572 



INSECTS AND ARACHNIDA. 



winding of a continuous spiral fibre through their interior, the 

 fibre is broken into rings, and these rings do not surround the 

 whole tube, but are terminated by a set of arches that pass from 

 one to another (Fig. 287, a). When a portion of one of the 

 great trunks with some of the principal branches of the tracheal 

 system has been dissected out, and so pressed in mounting that 

 the sides of the tubes are flattened against each other (as has 

 happened in the specimen represented in Fig. 291), the spire 

 forms two layers which are brought into close 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. That this appearance, however, is altogether 

 an optical illusion, may be easily demonstrated by carefully 

 following the course of any one of the fibres, which will be 

 found to be perfectly regular. 



392. 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 lai'vse, 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 regai'd 

 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. Gene- 

 rally speaking, they are furnished with some kind of sieve at 

 their entrance, by which particles of dust, soot, &c., which would 

 otherwise enter the air-passages, are filtered out ; and this sieve 

 may be formed by the interlacement of the branches of minute 

 arborescent growths from the borders of the spiracle, as in the 

 common Fly (Fig. 292), or in the Bytisous ; or it may be a mem- 

 brane perforated with minute holes, and supported upon a frame- 



Fio. 293. 



Spiracle of common Fly. 



Spiracle of Larva oi Cochchafer. 



work of bars that is prolonged in like manner from the thickened 

 margin of the aperture (Fig. 293), as in the larva of the Melo- 



