434 THE INSECTA. $ 842. 
The tracheae are cylindrical tubes of variable size, which often form, in 
their course, vesicular dilatations and numerous anastomoses. They divide, 
like blood-vessels, into many branches which gradually decrease in size, 
ending, at last, caecally, so that the expired air passes out by the same. way 
that it entered. 
The intimate structure of these organs is remarkable, and has always 
attracted the attention of anatomists.” When filled with air they present 
a beautiful, silver appearance. Externally, they are invested with a thin 
transparent, colorless, or very rarely brownish membrane, corresponding 
toa peritoneal envelope.” Internally, they are lined with another mem- 
brane still finer, which presents a lamellated epithelial structure.” Between 
these two membranes is situated a solid spiral filament whose turns are 
usually near together. This filament is sometimes cylindrical, sometimes 
flattened, usually transparent and colorless, and in a few instances only, 
of a dark color.” Often, its course is unbroken for a long distance, and 
rarely is its extremity forked. The new threads always begin between the 
turns of the preceding one, as,may be easily observed at the commence- 
ment of each trachean ramification, In the ultimate trachean branches, 
these threads gradually decrease in size, and at last become indistinct. In 
the vesicular dilatations of the tracheae, with many Insecta, the spiral thread 
is often wholly wanting. * 
§ 842. 
The Branchial tracheae are found only in certain aquatic larvae and 
pupae, and never in the perfect Insecta. The absence of stigmata here is 
compensated by the existence of false branchiae (Branchiae spuriae sew 
zracheales), which are cylindrical, or riband-like organs covered by a very 
2 For the internal structure of the tracheae, see, be- 
side the works of Burmeister, Lacordaire, and 
Newport, that of C. Sprengel, Comment. de partib. 
quibus Insect. spiritus ducunt, 1815; Suckow, in 
Heusinger’s Zeitsch. II. p. 24, Taf. I. fig. 10; 
Straus, Consid. &c. p. 315, Pl. VI. fig. 5; New- 
port, Philos. Trans. 1836, p. 529; and Platner, in 
Miiller’s Arch. 1844, p. 38, Taf, III. 
8 This membrane is brown in the Libellulidae 
and Locustidae ; this coloration is due to a finely- 
granular substance contained in.the membrane. 
4 See Platner, toc. cit. Most a ists regard 
body of insects, there is no trace of ciliated epi- 
thelium, which, indeed, would be incompatible 
with the presence of chitine. Peters (Muiller’s 
Arch. 1841, p. 233) was certainly deceived when he 
thought he observed ciliary movements in the 
tracheae of Lampyris, Coccinella, Musca, and 
other Insecta. He has himself admitted that 
he was not able to distinguish the cilia. For my 
part, I have sought in vain for this movement in 
the tracheae, and Stein (Vergleich. Anat. u. Phys- 
iol. d. Insekt. 1847, p. 105) has been equally un- 
ful. 
this internal membrane as mucous. This being ad- 
mitted, it was very natural to suppose that it, like 
that of the lungs of the Vertebrata, is covered with 
cilia. But here, as well as in other regions of the 
*[§ 3841, end.] See, also, for investigations upon 
the intimate structure of the tracheae, Dujardin 
(Comp. rend. 1849, p. 674), and Mayer (Ueber die 
Entwickelung, des Fettkérpers, der Tracheen, &c. 
&c., bei den Lepidopteren, in Siebold' and Kailli- 
ker’s Zeitsch. I. p. 175). The views of Dujardin 
are different from those usually received, for he 
regards the spiral thread not as a special forma- 
on, but only a fold like thickening of the internal 
membrane, — which membrane is not composed of 
cells but is a structure analogous to the wing-mem- 
‘rane, and is covered with hairs and points. On 
5 The tracheae of the larvae of the Dytiscidae owe 
their black color to the spiral filaments. 
6 With the Muscidae, Syrphidae, Vespidae, Api- 
dae, and Melolonthidae. 
the other hand, Mayer, who has studied the embry~ 
onic development of these organs, states that the 
spiral thread ‘is originally a homogeneous mem- 
brane, which ultimately splits up into the threads. 
This subject of the structure of tracheae has now 
an additional point of interest, from its relations to 
Blanchard’s views of a peritrachean circulation in 
the Insecta. In tnis connection see especially Fi- 
lippi (Annali della R. Accad. d’ugricoltwro di 
Torino. V., also Wiegmann’s Arch. 1851, Th. II. 
p. 145). — Ep. 
