405 



Note on the Tracheal Tubes of Insects, etc. 



By a. a. Merlin, F.R.M.S. 



{Read March IGth, 1900.) 



Some few years ago, while examining a balsamed preparation of 

 the tracheae of the Dytiscus larva, employing critical illumination 

 and an apochromatic objective, it was found that the internal 

 strengthening thread did not consist of a long, continuous fibre of 

 equal breadth throughout, spirally wound round and round in the 

 tube, as invariably described and figured in the text-books, but, 

 on the contrary, the thread was seen to consist of a number of 

 short and varying lengths, sometimes suflScing to form a couple of 

 complete loops within the circumference of the pipe, but in many 

 instances falling short of one complete coil. 



Hoping that some one more competent than myself would take 

 the matter up, I at the time alluded to it in a short letter to the 

 *' English Mechanic," but apparently without result. 



The structure is easily observable in the tracheae of a Trinidad 

 centipede (Scolopench'cc morsitans). In this the chitin fibres are 

 deeply coloured and very distinct, but, so far as my observations 

 extend, and a considerable number of various insect tracheae have 

 been examined for the purpose, a precisely similar general 

 arrangement is to be seen in all such tubes, even the smallest, 

 although in some of the latter the lengths of the continuous 

 thread frequently suffice for two or three complete turns, which 

 is more rarely the case in the larger kinds. 



The task of elucidating the structure referred to, in the case of 

 Scolopench'a morsitans, cannot be regarded as in any way a difficult 

 one, an ordinary achromatic |inch objective rendering all the 

 existing features conspicuous, indeed they are so obvious that it 

 is puzzling to account for the lack of truthful descriptions or en- 

 gravings, and I consequently feel some diffidence in calling your 



