INSECTS AND ARACHN1DA. 



239 



apposition; and a very beautiful appearance, resembling that of watered 

 silk, is produced by the crossing of the two sets of fibres, of which one 

 overlies the other. iThat this appearance, however, is altogether an opti- 

 cal illusion, may be easily demonstrated by carefully following the course 

 of any one of the fibres, which will be found to be perfectly regular. 



FIG. 431. 



FIG. 432. 



Portion of a large Trachea of Dytiscus, with some 

 of its principal branches. 



FIG, 433. 



;- 



Tracheal system of Nepa (Water-scor- 

 pion): a, head; 6, first pair of legs; c, first 

 segment of thorax; d, second pair of wings; 

 e, second pair of legs; /, tracheal trunk: gr, 

 one of the stigmata; h, air-sac. 



Spiracle of Common Fly. 



635. The ' stigmata' or 'spiracles' through which the air enters the 

 tracheal system, are generally visible on the exterior of the body of the 

 insect (especially on the abdominal segments) as a series of pores along 

 each margin of the under surface. In most larvae, nearly every segment 

 is provided with a pair; but in the perfect insect several of them remain, 

 closed, especially in the thoracic region, so that their number is often 

 considerably reduced. The structure of the spiracles varies greatly in re- 

 gard to complexity in different insects; and even where the general plan is 

 the same, the details of conformation are peculiar, so that perhaps in 

 scarcely any two species are they alike. Generally speaking they are fur- 

 nished with some kind of sieve at their entrance, by which particles of 



