DISCOVERY 



189 



and its non-occurrence is due to mixing by wind. In 

 water with much vegetation the conditions are again 

 changed. The plants give off oxygen, and in absence 

 of mixing, the tension may rise as high as 40 per 

 cent, in the lower depths, a fact that may be of great 

 importance to animal life. 



Pupal Respiration 



With this short review of the general conditions, 

 let us take the simplest case, the respiration of a pupa in 

 air. Taking demand first. Professor Krogh has shown 

 that chrysalides of Tenebrio molitor (the larva of 

 which is the "meal worm") show first, reckoning 

 from the commencement of pupation, a rapidly de- 

 creasing o.xygen consumption, corresponding to the 

 breaking up of the larval tissues. The decrease takes 

 not much longer time at low than at high tempera- 

 tures. Then there is a resting period with a nearly 

 constant metabolism, and the length of this pause 

 depends on the temperature. It lasts about 50 hours 

 at 33° ; 70 hoiu-s at 27° ; 100 hours at 24° ; and 170 

 hours at 21°. Thereafter there is a rapid increase in 

 the rate of oxygen use, until metamorphosis takes 

 place. It is interesting to note that the total amount 

 of oxygen used in the process of pupation is the same 

 at all temperatures ; that the process is not more 

 economically executed at one temperature than at 

 another, and there is, therefore, no " best " tem- 

 perature, except from the point of view of speed. 



Now the oxygen is available for the use of the pupa 

 only by diffusion over its surface. Without going 

 very deeply into the laws of diffusion, let us recall the 

 essential points. Gases diffuse much more rapidly in 

 air than in water, and still more rapidly through air 

 than through animal tissues. Carbon dioxide diffuses 

 twenty times more quickly than oxj^gen, and oxygen 

 three times as quickly as nitrogen. The biological 

 significance of the rapidity with which carbon dioxide 

 diffuses is great. The speed with which it diffuses 

 from animal tissues, where it is constantly being pro- 

 duced, into air or into water, provides that there will 

 never be any accumulation in the tissues, that the 

 chief waste-product of respiration is quickly and easily 

 eliminated. Whether diffusion will take place from 

 any one medium to any other depends on the tensions 

 of the gases present. Thus in atmospheric air over 

 water, if no oxygen is passing either w-ay, we say that 

 there is a tension of 20 per cent, of an atmosphere in 

 the w-ater, and air and water are in equOibrium as 

 regards oxygen. But if there is less oxygen in the 

 water, so little, for instance, that it would be in 

 equilibrium with air containing only 15 per cent, of 

 oxygen, then we say that there is a tension of 15 per 

 cent, in the water, and, in contact with atmospheric 

 air, there will be a pressure-head of 5 per cent, to 



drive oxygen into the water. The higher this pres- 

 sure-head, the more quickly will oxygen diffuse into 

 the water. Accordingly the rate of diffusion of air into 

 the body of the pupa will be proportional to the 

 difference in pressure between the external supply 

 and the tension in the body tissues. There will be 

 a point at which the amount of oxygen entering will 

 just counterbalance the consumption, and so, at 

 constant temperature, with constant metabolism, we 

 should expect the tension in the tissues to be at a 

 constant level below the tension in the surrounding 

 air. A rise of temperature — which we saw means an 

 increase in demand for oxygen — with the same oxygen 

 pressure outside, would mean that the tension in the 

 tissues would have to be lower in order that the needs 

 of the animal should be supplied. And, in fact. Dr. 

 Gaarder found that to provide for the needs of the 

 Tenebrio pupa during the resting period, at 20*^, a 

 pressure-head of about 5 per cent, of oxygen is re- 

 quired. It follows that a reduction in the percentage 

 of oxygen in the air (whUe the temperature remains 

 the same) will have no effect on the metabolism so 

 long as this pressure-head of 5 per cent, is maintained. 

 As soon as it fails (and then there will be a zero oxygen 

 tension in the tissues), the oxygen diffusing in will 

 be insufficient for the needs of the pupa. At a higher 

 temperature — Dr. Gaarder took 32° — the o.xygen con- 

 sumption of the pupa in the resting period is a little 

 more than twice as great as at 20°, and the tension 

 in the tissues is 10-7 per cent., giving a pressure-head 

 of about 10 per cent. Thus the pupa wOl suffer in- 

 sufficiency of oxygen at tensions lower than 10 per 

 cent, at this temperature. If the supply of oxygen 

 is reduced below- these critical tensions, what happens ? 

 The consumption of oxygen falls correspondingly to 

 the fall in the external pressure, but, when the pupa 

 is restored to a plentiful supply, the consumption rate 

 rises at once, not only to the normal, but above the 

 normal, and remains for a time at this high level, 

 finally coming down to the normal rate. When in- 

 come is not equal to requirements, since the demand 

 is already at its lowest level and cannot be further 

 reduced, reserve measures must be used. The tissue 

 stores are burned in the fire of life without being built 

 up again. The works must be kept going ; capital 

 is used, and when a plentiful income is again possible, 

 the capital must be restored. The income must be 

 abnormally high for a time to balance the period of 

 depression. 



That is the simplest case, and it contains all that 

 is essential to the understanding of the general problem. 

 The main arguments apply equally to all forms, to 

 larvffi and pupae alike, in air or in water, and to the 

 great class of adult insects. Diffusion may be over 

 the whole surface, or through an elaborate system of 



