THE CIRCULATORY SYSTEM. 109 



generalized insects, there should be a chamber to each segment, but 

 the heart is variously shortened from both ends so that the chaml^ers 

 are always fewer than the segments. The posterior end of the heart 

 is closed, but its anterior end is produced into a long narrow tube 

 called the aorta (fig. 1, Ao) which extends through the thorax and 

 ojjens by a few simple branches into the cavity of the head. 



The heart of the bee (fig. 1, lit) consists of only four chambers 

 {Iht-Jfht) lying in the third, fourth, fifth, and sixth segments of the 

 abdomen. In the front of this part of the body it bends downward 

 and forms a large convoluted loop {i) of about 18 folds where it 

 passes through the abdominal constriction. All of this convoluted 

 part really belongs to the abdomen, since it lies in the propodeal part 

 of the apparent thorax, which is the true first abdominal segment. 

 The aorta {Ao) extends forward from here as a very fine tube making 

 a large arch between the muscles of the thorax and then enters the 

 back of the head. According to Pissarew (1898) the convolutions 

 of the anterior end of the heart are peculiar to the honey bee, being 

 absent in its nearest relatives such as Bomhus and Megachile. The 

 heart walls, as before stated, are muscular and produce a rhythmical 

 contraction of the tube whose pulsations follow each other from be- 

 hind forward. Thus the contained blood is driven out of the anterior 

 end of the aorta into the head, where it bathes the brain and the other 

 organs of this region, and then flows backward, percolating through 

 the spaces between the organs of the thorax. 



From the thorax it enters the cavity of the ventral sinus — not the 

 general abdominal cavity, at least in the bee — and is pumped back- 

 ward by the pulsations of the ventral diaphragm and dorsally over 

 the inner walls of the thorax and through definite channels about all 

 the viscera, finally collecting in the dorsal sinus where it again enters 

 the heart through the lateral ostia. The lips of the ostia are pro- 

 vided with small membranous lobes Avhich project inward and con- . 

 stitute valves that prevent the expulsion of the blood, A similar 

 valve is placed at tlie anterior end of each chamber of the heart to 

 prevent a possible backward flow. 



In the bee, both the dorsal and the ventral diaphragms are well 

 developed, the former (fig, 1, DDph) extending from the third 

 abdominal segment to the seventh, inclusive, while the latter (VDph) 

 extends from the abdominal constriction to the eighth segment. 

 The ventral diaphragm is much more muscular than the dorsal and 

 its pulsations, which are very strong, follow each ether from before 

 backward. They may easily be observed by removing the top of 

 the alidomen from an aspliyxiated bee. The ventral sinus is very 

 ample, inclosing tlie nerve cord of the abdomen, and I'eceives into 

 its anterior end the blood channels of the thorax, so that the latter 



