572 The Structure of the Trachee of Insects. [June, 
enclosing older ones with well-developed spirals; a tube inside of a 
tube, the contrast between striz and spirals indicating the course 
by which increasing crenulation with thickening is turning the 
smooth membranous tube into the definitely strengthened 
trachea. 
4. Mode of Aération—The chitinous walls of the trachea are 
supposed to permit the transmission of air to the surrounding 
blood, but not the passage of fluids. If we bear in remembrance 
that the air within them is dry (sc., not dissolved in any fluid), I 
suspect that the direct passage through the wall is imaginary. 
Gegenbaur thinks that the fine extremities of the tracheæ my 
play some part in the function of aëration, and a consideration 
of the terminals of the tracheal branchlets may well make 
us ask if they do not perform the chief part. Boudelot 
has fixed the seat of the respiratory movements in the ab- 
dominal nerves, and therefore in the abdominal muscular move- 
ments.1 As the abdomen expands and contracts, the tracheal 
trunks rhythmically enlarge and diminish, causing inspirations 
and expirations of air; a partial vacuum and pressure being or 
nately produced around the trunks. The structure of the agi 
indicated above explains what was long a mystery to me, 
chitine, which is flexible but not tensile, can secure the ae 
- ment 
‘ment and reduction of volume essential to a tidal move with 
air. The opening and closing of the spiral ot om 
widening and narrowing of the external fissures, wer e 
oscillations of pressure, and hence the variations of volu 
the movements of air. 
The air reaching the numerous branchlets of the 
plies directly all the important tissues, the digestive 
nerves, limbs, eyes, mouth-parts, &c. The b i 
chitine at their extremities, enter directly into what “aed 100k : 
described (loc. cit.) as “ bags, which when highly Fee q whee 
like small lungs; the branchlets losing the spiral : res thes 
they enter these lungs.” Weismann descr ibes and e within 
terminals as long spindle-cells having a fine pes fi with aif- 
them ; and states that in the young larva they aa a 
Max Schultze has shown how in the glow-worm as and afè 
didula) the terminal cells of the tracheæ are yee combustio” 
abundant at the fat-bodies which by a process of slow 
‘Ann. d. Sci. Nat., Ser. 5, Vol. 11 (1864). 
canal, g 
