30 MR. LUBBOCK ON THE DISTRIBUTION 



It might at first be supposed tliat the points at which the air ceases are really the ends 

 of the trachese, and that where they can be traced further they are merely solid threads. 

 In some cases, however, the air disappears slowly from the fine tubules ; and a case of this 

 is represented in PI. III. fig. 8. Fig. 8 represents a smaU tracheal branchlet ramifying 

 over two egg-tubes of MpparcUa Jctnira, examined as soon as possible (perhaps a quarter 

 of an hour) after the death of the insect. Fifteen minutes later, the air had disappeared 

 from the fine ends, and no more could be seen than is represented in figs. 8' & 8". In an- 

 other quarter of an hom% still more of the tracheae had disappeared, and only the stump 

 as it were, F, remained. In this instance all the other tracheae on the organ ended in the 

 same abrupt manner ; but it is evident, from the preceding and many other similar obser- 

 vations, that the apparent ends are by no means the real terminations of the tracheae. 



This absorption or repulsion of the air from the finer tubules depends evidently, in a 

 great measure, on the nature of the wall of the tracheae and of the surrounding tissue, 

 because it proceeds with different rapidity in different parts of the same insect, so that 

 while, for instance, in Carabus I never found air in the finer tubules on the Malpighian 

 vessels, on the ovaries the air may be seen for a short time after death ; and in the gan- 

 glion, even after some hours, all the fine tubules still contain air. 



I am unable to offer any decided opinion whether this disappearance of the air from 

 the finer tubules is owing to its absorption by the tissues, or whether the surrounding 

 fluid forces its way through the delicate membrane of the tracheae. Probably, however, 

 the latter is the case ; because, if the air were simply absorbed by the surrounding tissue, 

 a fresh quantity would, I suppose, continually be supplied by the larger tracheae. 



Moreover I never found that the manner of death made much difference in the condi- 

 tion of the trachese, whereas, if the disappearance of the air were caused by vital action of 

 the surrounding tissue, this would hardly have been the case. 



It is difiicult, however, to understand why the small tracheae fill themselves with fluid so 

 much qu.icker in some insects, and in some organs, than in others. Probably, however, this 

 is o\\ing to some differences in the tracheal wall, which, though always permitting the 

 absorption of air by the tissues, may be more easily permeable by fluid in some parts than 

 in others. 



The surrounding fluid must probably have the same tendency to expel the air from 

 the finer branchlets during life as after death ; so that, whilst the insect remains alive, some 

 counteracting agency must be at work. In those insects which show no respiratory move- 

 ments, Prof. Graham's suggestion, above alluded to, seems to offer the best explanation. 



I have not yet paid much attention to the metamorphoses of the tracheae during the 

 change from the larva into the imago. The process seems, however, to be very curious ; 

 and I hope ere long to be able to devote some time to the investigation of it. 



In a Lepidopterous pupa, I found, in various parts of the body, knotted spiral tracheae 

 invested by a common membrane, as figured by Semper*. How these knots are formed 

 I know not. 



The structure of the tracheas has generally been described as consisting of an external 



* Zeitschrift fur Wiss. Zool. pi. xv. fig. 10. 



