CIRCULATION IN INSECTS. 225 



the frequency of the pulsations of the dorsal vessel 

 varies much, even in the same insect, at different 

 periods of its existence, and according as it is more 

 or less excited. Thus Mr. Newport found the 

 number of pulsations per minute in the larva of the 

 Sphinx ligustri, at an early stage of its existence, 

 to be 82 ; and that it gradually diminishes to 30, 



valve "by a reflecting inwards and upwards of the inner coat, or 

 coats of the artery," (by which he means the dorsal vessel) " and 

 the under one by a contraction or projection of the like parts of a 

 portion of the artery beneath, so as to come within the grasp of the 

 lower part of the valve above it." The whole vessel is made up of 

 three coats, the two innermost of which, the lining, or serous, and 

 the muscular, or principal portion of the vessel, constitute the re- 

 flected portions, or valves ; while the third, or outermost coat, which 

 is exceedingly thin and delicate, is continued over the vessel nearly in 

 a straight line, and does not appear at all to follow the reflections of 

 the other two. In the caterpillar, this portion of the vessel has eight 

 pairs of small suspensory muscles, (seen along the upper side of 

 Fig. 339,) which arise from the middle of the upper surface of each 

 valve, and are continued back to be attached over the middle of 

 the next valve: they seem to have considerable influence over the 

 contractions of the valves. The Aortal, or anterior portion of the 

 vessel, extends from the hinder part of the fourth segment to its ter- 

 mination and division into vessels, to be distributed to the head ; 

 which division takes place after it has passed the oesophagus, and 

 at a point immediately beneath the supra-oesophageal ganglion, or 

 brain of the insect. This portion of the vessel is much narrower 

 than the dorsal, has no distinct valves, or muscles ; nor do any ves- 

 sels enter it laterally ; but it is very delicate and transparent, and 

 gradually diminishes in size from its commencement to its anterior 

 termination. Its course, in the caterpillar, is immediately beneath 

 the integument, along the fourth and third segments, till it arrives 

 at the hinder parts of the second segment; when it gradually de- 

 scends upon the oesophagus, and immediately behind the cerebral 

 ganglion, gives off" a pair of exceedingly minute vessels. It then 

 passes beneath the ganglion ; and, in the front part of the head, is 

 divided into several branches, as noticed by Mr. Newport in the 

 anatomical description he has given of the nerves of this species of 

 Sphinx: (Phil. Trans. 1832, p. 385.) These branches are best ob- 



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