TRACHEAL RESPIRATION 



121 



show how important it is for many insects to keep down the 

 water loss, demonstrate the increase when the spiracles are 

 forced open, and point out the almost complete impermea- 

 bility of the integument of many insects to water vapour. 



Tod™? 



30 



20 

 W 



8 ID 12 

 Weeks 



B 



Fig. 67. The effect of opening the spiracles on the rate of loss of water. 

 A, rate of loss from a starving mealworm; B, rate of loss from a number of fleas 

 in 1/100 mg per mg per hour. Columns marked x obtained when the spiracles 

 were kept open with 5% CO.,. (Mellanby.) 



The experiment illustrated by Fig. 67 is very convincing. 

 The curve shows the rate of water loss from a fasting mealworm 

 over 18 weeks. It is gradually reduced with the falling 

 metabolism, but when the animal was forced by C0 2 to keep 

 its spiracles open (at X in the figure) the evaporation rose to a 

 high level, nearly the same after sixteen weeks as after two. 



By the mechanism now described, diffusion can only be 

 regulated downwards. Wigglesworth has discovered another 

 mechanism by which the insect can meet increased demands 

 and which is in its effects analogous to the opening up of 

 capillaries in active muscles in vertebrates. 



This mechanism is located in the non-chitinous tracheoles 

 of less than 1 micron diameter which branch out from the 

 smallest tracheae and form a rich network 1 surrounding cells 

 or penetrating into them, as in flight muscles where the 

 tracheoles may even invest the single fibrils and sarcomeres. 

 These final branches are normally invisible in the living insect, 

 because they are filled with fluid, and the central point in 



1 It seems difficult to decide whether true networks exist or the final branches 

 have free endings. In several cases where loops have been described, e.g., in 

 the tracheal gills of Odonata (fig. 83), a closer investigation has shown that each 

 tracheole ends by itself. From the point of view of oxygen supply the difference 

 is irrelevant. 



