434 THE INSECTA. <§« 342. 



Tlie 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 

 to a 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.'^' * 



§ 342. 



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 sen 

 tracheales), which are cylindrical, or riband-like organs covered by a very 



2 For the internal structure of the tracheae, see, be- body of insects, there is no trace of ciliated epi- 

 side the works of Burmeister, Lacordaire, and thelium, which, indeed, would be inC'impatilile 

 iVeuTJorJ, that of C. A';)rpn?e/, Comment, de partib. with the presence of chitine. Peters (Miillrr^s 

 quibus Insect, spiritus ducunt, 1815 ; Suckow, in Arcli. 1S41, p. 233) was certainly deceived when he 

 Heusin ger\^ Zeitsch. II. p. -Ji, Taf. I. fig. 10 ; thought he observed ciliary movements in the 

 Straus, Consid. &c. p. 315, I'l. VI. fig. 5; New- tracheae of Lampyris, Cnccine/la, Musca, and 

 port, Philos. Trans. 1836, p. 523 ; and Plainer, in other Insecta. He has himself admitted that 

 Miil/er^i Arch. 18-H, p. 38, Taf. III. he was not able to distinguish the cilia. For my 



3 This membrane is brown in the Libellulidae part, I have sought in vain for this movement in 

 and Locustidae ; this coloration is due to a finely- the tracheae, and Stein (Vergleich. Anat. u. Phys- 

 granular substance contained in the membrane. iol. d. Insekt. 18i7, p. 105) has been equally un- 



4 See Platner, loc. cit. Most anatomists regard successful. \ 



this internal membrane as mucous. This being ad- 5 The tracheae of the larvae of the Dytiscidae owe 



mitted, it was very natural to suppose that it, like their black color to the sjiiral filaments. 



that of the lungs of the Vertebrata, is covered with « With the Muscidae, Syrphidae, Vespidae, Api- 



cjlia. But here, as well as in other regions of the dae, and Melolonthidae. 



* [ § 341, end.] See, also, for investigations upon the other hand, Mayer, who has studied the embry- 



the intimate structure of the tracheae, Dujardin onic development of these organs, states that the 



(Comp. rend. 1819, p. 674), and Mayer (Ueber die spiral thread is originally a homogeneous mem- 



Entwickelung. des Fettkiirpers, der Tracheen, &c. brane, which ultimately splits up into the threads. 

 &c., bei den Lepidopteren, in Siebold and Kolli- This subject of the structure of tracheae has now 



fter'* Zeitsch. I. p. 1T5). The views of Dujardin an additional point of interest, from its relations to 



are different from those usually received, for he Hlanckard's views of a peritrachean circulation in 



regards the spiral thread not as a special forma- the Insecta. In this connection see especially Fi- 



tiun, but only a fold-like thickening of the internal lippi (Aimali della K. Accad. d'agricolturo dl 



membrane, — which membrane is not composed of Torino. V., also fViegmann^s Arch. 1851, Th. II. 



cells but is a structure analogous to the wing-mem- p. 145). — Ed. 

 brane, and is corered with hairs and points. On 



