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the right and left side and by the hollow rod above and below. 
Really the two tracheal trunks do not open separately to the outside, 
but, as stated by Raschke, through a single orifice. To the rod 
are attached: the long muscles which run through the whole of the 
sipho, further two slanting muscles going from the apex of the rod 
down to the two ventral, lateral flaps. The chitin rod is most 
clearly seen in fig. 30, drawn from a thrown-off skin of a Culex- 
larva. 
Fig. 31 shows a series of transversal sections through the Culex- 
tube. Fig. 31 a and b show the two tracheæ and the muscles 
between them; in c the apex of the chitin rod may be seen; d 
—g show some different sections of the rod and the muscles 
attached to it; the sections h—i show the edges of the respiratory 
cup and the flaps, which appear in the drawing as thickenings in 
the walls of the outer tube. The sections k—m are cut through the 
respiratory cup itself; in n the section has struck a little outside 
the two ventral lateral flaps; sections h--j show that stay-bars of 
chitin lead inwards from these flaps and serve for attachment of 
muscles. As shown by the section these muscles are arranged 
rosary-shaped around the one side of the inner tube. 
If we will try to understand the structure of the inner tube 
of the Mansonia-sipho, we only have to compare the transversal 
sections of this sipho with those of the Culex. It will then be 
obvious, that the only change which has taken place in the Man- 
sonia-tube is a considerable prolongation of the cup-shaped im- 
pression in which the tracheæ terminate in the case of Culex; 
this also holds good with regard to the chitin rod. The section 
fig. 31 of the Culex is exactly in accordance with the section fig. 28 
of the Mansonia. It must only be noted, that all muscles as well 
as the coarse hair cover of the inner lumen are absent from that 
part of the Mansonia tube used as a piercing organ; moreover 
the chitin of this part is very thick. 
This will be further confirmed by a comparison of the series 
of horizontal cuts of the Culex-tube (fig. 32) with the correspond- 
ing series of Mansonia (fig. 27). The chitin rod is seen in P- 
In. the Culex the felt cover is restricted to a very short part at 
the point of union of the two tracheæ. 
We may imagine ourselves the transformation from the common 
