DISTRIBUTION OF THE TRACHEAE 433 



organs, and upon the size of those whose size is variable. Around 

 the large, hollow organs (digestive canal, sexual organs) the tracheae 

 ramify in all directions, forking so that the branches diverge at a 

 wide angle. In the organs which have muscular walls, like the 

 oviduct, the tracheae run straight when the walls are distended, but 

 have a sinuous course when the walls are contracted. (Minot.) 



Around the organs of more elongated form the branches of the tracheae run 

 more longitudinally, as is shown by the air-tubes of the muscles, which also pre- 

 sent some peculiarities worthy of especial notice. 



"A short, thick trunk arrives at the muscular bundle, and dividing very 

 rapidly, breaks up into a large number of delicate tubes, which penetrate be- 

 tween the muscular fibres, then terminating in tubes of exceeding fineness, which 

 at first sight seem to form a network that might well be called a rete mirabile. 

 A closer examination, however, reveals that it is not a real network, but rather 

 an interlacing confusing to the eye. The longitudinal direction of the tracheae 

 of the muscles presents a striking contrast to the system of divarication repre- 

 sented in Figs. 13 and 14. The course of the tracheae of the Malpighian tubes is 

 also very curious. There is one large trachea which winds around the tube in a 

 long spiral, giving off numerous small branches which run to the surface of the 

 tube, upon which they form delicate ramifications. Each tube has but a single 

 main trachea, and I think the trachea continues the whole length of the tube, 

 but of this last point I am not quite sure." (Minot.) 



While in the nymphs of Orthoptera the tracheae very closely re- 

 semble those of the adult, in larvae of insects with a complete meta- 

 morphosis the tracheae differ very much in distribution from those 

 of the adult. The larval tracheae are also more generalized and 

 more like those of the original type than the tracheae of perfect in- 

 sects. (Lubbock.) 



In general there are two main tracheae, one passing along each 

 side of the body, near the digestive canal, connected with its mate 

 by a few transverse anastomosing branches, and sending off a branch 

 to each spiracle, this arrangement being most simple and apparent 

 in the maggots of Diptera. From these two main branches smaller 

 twigs branch off into every part of the body with its appendages, 

 passing among the different organs, often serving as cables to hold 

 them loosely in place ; they also penetrate into the component parts 

 of the organ themselves, passing into the fat-bodies, and among the 

 fibres of muscles, where they become finely attenuated and refined 

 like the capillaries of the vascular system of vertebrates. (Figs. 

 395, 396.) 



In the youngest larva of Corethra plumicornis Weismann ascertained the 

 thickness of the longitudinal stem to be 0.0017 mm. That of the finest tracheal 

 endings in the silk-glands of the silk-worm was found by Von Wistinghausen to 

 be 0.0016 inm. (Zeits. f. Wiss. Zool. xlix, 1890, p. 575.) Weismann states that 



