35 THE TR AC HEAL SYSTEM OF THE JMAGO. 



developed like the haemal system of vertebrates from the para- 

 blast, and that the closed system is the primitive form ; that 

 the spiracular openings originated subsequently as pouch-like 

 involutions of the epiblast. 



Palmen entirely disposed of the old view, that the tracheal 

 spiracles are modified gills, or originate by the shedding of the 

 branchial appendages ; they originate independently, and are 

 sometimes segmental, sometimes inter-segmental, and vary 

 greatly, both in position and number, in different insects. 



If, as I have supposed, such is the origin of tracheal vessels, 

 they probably at first contained a pseudo-haemal fluid, like the 

 pseudo-haemal vessels of Annelids, as it is exceedingly difficult 

 to understand how these vessels can have contained air in 

 forms in which they are permanently closed. Dewitz has 

 shown [159] that even in insects with tracheal gills there are 

 temporary spiracles at some stage of development by which 

 the tracheae are first filled with air. In the young embryo the 

 tracheae are always filled with fluid, and it is only in the later 

 stages of development, when the tracheae are provided with 

 one or more pairs of spiracles, that they contain air. 



2. THE TRACHEAL SYSTEM OF THE BLOW-FLY, 

 a. The Spiracles and Spiracular Sacs. 



In the adult Blow-fly there are three pairs of thoracic, and 

 seven or eight pairs of abdominal spiracles. 



These spiracles may be classed in three groups : The great 

 inter-segmental thoracic spiracles, the tympanic spiracles, and 

 the abdominal spiracles. 



The Nomenclature and Position of the great thoracic spiracles 

 has been already discussed (p. 180). The tympanic spiracles 

 are small and exclusively expiratory openings connected 

 with the auditory and sound-producing mechanism. The 

 abdominal spiracles are comparatively simple inspiratory 

 orifices ; there are five pairs opening externally on the 

 ventral aspect of the lateral plates of the abdomen ; and two 



