DISTRIBUTION OF THE TRACHE/E 



435 



in the larvae of Corethra and Chironomus the tracheal system is only incom- 

 pletely developed ; the tracheae are not united with each other, and in the young- 

 est larvae they do riot contain air. 



Each of the two main tracheae, as Kolbe states, 

 sends off into each segment of the body three 

 branches. 



1. An upper or dorsal branch, which supplies the 

 muscles of the dorsal region. 



2. A middle (visceral) branch, whose twigs pass 

 to the digestive canal and back to the organs of 

 reproduction. 



3. A lower (ventral) branch, whose twigs are 

 distributed to the ganglia and to the muscles of the 

 ventral region. 



In certain Thysanura, as a species of Machilis 

 (Fig. 397), we probably have the primitive condition 

 of the tracheal system, the longitudinal and trans- 

 verse anastomoses being absent, but in other Thy- 

 sanura (Japyx, Nicoletia, Lepisma, and a few species 

 of Machilis) they are present. 



As Kolbe remarks, whether the fine ends of the 

 tracheae are closed or open, whether after the analogy 

 of the blood capillaries of vertebrates they anastomose 

 with each other, whether the ends of the air-tubes 

 pass between the cells or penetrate into them, these 

 questions are not fully settled. According to Ley- 

 dig's l latest views the tracheae penetrate into the cells 

 and unite with the hyaloplasma. Hence the process 

 of respiration in the last instance takes place in the 

 hyaloplasma. This assumption accords with the fact 

 that in the tracheate Arthropods the terminations of 

 the tracheae carry the atmospheric air into the space 

 bounded by the cellular network, also to the hyalo- 

 plasma filling the spaces. Leydig 2 also thinks that 

 the finest tracheal endings penetrate into the muscular 

 tissue and unite with the primitive muscular fibres. 



FIG. 397. Traclieal system 

 of the right side of JfachiUt 

 maritima : k, head ; I, II, III, 

 thoracic segments ; l-l'O, ab- 

 dominal segments ; *, stigma. 

 After Oudemans, from Lang. 



Kupffer is likewise of the opinion that the 

 fine tracheae penetrate into the cells, and Lidth 

 de Jeude asserts that they enter the- epithelial cells, "each cell con- 

 taining several branches." Kolliker, Emery, etc., maintain, how- 



1 Untersuchungen zur Anatomie und Histologie der Tiere, 1884, p. 72. 



2 Zelle und Gewebe, 1885, p. 43. (See also our p. 217.) 



FIG. 395. Nelanoplus femur-rubrnm, showing distribution of air-tubes (trachea-) and air-sacs ; 

 T'. main ventral trachea (only one of the two shown) ; S, left stigmatal trachea, connecting by vertical 

 branches with D, the left main dorsal trachea ; e, left cephalic trachea ; oc, ocular dilated trachea. 

 From the first, second, third, and fourth spiracles arise the first four abdominal air-sacs, which are 

 succeeded by the plexus of three pairs of dilated tracheae, I, II, III, in Fig. 396. Numerous air-sacs 

 and trachea; are represented in the head and thorax. The two thoracic spiracles are represented, 

 but not lettered. 



FIG. 396. />, left dorsal trachea; S, left stigmatal trachea; I, II, III, first, second, and third 

 pairs of abdominal dilated tracheae, forming a plexus behind the ovaries; 1, pair of enormous 

 thoracic air-sacs ; 2, pair of smaller air-sacs ; 3-7, abdominal air-sacs ; oc, ocular dilated trachea 

 and air-sacs ; c, cephalic trachea. The relations of the heart to the dorsal trachea; are indicated. 

 Drawn by Ernerton from dissections by the author. 



