XIl] COCKROACH 241 



The tracheal system opens to. the outer air by ten pairs of oval 

 pores or stigmata. These lie in the soft integument between the 

 terga and sterna, one pair just in front of the mesothorax, one pair 

 just before the metathorax and eight pairs just in front of each of 

 the first eight abdominal segments, so that they seem to be inter- 

 segmental in position. These openings lead into tubes or tracheae 

 which soon bifurcate and divide. The larger branches have a 

 definite and symmetrical arrangement. There are dorsal arches 

 running up towards the heart and ventral arches descending towards 

 the nerve-cord. These arches are connected with one another 

 longitudinally by trunks which run on each side of the pericardium 

 and the nerve-cord. Large trunks also are given off' to the 

 alimentary canal It follows that should one stigma become 

 blocked the organs which its tracheae supply are still provided 

 with air. The finer branches become smaller and smaller until they 

 become veritable capillaries which penetrate every tissue. 



The tracheae being full of air present a glistening silvery 

 appearance which is unmistakable. They are prevented from 

 collapsing by the presence of a spiral thickening of the chitinous 

 lining which runs round the interior of the tube just as the wire 

 spiral strengthens the india-rubber tube of some kinds of garden hose. 

 Respiration is effected by the alternate arching up and flattening of 

 the abdomen, resulting in an alternate increase and diminution of its 

 volume, which, since the blood is incompressible, secures an alternate 

 inrush and expulsion of air from the tracheae. In all probability 

 only the contents of the larger tracheae are affected by this process; 

 by diffusion the oxygen from this "tidal air" is handed on to the 

 finer tracheae. 



In Insects the tracheal mode of respiration reaches its highest 

 development and it is accompanied by a correlative diminution and 

 simplification of the circulatory apparatus, the oxygen of the air 

 being conveyed directly to and the carbon dioxide removed directly 

 from the cells to the outer air, whilst the blood loses its respiratory 

 function. It is doubtful how far this state of things is connected 

 with the peculiar disappearance of the coelom which presumably 

 exists only in the cavity of the reproductive organs because a 

 similar replacement of the coelom by a haemocoele is found in 

 Crustacea and Mollusca, where the blood is respiratory and the gills 

 are compact and active organs ; but probably it is correlated with 

 the modification which the excretory system undergoes and this again 

 is undoubtedly influenced by the state of the body-cavity. 



S. & M. 16 



