chap, vi.] RESPIRATION OF INSECTS. 217 



has been observed by Lowne that the blowfly makes 

 vigorous movements with its legs from, sixteen to 

 thirty times a minute, and he judges that these are 

 respiratory movements from the consideration that, 

 as the valves of the anterior spiracles are closed for a 

 short time, the air must be necessarily driven through 

 the small tubes ; these movements are accompanied 

 by a contraction and a dilatation of the abdomen. 

 If placed under an exhausted receiver of an air-pump 

 and removed before death ensues, the renfling of the 

 tracheal system is observed to be accompanied by 

 violent movements of the legs and wings, so that 

 here, as in the crayfish (see page 225) and elsewhere, 

 the activity of the locomotor organs is seen to be in. 

 direct relation with an increased supply of oxygen. 

 In the Iarva3 of some Insects the tracheal system is 

 completely closed, or, in other words, there are 

 tracheal tubes but no stigmata ; in these cases the 

 tubes are either placed not far from the surface of 

 the body, when the respiration may be said to be 

 vague, or the tracheae form out-growths (the so- 

 called tracheal gills) which project into the water 

 (e.g. Culicidre), and offer the exact converse of what is 

 the essential point in tracheal respiration ; in it the 

 air comes to the organs by internal tubes, in such 

 larvae tubes go to the water in order to obtain the 

 oxygen. 



We may appropriately pass from these external 

 tracheal tubes to the form of respiratory organ in 

 which there is an out-pushing of the body wall to 

 form a so-called external gill. Physiologically this 

 condition is preceded by what obtains in the Starfish, 

 where the respiration is said to be vague ; from 

 among the spaces left between the ossicles of the 

 dorsal surface (Fig. 23) there project thin membranous 

 out-pushings of the lining of the body cavity, and 

 the fluid within is thus only separated from the 



