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 known in which the biliary vessels 
open elsewhere than at the anterior end of the ilenm (excepting perhaps the Homoptera); 
and we 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 Pentatoma, Scutellera, &c, the so-called " cordons valvulaires " 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 sufficiently 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, while 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 tracheae. 
In the different species of one genus we generally find the tracheae 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 tracheae, for instance, Musca (PL III. 
fig. 2) much resembles Bombus ; while Tipula (PI. III. figs. 4 & 5) and Tenthredo are 
quite different from either, but resemble one another. A third genus of Hymenoptera, 
Ophion (PI, III. fig. 7), again, is dissimilar from either Bombus or Tenthredo, and 
agrees very nearly with Acheta (PI. III. fig. 12), which, for its part, differs entirely 
from Locusta. It would seem, therefore, that the distribution of the finer tracheae 
cannot have any bearing on the question of ordinal, or even of family affinities. 
In some cases tracheae, which at first sight are very dissimilar, present in reality no 
difference. Thus, in PI. Ill, the figures 4 & 5 represent the tracheae on the ovary of 
Tipula : in fig. 5, the tracheae are represented as they appear when expanded by the 
egg ; 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 tracheae 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 tracheae 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 Necrophorus, Locusta, Chrysopa, Tipula, &c, the ovaries 
present more decided tufts of tubules than most of the other organs ; and in Musca and 
