92 = MR. NEWPORT ON THE ANATOMY AND DEVELOPMENT 
only a very few longitudinal and transverse fibres, which are separated by wide inter- 
spaces (fig. 12 a); the one extending throughout the whole length of the organ, and the 
other encircling every part of it; and these are crossed obliquely by a few fibres which 
attach the canals loosely to the tegument, and aid its peristaltic movements. But the 
glandular or middle tissue is more complete. It is composed of the cells before alluded 
to*, which are large, hexagonal in shape, and, in most instances, correspond to the inter- 
spaces formed by the decussation of the muscular fibres (b). Each cell in its interior has 
a very large granular nucleus. The mucous, or lining tissue of the canal, is formed by a 
layer of somewhat flattened cells, which have small granulated nuclei, are loosely aggre- 
gated together, and have all the characters of epithelial cells (c). In the imago, however, 
the muscular tissue is composed of very strong longitudinal and transverse bands, crossed 
as in the larva by a few delicate oblique ones; while the glandular tissue is less marked, 
the mucous tissue being most developed; thus, preceding in its changed condition, the 
change of food of the perfect insect. | 
Every part of the canal is supplied with tracheæ, the trunks of which, one in each 
segment, passing transversely inwards, divide into branches, which, again subdivided, 
penetrate into and ramify through the structure. These, like all other tracheæ, are formed, 
as described by Sprengel, of three tissues, an external membranous, and an internal 
mucous, which enclose between them a strong spiral fibre. The external, as I formerly 
showed, so loosely invests the middle, or spiral, that, usually, there is some interspace 
between them; and, as also mentioned}, this external tissue is simply a reflexion and 
extension of the common peritoneal membrane. The ramifications of tracheæ which 
penetrate the structure of the canal, or of any other organ, become, as I have since found, 
denuded of this covering as they enter, and then seem to be formed only of two tissues, 
_ the spiral and mucous,—if, indeed, there be not also, as I have reason to think there is, 
an extremely delicate serous, or basement membrane, closely adherent to, and uniting the 
coils of fibrous tissue, on its external surface. The tracheze which penetrate the muscular 
layer of the canal terminate in the glandular or adipose layer, where a few of the branches 
anastomose; but, as elsewhere stated$, « they do not ramify in the internal or mucous 
membrane.” The ultimate divisions of these tracheæ are always distributed separately, 
and do not anastomose. They end in extremely minute, filiform, blind extremities, as 
noticed by my friend Mr. Bowerbank, F.R.S. This I find is their condition in all struc- 
tures, in the nervous and tegumentary equally as in the glandular and muscular. 
These facts may, perhaps, assist us to understand the nature of the injection of the 
tracheæ by M. Blanchard|, and also the mode of nutrition in insects;—the ultimate 
branches of trache in the tissues of the alimentary canal operating, possibly, as absorbent 
structures, and inducing the chylific fluid elaborated around them to flow, in its transit 
outwards, along the channels formed by their loose peritoneal covering, into the regular 
circulatory currents {],—a view which, in part,—so far as refers to the presumed absorbent 
operation of the trachew,—was long ago held by Dr. Kidd, in his paper on the Anatomy 
* > . 
: E. d á : t On the Respiration of Insects, Phil. Trans. 1 836, p. 530. pl. 36. fig. 1. 
t Article usecta,’ loc. cit. p. 965. $ Loc. cit. Phil. Trans. 1836, p. 532 and 564, pl. 36, fig. 4. 
|| Annales des Sciences Nat. 3™ Série, tom, xi. p- 372, et seq. € Article * Insecta,’ p. 979. 
