The Structure of the Trachee of Insects. 571 
1884. ] 
They are so minute that we need not be surprised if direct obser- 
vation has hitherto failed to detect this structure. A cross sec- 
tion of them in my possession (prepared by W. M. Rankin) seems 
to me to confirm this view; but the outside diameter of the spiral 
is only .0025™™ (about a twenty-thousandth of an inch), and inter- 
ferences by. diffraction lessen the value of direct observations ex- 
cept as to general outline.1 I think that I can also see evidence 
of a lumen within the spiral, and of the fissure, in stained speci- 
mens of trachez prepared in the usual way.” Carl Chun’s figure, 
though not so understood by its author, is in accordance with 
our view (Fig. 3). What he supposes to be solid fibers cannot 
arise in the way suggested by him, inside and independently of the 
6 46 
intima, which is really the bound- 
Supposed to be the optical expres- _Fic. 3.—Cross-section of two spirals 
I 
Re re se Cee 
Beet eternal fissure... vor comeing menteney, B 
think I have also got the explana- outer membrane of later origin. After 
a the “outer membrane of 7 
ter origin “represented at M?, It is the first appearance of a 
intima Prepared to replace the functioning one after the pró- 
"ieot molting, 
oot of genesis of the spirals has been found by different 
Si to be first by the formation of homogeneous membrane. 
- (in these, i riom in fine branches and also in large air-sacs 
x brane soo, ‘owever, striation is usual). The homogeneous mem- 
Ter anes transversely striated by incipient crenulation. 
We can easily see new tracheæ in the striated condition 
So, ESAS 
Plined by } m What I take to be their lumen appears to me too sharp to be ex- 
a ù : on in solid fibrils, 
: eine Specimens viewed, as Fig. A, the circle of light at the margin of the 
bright and well defined as to indicate an inner cavity. 
