260 



Apple and Pear Sucker 



VII. Tracheal System. (Figs. 15, 16). 



From the economic point of view none of the internal organs of an 

 insect is of more importance or deserving of more attention than the 

 tracheal organs. They constitute the chief medium through which a 

 contact-poison insecticide acts. Prof. Lefroy has proved this point 

 by his experiments on meal worms. The tracheal system differs in 

 different insects, as also in the larva and the imago of the same insect, 

 specially with regard to the way the respiratory organs open to the 

 exterior. In some insects the spiracles lead directly to the tracheae, 

 while in others there are various modifications for closing and opening 

 the mouths of the tracheae. This difference in the arrangement of the 



Fig. 15. 



Fis. IG. 



spiracles and the tracheae may explain why a certain insect with simple 

 tracheae succumbs to an insecticide more easily than others Avith 

 modifications of the tracheal apparatus. In Psylla mali there are two 

 kinds of trachea, one in the imago and the other in the larvae, 

 including the nymphs. 



(i) Tracheal system in the imago (Fig. IG). It is of the simplest. 

 The outer integument is invaginated to form a spiracle which leads 

 directly to a tracheal trlmk. There is a structure in the spiracles which 

 may function as a strainer of the air which is sucked through the spiracles. 

 The structure of a spiracle of this kind may be easily understood from 

 the figure. 



