24 MR. LUBBOCK ON THE DISTRIBUTION 



admitting that this is apparently the case, seems to think that it must be an illusion, and 

 that there cannot really be any such communication. 



There is however, I believe, no other group of insects kno^vn in which the biliary vessels 

 open elsewhere than at the anterior end of the ileum (excepting perhaps the Homoptera); 

 and Ave find in certain Hemiptera that the posterior part of the stomach is much elon- 

 gated, and that the Malpighian vessels open at the end of it. I am therefore inclined to 

 believe that in Tentatoma, Scutellera, &e., the so-called " cordons vahoilaires " represent 

 the posterior part of the stomach, and that if the chamber into which the Malpighian 

 vessels fall is really the " colon," we may infer that the ileum is not developed. 



In examining any given organ or membrane, it is of course necessary to have before 

 one a surface suificiently large to give a good idea of the type of distribution, in order to 

 feel sure that the arrangement of the tracheae which is seen is really characteristic of the 

 organ. I have therefore chosen the largest insects I could obtain, because in them we 

 see the same type of distribution repeated over and over again in the field of view ; and 

 I have also compared different specimens together. 



The mode of branching is in many respects comparable to that of trees. As we find, for 

 instance, no two oaks exactly alike in their mode of branching, wliile yet the species 

 possesses a well-marked type of its own, so in the tracheae, though no two branchlets 

 divide in exactly the same manner, still they possess a well-marked character. And 

 though in numerous insects many organs are alike in this respect, there are others in 

 which any fragment of an organ could at once be recognized if it were large enough to 

 show the mode of branching of the trachetB. 



In the different species of one genus we generally find the trachese very similar. In 

 comparing together, however, insects belonging to different families of the same order, 

 this is by no means the case. In the ovarian trachese, for instance, Ilnsca (PI. III. 

 fig. 2) much resembles Bombtis; while Tipula (PL III. figs. 4 & 5) and Tenthredo are 

 quite different from either, but resemble one another. A third genus of Hymenoptera, 

 OjjJdon (PI. III. fig. 7), again, is dissimilar from either Bomhus or Tenthredo, and 

 agrees very nearly with Aclieta (PI. III. fig. 12), which, for its part, differs entirely 

 from LoGusta. It would seem, therefore, that the distribution of the finer trachese 

 cannot have any bearing on the question of ordinal, or even of family affinities. 



In some cases trachese, which at first sight are very dissimilar, present in reality no 

 difference. Thus, in PI. Ill, the figures 4 & 5 represent the trachese on the ovary of 

 Tipula : in fig. 5, the trachese are represented as they appear when expanded by the 

 e^g ; while on the parts of the tube which lie between the eggs, and are much narrower, 

 they resemble fig. 4. It may probably be stated, as a general rule, that the waved 

 course of many trachese is a provision to allow for the expansion and movements of the 

 organ to which they are attached. 



There seems to be some special tendency in the ovarian trachese to arrange themselves 

 in tufts, perhaps because in this way they are better able to adapt themselves to the 

 alterations which occur in the diameter of the egg-tubes during the rapid growth 

 and passage of the eggs. In Necrophori^s, Locusta, Chrysopa, Tipula, &c., the ovaries 

 present more decided tufts of tubules than most of the other organs ; and in Musca and 



