ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 409 



3. Those with tracheal gills. — These, comprising the 

 larger larvae of all the more generalized orders of aquatic 

 insects, have adhered more strictly to the tracheate type of 

 respiratory apparatus. Tracheal gills are protrusions of the 

 body-wall with fine tracheal tubes grown out into them, and 

 the exchange of gases in respiration is between the air in the 

 tubes and the water outside the gill. The tracheal system, 

 therefore, instead of being reduced, is increased by the out- 

 growth of the additional parts that penetrate the gills. 



Fig. 239. Larvae of dipterous insects, x, the pnnkie {Ceratopogon). y , the 



phantom larva of Corethra; s, a "blood worm" — ihe larva of a midge 



{Chironomus) ;/, floats (expansions of the main air tubes) ; g, g. g, blood gills. 



Tracheal gills may be external as in the case of the damsel- 

 fly nymph shown in fig. 225, or internal, as in the case of the 

 larger dragonfly nymph shown in figure 240. Whatever 

 their position number or arrangement, they conform more 

 or less closely in shape to two types, filiform or cylindric, and 

 lamelliform or flat. 



Their diversity in form, position, arrangement and num- 

 ber and size will be seen in the series of larvae selected for 

 study. 



