THE TRACHEAE 97 



areas are much reduced, and the spiracles are displaced so as to lie 

 between contiguous lateral plates, to which they are closely connected by 

 strong bars of chitin. Each spiracle is an oval or dumb-bell shaped open- 

 ing, the margin of which is maintained by a rim of chitin, more or less 

 thick in accordance with the density of the rest of the thoracic wall. 

 The opening is guarded by a series of teeth of various forms, designed to 

 prevent the ingress of solid particles ; they may be simple and comb- 

 like, or extremely minute, thin, and numerous, arising from a large 

 number of short common trunks and bending in all directions across the 

 opening like the branches of a tree. (Plate XX, fig. 5.) Posterior to the 

 opening there is a small vestibule, provided with a simple valve and 

 muscle, by means of which the size of the inlet can be regulated. 



The abdominal spiracles lie in the membrane between the sternites and 

 tergites. Each is placed a little anterior to the segment to which 

 it properly belongs, and the number is usually less than the number 

 of visible segments. The first pair lie close to the distal end of the 

 metathorax. They resemble the thoracic spiracles in structure, but are 

 generally smaller. The abdominal spiracles do not, however, diminish 

 progressively in size as one might expect, the largest being usually 

 that which admits the air destined for the supply of the ovaries. 



In a primitive arrangement of the parts each pair of spiracles would 

 give rise to a pair of branching tracheal tubes for the supply of the 



segment, but in the complex imago of the Diptera 



" . Arrangement 



there is a considerable deviation from this type, due O f Tracheae 

 partly to the suppression of some spiracles and their 

 tracheae, and partly to the dislocation of others from their segments 

 in a forward direction. The system is rendered more complex by 

 the anastomosis of the branches of one tracheae with those of its 

 fellow of the opposite side and with those of the adjacent segments ; the 

 anastomosis takes place, not between the ultimate divisions, but between 

 tracheae of considerable size. In the main features, however, the segmental 

 arrangement is preserved, and most of the branches from a spiracle go to 

 supply the tissues of the segment to which it belongs. 



The distribution of the main branches will be understood sufficiently 

 well from the figures given. (Plate XX.) The head receives its supply 

 from the anterior or mesothoracic spiracles by means of a pair of 

 tracheae which pass through the neck. The two thoracic spiracles 

 supply the huge mass of muscle contained within the thoracic cavity, 

 and also the legs; some branches from the posterior spiracle pass into the 

 abdomen and anastomose with the abdominal tracheae. Each abdominal 

 13 



