventricular valves have, appa- 

 rently, only been observed by recent investigators in the larval 

 stage. 



The Position of the Ostia has been the subject of much 

 difference of opinion. Verloren places them at the junction 

 of the several ventricular cavities, and Graber midway between 

 these points. My own observations on the Blow-fly agree 



