80 RESPlRATOliY ORGANS. 



one of the loveliest objects in nature. In searching, 

 therefore, for tracheae, we are very much assisted by 

 the presence of the air ; and while in preserved speci- 

 mens these organs are not easy to trace, at least to- 

 wards their extremities, in fresh specimens they are 

 generally very conspicuous. 



Yon Olfers repeats the statements of Nicolet, as 

 regards Acliorutes and the allied genera. His own 

 observations were made on Smyntlmrus ohlongus, Tomo- 

 cerus plumbeus, and Orchesella fastuosa. In these he 

 found the respiratory system formed on one plan. 

 They have, he considers, two spiracles only, and these 

 are situated on the inferior side of the thorax, close to 

 the first pair of legs. From each spiracle start three 

 principal trunks, one of which goes forwards, one up- 

 wards, and one backwards. The first two are immedi- 

 ately divided into a great number of branches, which are 

 extremely narrow. The other passes to the middle 

 line, lies close to the corresponding trachea of the 

 other side, without, however, apparently uniting with 

 it, and then also breaks up into a number of fine 

 branchlets. 



I have again examined Smynthurus, and can only 

 confirm my previous statements. Yon Olfers, without 

 being acquainted with my memoir, agrees with me, as 

 we have seen, that there is only one pair of spiracles. 

 He considers, however, that they are situated in the 

 thorax, close to the place of insertion of the anterior 

 legs, while I think they are in the head. 



I will not be so bold as to say that there certainly 

 is no spiracle in the place indicated by Yon Olfers ; the 

 animal is so small and so inconvenient for dissection, 

 that I will not venture to make a positive assertion in 

 opposition to one who has evidently worked with much 

 care and skill. Nevertheless, I think he has been 

 misled by the fact that at this part the tracheae are 

 held in place by the rather large branch given off" to 

 the anterior leg. I think I have traced all the tracheae 

 up into the head, and that the only spiracles possessed 



