38 THE CAUSES WHICH PROPAGATE 



Hover-fly I ouce by accident decapitated over night, standing 

 immovably on the window-sill the ensuing morning, and occupy- 

 ing itself, while I was engaged in writing, between the hours of 

 nine and twelve, in cleaning its wings Avith its hind legs and its 

 three pairs of legs by rubbing them in a determined manner 

 together, raising its foi'elegs vainly in air as if searching for its 

 head to brush up. Whether such vitality in insects be an 

 instance of nervous reaction, continued respiration, or a pheno- 

 menon of life, is not so clear. All but vegetative processes are 

 held to cease in the animal with the suspension of circulation ; 

 and as regards circulation in insects I believe much remains to 

 be learnt. The dorsal vessel we notice pulsating along the back 

 of a caterj^illar was not held by Marcel de Serres to constitute 

 a true insect heart. He describes it as a closed vessel with no 

 visible opening, composed of two membranes, one internal and 

 muscular, the other external and cellular, and pervaded by a 

 close interlacement of tracheae or air-vessels. When opened, 

 its interior presents a trans2:»arent coagulable liquid, which dries 

 rapidly, and then exhibits the aspect of gum, of a colour seldom 

 deeply defined, but sometimes greenish, orange yellow, or sombre 

 brown. Its contractions vary singularly in different species. 

 Thirty-six per minute in the caterpillar of the large Emperor 

 Moth, eighty-two at least in grasshoppers,, and a hundred and 

 forty in one of the ground bees. 



Very various have been the subsequent surmises regarding 

 this vessel and its function, until Dr. Eowerbank in the old 

 Entomological Magazine, No. 3, p. 239, at length fairly estab- 

 lished a true circulation existing in the immature state of a 

 May-fly. " The blood, abounding in flattened oat-shaped par- 

 ticles, may be seen circulating in every part of the body, not in a 

 continuous stream, but at regular points, in accordance with the 

 pulsations of the great dorsal vessel. The latter, which is of 

 great comparative magnitude, extends nearly the whole length of 

 the body, and is furnished at regular intervals with double 



valves The structure of the upper valve appears to 



consist of a duplication inwards and upwards of the inner coat 

 of the artery ; that of the under of a contraction and 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 



