268 1NSECTA. 



in the first place, that their blood is all arterial ; they can have no 

 occasion for veins, as they have no venous blood, the whole of the 

 circulating fluid being continually oxygenized as its principles be- 

 come deteriorated. The perfection of their muscular power, their 

 great strength and indomitable activity, are likewise intimately 

 related to the completeness of their respiration ; so that the vital 

 energies of the muscular system are developed to the utmost, en- 

 dowing them with that vigorous flight and strength of limb which 

 we have already seen them to possess. It must likewise become 

 apparent, that, as the blood is freely exposed to the influence of 

 oxygen in every portion of the insect to which the air-tubes reach, 

 one great necessity for the existence of a circulatory apparatus is 

 entirely done away with, and as we have observed before, all those 

 parts of the vascular system required in other animals for the pro- 

 pulsion of the vitiated blood through pulmonary or branchial organs, 

 are no longer requisite ; so that, by dispensing with the compli- 

 cated structures usually provided for this purpose, the body is 

 considerably lightened. The circulation of the nutritive fluids is 

 in fact limited to their free diffusion amongst all the internal 

 viscera, and is effected in the following manner: If we ex- 

 amine the back of a silkworm, or of any transparent larva, a long 

 pulsating tube is seen running beneath the skin of the back, from 

 one end of the body to the other ; its contractions may readily be 

 watched ; they are found to begin at the posterior extremity, and 

 are gradually continued forwards, so that the vessel presents a 

 continual undulatory movement, by which the fluid contained in 

 its interior is pushed from the tail towards the head. This dorsal 

 vessel, which may be so well observed in the thin-skinned larva, 

 exists likewise in the perfect insect, although from the opacity of 

 the integument, its movement is no longer apparent, except by the 

 vivisection of the animal. 



(310.) This dorsal vessel, or heart as we shall call it for the sake 

 of brevity, is organized in a very singular manner ; for, instead of 

 being a closed viscus, it communicates most freely, through several 

 wide lateral apertures, with the cavity of the abdomen, and from 

 thence derives the blood with which it is filled. The dorsal vessel 

 is widest in the abdominal region ; but is continued, nevertheless, 

 through the thorax into the head, where it terminates as a simple 

 or furcate tube, that is not closed, but open at the extremity. 



The structure of this remarkable heart has been fully investi- 

 gated by Strauss Durckhcim,* and is extremely curious ; it con- 



* Op. cit 



