996 



INSECTS AND ARACHNIDA 



by the crossing of the two sets of fibres, of which one overlies the 

 other. That this appearance, however, is altogether an optical illu- 

 sion may be easily demonstrated by carefully following the course 

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

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

 tracheal system are generally visible on the exterior of the 



body of the insect (espe- 

 cially on the abdomi- 

 nal segments) as a series 

 of pores along encli 

 margin of the under sur- 

 face. In most larv.-e. 

 nearly every segment is 

 provided with a pair, bur 

 in the perfect insect 

 several of them remain 

 closed, especially in the 

 thoracic region, so that 

 their number is often con- 

 siderably reduced. The 

 structure of the spiracle> 

 varies greatly in regard to 

 complexity in different in- 

 sects ; and even where the 

 general plan is the same 

 the details of conforma 

 tion are peculiar, so that 



perhaps in scarcely any two species are they alike. Generally speak- 

 ing, they are furnished with some kind of sieve at their entrance by 



v / 



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 arbo- 

 re.Mvnt growths from the 

 border of the spiracle, as 

 in the common fly (fig. 

 743). or in the I ti/tiscti.-; : 

 or it may be a membrane 

 perforated with minute 

 holes, and supported upon 

 a framework of bars that 

 is prolonged in like manner 

 from 1 lie thickened margin 

 of 1 he aperture (fig. 744), 

 as in the larva- of the 

 Melolontlta (cockchafer). Not iinfrei|iient ly the centre of t he aper- 

 ture is occupied by an impervious disc, from which radii proceed 

 to its margin, as is well seen in the spiracle of Ti/mln (craiie- 

 lly). 1 Ill those aquatic larva- which breathe air \\ e often find one 



1 Consult Landois and Thiele, ' Der,Tracheenverschlusa lu.'i den Insecti-n,' 



m 'Itrifl f. WISS. /.lint, xvii. p. 187. 



FIG. 742. Portion of a large trachea of Dytiscus, 

 with some of its principal branches. 



FIG. 743. Spiracle of common tly. 



