INSECT VARIETY. 39 



writer inclines to the opinion that a much greater portion of the 

 circulation than we can clearly define is carried on within special 

 vessels, as the blood may be frequently seen flowing within 

 curved and other lines, as if confined within very narrow limits, 

 and in the caudal extremity the ascending and descending vessels 

 are seen, like vein and artery, to accompany each other, and at 

 the same moment that the fluid passes up the one with, the usual 

 pulsatory motion it descends the other. There is, however, no 

 perceptible pulsation of these minuter vessels themselves, and the 

 motion of their fluids therefore results from the action of the 

 great dorsal heart." 



Possibly our imperfect knowledge of the circulatory system 

 of insects may be due to a very natural objection to vivi- 

 section, which does not affect us in any degree in the case 

 of an oyster or whelk ; for, on lately dissecting an example of 

 the Privet Hawk Moth I had killed in the usual manner and 

 had assumed to be dead, a very different arrangement presented 

 itself from what the elder anatomists had previously led me to 

 conceive. I traced first the well-known dorsal tube (Plate VI., 

 Fig. 11, b), yet clinging by its length to the upper portion of the 

 abdomen, and marked throughout by a green tint, doubtless 

 derived from the fluid enclosed ; and I noted down its waning 

 valvular movements. I then gently removed the intestines, 

 when to my surprise there appeared another quite similar tube 

 (c), ventrally, and of an anther colour, whose upper fold moved 

 in similar fashion to the valves of the dorsal vessel. I traced 

 this forward to the junction of the thorax and abdomen, where 

 it could be plainly observed to unite with the upper vein in a 

 dilatation (a) , that constituted a distinct vessel of a flat-roundish 

 form. I next noted down a two-fold alternating pulsation in 

 this vessel, that indicated a circular flow of the fluid as shown by 

 the double-headed arrows; and which certainly impressed me 

 with the notion of a rudimentary heart composed of an auricle 

 and ventricle, such as exists in mollusca. 



The two main tubes had besides several efferents, a, a* and 



* Dr. Vitis Grabcr (" Die Insecten," 1877, T. 1, s. 328—345) also mentions the 

 ventral vessel which he has noticed in Dragon-flies and Grasshoppers, and he 

 is of opinion it was first observed by Reaumur in the Saw-fly of the rose. It 

 should be regarded in the light of an artery to a dorsal vein. 



