RESPIRATION 



129 



to the tubes, when examined with the microscope, a transversely, 

 closely striated appearance. Packard considers 1 that in some 

 tracheae this fibre is not really spiral, but consists of a large num- 

 ber of closely placed rings. Such a condition has not, however, 

 been recorded by any other observer. The spiral fibre is absent 

 in the fine capillary twigs of the tracheal system, as well as from 

 the expanded sacs. The mode of termination of the capillary 

 branches is not clear. Some have supposed that the finest twigs 

 anastomose with others ; on the other hand it has been said that 

 they terminate by penetrating cells, or that they simply come 

 to an end with either open or closed extremities. Wisting- 



\ x 



FIG. 71. Portion of the abdominal part of tracheal system of a Locust (Oedipoda): 

 a, spiracular orifices ; b, tracheal tubes ; c, vesicular dilatations ; d, tracheal twigs 

 or capillaries. (After Dufour.) 



or capillaries. (After Dufour.) 



hausen 2 states that in the silk-glands the tracheal twigs anas- 

 tomose, and he is of opinion that the fine terminal portions 

 contain fluid. However this may be, it is certain that all the 

 organs are abundantly supplied with a capillary tracheal net- 

 work, or arboreal ramification, and that in some cases the tubes 

 enter the substance of tissues. Near their terminations they 

 are said to be ^ to -^ millimetre in diameter. 



We must repeat that such a system as we have just sketched 

 forms a striking contrast to the imperfect blood- vascular system, 

 and that Insects differ profoundly in these respects from Verte- 

 brate animals. In the latter the blood-vessels penetrate to all 



VOL. v 



American Naturalist, xx. 1886, pp. 438 and 558. 

 2 Zeitschr. wiss. ZooL, xlix. 1890, p. 565. 



