Respiration and Metabolism 223 



presumably after the period of exercise (Fig. 43). In Drosophila larvae the 

 entire tracheal system is permeable to respiratory gases and water, and various 

 mechanical stimuli may cause Huid uptake by the tubes.'"^ In Sciara larvae, 

 however, the tracheoles, even in the terminal regions, never normally contain 

 fluid after their initial filling with gas.-"' The filling process can begin at a 

 point within a main tracheal trunk, can occur in the near absence of oxygen, 

 but is inhibited by totally anoxic carbon dioxide, carbon monoxide, and nitro- 

 gen, and by low temperature (0° C). The filling seems to involve an internal 

 source of gas produced metabolically.-'*^ 



Special tracheal systems permit aquatic respiration in certain insects (see 

 page 21 5). Many aquatic insects possess hydrofuge faculties which permit them 

 to make contact with air through specially modified structures of the body. 

 The water-air surface film can thus be penetrated, and unwettable spiracles 

 exposed to the atmosphere, to facilitate gas exchange. Hydrofuge hairs enable 

 some insects {Dytisciis, Notonecta) to trap considerable quantities of air, which 

 is then carried below the surface and used as an oxygen store.'^''-'' The very 

 great efficiency of such surface-borne air in the respiration economy of diving 

 insects (e.g., Amphelochcinis, Hemiptera) has recently been pointed up in 

 regard to the "plastron" mechanism. •^^•* The plastron ("Lufthulle") is essen- 

 tially an epicuticular hair-mat with its trapped air serving as an oxygen supply 

 while the insect is submerged. Not only is oxygen which was originally 

 obtained at the surface given up by the air, but, as a result of the invasion 

 coefficients between the gases and water, nitrogen leaves the plastron slowly, 

 while additional oxygen diffuses in from the surrounding water. Such a system 

 was noted by Ege'"'' as bestowing on the plastron air an oxygen supply about 

 thirteen times as great as that actually afforded by its original volume. 



Tracheae may be regarded as the arthropod contribution to respiratory regu- 

 lation—an efficient mechanism, but restricted by the limits such a system 

 imposes on the size attainable by the organism. No other group of animals 

 has met the respiratory problem in quite this way. 



Summary. We see then a number of respiratory' adaptations which have 

 developed to facilitate gas exchange. Diffusion per se through the body surface 

 is sufficient for the smaller organisms and accounts for gas exchange across 

 special respiratory surfaces. The structural modifications in respiratory mech- 

 anisms essentially have been along the lines of increasing surface area, improv- 

 ing ventilation, and bringing the respiratory gases into more immediate contact 

 with the internal transporting system. Gills are characteristic of aquatic forms; 

 lungs and tracheae have made possible the complex development of land 

 dwellers. 



THE LEVELS OF OXYGEN CONSUMPTION 



The metabolic economy of animals shows a wide variation in adjustment 

 to oxygen availability. Most organisms ha\'c plenty of oxygen at their disposal, 

 but some are adjusted to oxygen deficiencies and others to what constitutes 

 essentially an oxygen-free environment. I he rate of oxygen consumption 

 reflects the metabolic activity of aerobic organisms and is modified by a number 

 of intrinsic and extrinsic factors. Such physiological changes as hyperthyroid- 

 ism, aging, hibernation, estivation, the recovery from oxygen debt after exer- 

 cise, and the like, may alter the oxygen uptake. Such environmental conditions 



