Tltc Tracheal Sf/sftm of Simulium. 



703 



three the head. The 1st unites with the corresponding 

 trachea of the other side by the head-commissure already 

 mentioned. The 2nd and 3rd branches of the same side are 

 connected together iu the head by a short vertical trachea. 



The tracheal system is renewed at each moult, the old 

 one being cast with the skin. While the new tracheae are 

 developing, they enclose the old ones. It is evident that 

 the new initial threads, which eventually become solid, must 

 at this time be hollow, As the larva gradually disengages 



