DISTRIBUTION OF THE AIR-TUBES. 
193 
placed. The nuclei can be seen in Anabrus in tracheae that have been 
mounted in balsam, without being stained, for the flat cells are sur- 
charged with unusually large, highly refringent, reddish-brown pigment 
granules of nearly uniform size (Fig. 01), hence the nuclei appear as clear 
ovals in the midst of the dark bodies of the cells. 
The spiral threads are unusually delicate in Anabrus (Fig. 57), and 
lie quite close together. 
In the finer branches of the tracheal system the structure is slightly 
modified (Fig. 8). In the first place, the nuclei are farther apart, 
showing that the cells are much larger than in the main tubes, and the 
nuclei appear much elongated, though their volume does not seem much, 
if at all, changed. The fine branches divide mainly by forking. In 
the crotch of the fork there often sits a triangular nucleus of entirely 
different shape from those on the other parts of the respiratory appa- 
ratus (Fig In'). 
The peculiar elongated shape of the nuclei on the finer tracheae ren- 
ders it possible to follow the course of the delicate air tubes (in stained 
preparations) through the other tissues with considerable ease. Never- 
theless it is advantageous in studying the distribution of the tracheae in 
the various organs to examine them immediately after the insect is killed, 
because they are then injected with air, so that under the microscope the 
large tubes appear silvery and the fine branchlets as dark lines in the 
fresh tissue. It will then be found that their distribution is almost as 
characteristic of the single organs as is the course of the blood-vessels 
in vertebrates. Williams 247 has reported some observations on this sub- 
ject, but his statements are generally received with some questioning. 
Sir John Lubbock 248 has published a valuable and extensive memoir 
on the distribution of the tracheae, containing the results of observations 
on a very large number of iusects of all orders. As far as I have been 
able to express my own results in general terms, I believe they are 
confirmed by the facts recorded by Lubbock. I find that the distribu- 
tion of the tracheae depends, first, upon the shape of the organs, and, 
secondly, upon the size of those whose size is variable ; whereby it must 
be remembered that the trachcce, as far as at present known, are exclusively 
confined to the connective tissue, including, of course, the fat body. No 
epithelium is ever penetrated by the air tubes in any instances known 
to me, through either my own observations or the writings of others. 
I give descriptions of the distribution of the trachea} in certain organs 
of Caloptenus and CEdipoda. Around the large organs (intestine, sexual 
organs), with interior cavities, the tracheae ramify in all directions, as 
on the ovary, for instance, Fig. 13, forking so that the branches diverge 
at a wide angle. In the organs which have muscular walls, like the 
oviduct (Fig. 14), for example, the tracheae run straight when the walls 
are distended, but have a siuuous course, as in the figure (14), when 
M ' Williams: Ann. Hag. Xat. Hist. (1854), vol. xiii, p. 194. 
248 Lubbock: Distribution of the trachea} of insects. Phil. Trans. 18C0, vol. xxiii, p. 194. 
13 L 
