1182 
Journal of Agricultural Research 
Vol. XXVIII, No. 12 
dorsal side of the tracheal trunk on each side close to the spiracular branches is 
a stout branch which runs dorsad for a short distance, and then divides into two 
secondary branches. One of these (fig. 2, A, h ) runs dorsad along the line of 
separation from the segment next preceding and supplies the pericardial cells 
of this region. The other secondary branch ( i) takes an oblique course along 
the suture separating the prescutum and the scutoscutellum, and enters the 
pericardial cavity in the middle of the segment opposite the valves of the heart 
in the pericardial intersegmental sinus. Here the branch breaks up into a few 
tracheoles which are distributed to the walls of the heart. The dorsal branches 
just described may be designated as “superficial,” in contradistinction to other 
(“deep”) branches which supply the viscera. They are branches of consider¬ 
able size springing from the dorsal side of the tracheal trunks in the intervals 
between the spiracular branches. In addition to these, each of the superficial 
branches usually gives off near its base a twig to the viscera. The viscera in 
Fig. 3—A, posterior end of a honeybee larva treated with caustic potash, seen from right side, showing 
distribution of tracheae to hind-intestine ( HInt),X ca 33; B, distal end of tracheal branch ( Tra ) 
showing tracheoles (tra) and nuclei of tracheal end cells (Nil), X 195 
the lower half of their segments are supplied by small twigs given off by the 
transverse commissures and not shown in the figures. In the 9th abdominal 
segment a superficial branch is given off caudad of the last pair of stigmata, 
while from the two ends of the posterior transverse commissure two or three 
deep branches are given off which supply the hind-intestine, as shown in Figure 
3, A. 
As in all insects, the tracheae are simple in structure, being merely thin-walled 
tubes composed of small flat epithelial (pavement) cells and lined with a chitinous 
intima which is thickened in the form of fine transverse spirally-wound threads, 
the taenidia. The size of the taenidia is at least approximately proportional to 
the size of the trachea of which they form a part, while the size of the tracheal 
cells themselves remains nearly constant in the same individual. Taenidia are 
found in the tracheal branches up to their termination in tracheal end cells. The 
structure of the spiracular branches, however, differs somewhat from that of the 
tracheae proper. At each point where the hypodermal epithelium turns inward 
to become the wall of a spiracular branch it forms a sharp fold, thus somewhat 
reducing the mouth of the aperture (PI. 3, D). The chitinous cuticle also folds 
