Miscellaneous, 75 



them presented rose-coloured, and others blue tracheae ; the viscera 

 and the muscles had preserved their natural colour. This was M. 

 Bassi's experiment, and I had verified his result ; but I wished to 

 follow out this experiment in a more complete manner. Many cater- 

 pillars have the blood of a slight greenish tint, or of a brownish green, 

 which appears in some respects unfavourable for showing the effects of 

 coloured substances introduced by the alimentary canal. So, although 

 I did not doubt in the least that in my caterpillars the blood was 

 charged with the colour of the indigo or madder, and that this tint 

 only appeared in the tracheae because there alone the liquid was im- 

 prisoned, I thought it better to choose for my further experiments 

 insects in which the blood is colourless and of which the white inte- 

 guments would render observation more easy. The larvae of the 

 cockchafer {Melolontha vulgaris) appeared to me to fulfill these con- 

 ditions pretty well. I placed several of these in some earth filled with 

 vegetable matter and mixed partly with madder and partly with in- 

 digo. At the end of several days, the blood of those which had eaten 

 madder had acquired a rosy tint, although this colour was but faint, 

 the digestive juices having acted less powerfully upon this substance 

 in these insects than in the larvae of Lepidoptera ; but the blood of 

 those which had eaten indigo had acquired a very marked blue tint, 

 this colour being perfectly distinct through the integuments of the 

 insects. It was evident immediately that the dorsal vessel was filled 

 with perfectly blue blood, and the nutritious fluid was distinctly visi- 

 ble in all the cavities of the body, coloured in a similar manner. 



When an insect has been subjected to an indigo diet only for a few 

 days, the blood becomes perceptibly blue ; this is particularly mani- 

 fest in those parts where it is present in considerable quantity, as in 

 the abdominal cavity and even in the dorsal vessel ; but in the space 

 between the membranes of the tracheae where it can only form a very 

 thin stratum, the tint is still very faint. By continuing the same diet, 

 the blood becomes more and more coloured, and then shows itself 

 throughout in the most distinct manner ; but still, neither the muscles 

 nor the viscera become tinged, these parts retaining their usual white- 

 ness. The tracheae constantly present the deepest tint at their base, 

 becoming gradually paler to the extremity ; this is readily explained 

 by the gradual diminution of the quantity of fluid interposed between 

 the trachean membranes. 



Thus the passage of the blood between the walls of the tracheae, 

 as well as the entire course of the circulation, as I had proved by 

 means of injections, made either through the dorsal vessel or through 

 one of the large cavities of the body into which the blood flows, be- 

 comes as evident as possible ; for it is the blood itself, in the living 

 insect, which, charged with colouring matter, renders the facts visible 

 to every one. 



It is clear that observations on these processes of alimentation will 

 soon leave not the least doubt on the course of the blood in insects ; 

 for perhaps no demonstration can render the facts more evident, than 

 that furnished by the experiments that I have just detailed. We see 

 clearly that nutritive matters pass into the blood contained in the 



