CIRCULATION OF THE BLOOD. 403 



may be thence concluded than that this warmth is produced only in 

 their social assemblage. Mere mechanical motion is, however, not 

 sufficient;, for this produces in summer a lower temperature ; the 

 single insect, on the contrary, produces no warmth, but is exposed 

 to the varieties of the external temperature, and dies when this sinks 

 below zero. Hence it merely remains possible to suppose that warmth 

 is developed by respiration. 



We have learnt from a preceding paragraph that respiration increases 

 upon motion, and especially on flight, and that consequently there must 

 be a greater quantity of oxygen absorbed by the body. But the 

 condensation which the oxygen necessarily undergoes upon intermix- 

 ture with the blood, as well as the whole process of combustion, must 

 evolve heat, and this heat upon expiration must pass from the body of 

 the insect to the surrounding medium. If, therefore, many breathing 

 insects are collected together in a small space, heat must be produced 

 even during their quiet slow respiration, which the thermometer evinces; 

 but if the swarm be put in motion, and if the bees flutter with their 

 wings, they breathe, consequently, more strongly and more intensely, 

 and, therefore, a greater quantity of earth is necessarily evolved. 

 Hence even every individual breathing insect would develope some 

 heat, which, however, from its rapid assimilation with the external 

 temperature, is not perceived. But in small spaces, and where many 

 individuals are inclosed together, this evolution of heat would certainly 

 be detected in other insects *. But the reason why the temperature 

 of the hive in summer is even less, or, at least, equal, upon the same 

 motion, to that of the external atmosphere, is to be explained by the 

 current of air produced by the motion by means of which fresh air is 

 introduced and the warmed air removed, as well as that each draught, 

 even upon the introduction of warm air, produces coolness. 



237. 



III. FUNCTION OF THE DORSAL VESSEL. CIRCULATION OF 



THE BLOOD f. 



THE most general physiological importance of the circulation of the 

 juices has been stated in the introduction to this chapter, and indicated 



* Compare Hausmann de Aniin. Ex. Respirat., pp. 68, &c. 



f- It is quite impossible that we should here repeat all the different opinions of earlier 

 anatomists and physiologists upon the function of the dorsal vessel : we hope it will suffice 

 to assure our readers that all the most important treatises upon this subject have been 

 resorted to, and their most useful facts inserted. 



D D 2 



