188 



DISCOVERY 



but actual sources of most of the scourges of mankind 

 have been discovered. 



We may say in conclusion that the principle of 

 the incarnation of ideas, of the realisation in the 

 world of substance of what had been vaguely fore- 

 shadowed in the world of mind, is a process which 

 has gone on in science as surely, if not perhaps so 

 conspicuously, as in art. The artist succeeds more 

 or less perfectly in incarnating his ideas of beauty in 

 stone, pigment, words, and sounds ; but it is sometimes 

 the privilege of the scientist to extract, as it were, the 

 concrete from the abstract, to isolate in material form 

 what was once only a notion, a suggestion, a forecast. 



The Respiration of 

 Insects 



By I. Leitch, D.Sc. 



To the ancient physiologist the problem of respiration 

 had not presented itself as a problem. To Aristotle 

 it was very simple. " For the fact is, some aquatic 

 animals (as fish) take in water and discharge it again, 

 for the same reason that leads air-breathing animals 

 to inhale air ; in other words, with the object of 

 cooling the blood." Leonardo, the greatest biologist 

 between the Greeks and modern times, had perhaps 

 a glimpse of the fact that the process is not so simple. 

 " The air which is inhaled by the lung continually 

 enters dry and cool, and leaves moist and hot. But 

 the arteries which join themselves in continuous 

 contact with the ramifications of the trachea distri- 

 buted through the lung are those which take up the 

 freshness of the air which enters such lung." But it 

 is only to the biologist of the last few years that the 

 immense complexity and exquisite refinement of the 

 process have become plain. 



The nature of the problems involved and the manner 

 of dealing with them are well illustrated by the latest 

 work on insect respiration from Professor August 

 Krogh's laboratory in Copenhagen. Let us take the 

 problem in its threefold aspect : (i) the nature of 

 the demand for oxygen in insects ; (2) the avail- 

 able supply of oxygen ; and (3) the means by which 

 supply is adapted to demand, or, as must happen 

 when the supply falls very low, demand adapted to 

 supply. 



The Demand for Oxygen 



With regard, then, to the demand for oxygen, there 

 are two factors of prime importance, that of activitv 

 and that of temperature. Any animal or part of 

 one, such as a gland or a muscle, will consume much 



more oxygen when active than when at rest. When 

 all voluntary muscular activity ceases, and other pro- 

 cesses are reduced to a minimum (digestion and ab- 

 sorption, for instance, are not taking place), there 

 will still remain a certain residual consumption of 

 oxygen which will just be enough to maintain a com- 

 pletely quiescent state of life. Now the amount of 

 oxygen required by any animal in this resting state 

 is of great importance. The fire of life is burning 

 low; it is "damped down " to the lowest practical 

 limit. If we can measure the oxygen consumption of 

 any animal in this resting condition, when its meta- 

 bolic processes — that is, the processes of using the 

 living tissues and their stores, and of building them 

 up again — are at their lowest level, and if we do so 

 under different conditions, say at two different tem- 

 peratures, then the difference in the amount of oxygen 

 used will measure the effect of the change of tem- 

 perature. Again, if we measure it with two different 

 oxygen supplies, then the difference in the amount 

 used (if there is any) will measure the effect of the 

 rise or fall of available oxygen. We can, in this way, 

 estimate the influence of summer and winter tem- 

 peratures, and the influence of a plentiful or a re- 

 stricted supply of oxygen. 



The Supply of Oxygen 



With regard to the supply of oxygen, in atmo- 

 spheric air the percentage is very constant. At sea- 

 level, that is to say, in the habitat of most insects, 

 we find 20-9 per cent, of oxygen, roughly a fifth of the 

 air ; there is just a trace of carbon dioxide, and the 

 remainder may be taken as nitrogen. Under earth, 

 where we find beetles and other insects, the amount 

 of oxygen will vary according to the vegetation and 

 the nature of the soil. At four inches down almost 

 normal percentages of oxygen have been found, but 

 in stagnant soil, or badly drained soil after rain, the 

 percentage of oxygen may be greatly reduced ; values 

 as low as 6 per cent, have been found. 



In water the amounts of oxygen, carbon dioxide, 

 and nitrogen present will be proportional to their 

 pressures in the atmosphere, and in nature they will 

 depend in the highest degree on the mixing to which 

 the water is subjected. It will absorb air at the 

 surface, but the air will spread downwards only very 

 slowly by diffusion. Now in summer the surface 

 water is warmest and therefore lightest, and, apart 

 from wind, no mixing will take place. In autumn 

 the surface water is coldest, the water will be well 

 mixed and will be practically saturated with air. At 

 temperatures below 4°, the point of greatest density 

 of water, there will again be no spontaneous mixing, 

 so that a winter minimum would also be expected, 



