CIRCULATION OF BLOOD. 188 



B, nor was it of equal steadiness, for in this latter canal, any 

 reflux or interruption to its progress was comparatively rare. 



On first viewing the circulation of the blood in these canals, 

 I imagined I perceived an irregular pulsation, but am now 

 inclined to think no such motion exists naturally in the wing, 

 but that the proper flow of the blood is in a steady, uniform 

 stream. This apparent pulsation I think may be attributed 

 to momentary obstructions, which the large elongated globules 

 meet with in their progress through their comparatively small 

 channels, as in several instances I observed a sudden stoppage 

 of the circulation, and consequent accumulation of globules 

 within the range of the field of view, which was followed by 

 an equally rapid disengagement upon the blood resuming its 

 course ; slight struggling of the insect likewise produces 

 momentary interruptions very similar to pulsations. 



Within the whole of the excurrent canals, branches of the 

 trachea are seen that in the canal A is comparatively small 

 compared with the like vessel in the canal B. Upon mea- 

 suring these canals and the trachea contained within them, I 

 found the former at the point d, Figs ! and 2, to be ~, and 

 its trachea 2222 of an inch in diameter, while the diameter of 

 the latter was ~- Q , and of its trachea i. Upon examining the 

 wings of several other specimens of the insect, I found the 

 same disparity in the proportions of the trachea to the canals 

 to exist in the whole of them, and this also appears to be the 

 case on the corresponding canals of the upper wings, which 

 have hitherto been but slightly examined, the under wing 

 having been selected as preferable for examination on account 

 of its superior delicacy and transparency. 



The trachea in the canal A, throughout nearly the whole of 

 its course, runs in a straight line along the anterior part of the 

 cavity which contains it, as at Fig. 2, A, and lessens gradually 

 in diameter, until at the termination of that canal it can no 

 longer be traced in consequence of its extreme tenuity. Near 

 its proximal extremity it gives off a small branch to the 

 transverse canal n, which terminates in a fine point at its 

 junction with the canal /; excepting in this instance, I could 

 not, after a careful examination, detect any other branch given 

 off from it to the transverse canals which connect it with the 

 canal /, although from its position in the canal A we might 

 naturally be led to suspect that was the case ; for, although 

 these small transverse canals, throughout nearly their whole 



