434 



THE INSECTA. 



^342. 



The tracheae are cylindrical tubes of variable size, which often form, ia 

 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 ju-esents 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 {Bra7ichiae spuriae sen 

 trackeales), 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 Biirmeister, Lacordaire, and 

 Neivport, that of C. Sprengel, Comment, de partib. 

 quibus Insect. Spiritus ducunt, 1815 ; Suckow, in 

 Heusinger^s Zeltsch. II. p. 24, Taf. I. fig. 10 ; 

 Straus, Consid. &c. p. 315, PI. VI. fig. 5 ; New- 

 port, Philos. Trans. 1836, p. 529 ; and Plainer, in 

 MüllerKi Arch. 1844, p. 38, Taf. III. 



3 This membrane is brown in the Libellulidae 

 and Locustidae ; this coloration is due to a finely- 

 granular substance contained in the membrane. 



■* See Plainer, loc. cit. Most anatomists regard 

 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 



body of insects, tliere is no trace of ciliated epi- 

 thelium, which, indeed, would be incompatible 

 with the presence of chitirie. Pelers {Muller''s 

 Arch. 1S41, 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 Slein (Vergleich. Anat. u. Phys- 

 iol, d. Insekt. 1847, p. 105) has been equally un- 

 successful. 



5 The tracheae of the larvae of the Dytiscidae owe 

 their black color to the spiral filaments. 



i; With the Muscidae, Syrphidae, Vespidae, Api- 

 dae, and Melolonthidae. 



* [ ^ 341, end.] See, also, for investigations upon 

 the intMnate structure of the tracheae, Dujardin 

 (Comp. rend. 1849, p. 674), and Mayer (Ueber die 

 Entwickelung. des Fettkörpers, der Tracheen, &c. 

 &c., bei den Lei)i(lopteren, in Siehold and Kölli- 

 ker\i Zeitsch. I. p. 175). The views of Dujardin. 

 are different from those usually received, for he 

 rejiaiys the spiral thread not as a special forma- 

 t'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- 

 brane, and is covered with hairs and points. On 



tlie otlier hand, Mayer, who has studied the embry- 

 onic dsvelopment 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 jjoint of interest, from its relations to 

 I}lancliard\s views of a peritrachean circulation in 

 the luaecta. In this connection see especially f'i- 

 lippi (Annali della R. Accad. d'ugricoltitro di 

 Torino. V., also fVieginann's Arch. 1851, Th. II. 

 p. 145). — Eu. 



