1891.] MICKOSCOPICAL JOURNAL. 181 



Some Observations upon Insect Tracheae. 



By Dr. EDWARD GRAY, 



BRNICIA, CAL. 



"• Opposed to the general opinion of anatomists since Sprcngel, the 

 tracheic are not composed of a ilouble membrane with a spiral fibre be- 

 tween them, which may be rolled up by itself; but the spiral fibre is 

 only the result of the thickening of the inner membrane. * * * Xhe 

 inner surfiice is not formed of a mucous membrane, and shows no cells ; 

 it corresponds to that of the wings, and may indeed show hairs or 

 prickles like the external coverings and membranes. In fact, these epi- 

 dermal appendages * * * prove at once that the spiral thread is 

 raised internally, and that it actually forms a part of the inner mem- 

 brane." 



Who is it who thus speaks with authority, and sets at naught the 

 teachings of the books? It is no less a writer than Dujardin, and the 

 original of the quotation appeared in the Coniptes Rendus for 1849. It 

 is therefore remarkable that these statements, if correct, have not long 

 ago been duly incorporated into the text-books. That Dujardin is right 

 is not difficult to prove when one will examine for himself. First, 

 very recent authorities, such as Graber and Lowne describe the struc- 

 ture of the trachea similarly to Dujardin. To quote from Lowne only : 

 '' The main trunks and larger vessels exhibit an external coat of thin 

 polygonal cells closely united by their edges. * * * Internally to 

 these cells there is a thick cuticular intima, with a distinct spiral struc- 

 ture, which gives the vessels their well-known appearance. In the 

 smaller tracheal the intima is apparently structureless." ("■Blow-fly" 

 revised ed.) 



Graber's statement tallies with this, and these writers, be it observed, 

 are practical insect anatomists and physiologists and original observers. 



Second, the intelligent microscopist may demonstrate the matter for 

 himself. It is, to be sure, a problem of some delicacy, and not to be 

 solved by ofl-hand and careless methods. Still, the average observer 

 need not feel unequal to the task. No result will be attained by soak- 

 ing or boiling the trachea in caustic potassa and mounting in balsam ; 

 every vestige of epithelium, external or internal, will assuredly be de- 

 stroyed in this way. Fresh specimens only, examined in fluid, are fit 

 to decide such a question. Inferences from ordinary balsam slides, in 

 particular, are not to be trusted except as corrected by fresh untreated 

 specimens. Quite recently a tracheal trunk from a beetle — species un- 

 known — with no other treatment than carmine staining, showed the 

 polygonal cells clearly with the spiral intima, but with no internal mem- 

 brane. Very commonly, however, the inquirer will find it not easy to 

 demonstrate the epithelial layer. 



Dujardin's words are of interest in another direction, for Mr. F. 

 Dienelt lately re-discovered the intra-tracheal hairs mentioned in the 

 quotation, and editorial mention was made of the fact in The iV/Zcro- 

 5co/<? for August, 1890. It certainly appears remarkable that with the 

 improved instruments of modern days a period of forty years should 

 have passed before the re-discovery of these hairs. The largest devel- 

 opment thus far known occurs in a water scorpion of the Nepa family, 

 named Zaltha Jlamlnea. In this insect the intra-tracheal hairs are 

 not distributed everywhere, but only in certain of the larger trunks. 



