358 THE TRACHEAL SYSTEM OF THE IMAGO. 
developed like the hemal 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. 
Palmén 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-hemal fluid, like the 
pseudo-hzmal 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 trachez are first filled with air. In the young embryo the 
trachez are always filled with fluid, and it is only in the later 
stages of development, when the tracheze 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 
