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 tin- 

 body of the insect (espe- 

 cially on the abdomi- 

 nal segments) as a series 

 of pores along each 

 margin of the under sur- 

 face. In most larva-, 

 nearly every segment is 

 provided w T ith a pair 

 in the perfect 

 several of them remain 

 closed, especially in the 



but 



insect 



thoracic region, so that 



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

 with some of its principal branches. 







^m ^tflP^B ^ their number is often con- 



T $Jf^ gprv.^ siderably reduced. The 



gM \ structure of the spiracle- 



nj ; H varies greatly in regard t< > 



complexity in different in- 

 sects ; and even where 1 he 

 general plan is the same 

 the details of conforma- 

 tion are peculiar, so thai 



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

 ing, 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 arbo- 

 rescent growths from the 

 border of the spiracle, as 

 in the common fly (fig. 

 743), or in the J)i/tiscus ; 

 or it may be a membrane 

 perforated with minute 

 holes, and supported upon 

 a framework of bars that 

 is prolonged in like manner 

 from the thickened margin 



FIG. 743. Spiracle of common fly. of the aperture (fig. 744), 



as in the larva? of the 



Melolontha (cockchafer). Not unfrequently the centre of the aper- 

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

 to its margin, as is well seen in the spiracle of Tipula, (crane- 

 fly). 1 In those aquatic larvae which breathe air we often find one 



1 Consult Landois and Thiele, ' Der Tracheenverschluss bei den Insecten,' Zcit- 

 schriftf. wiss. Zool. xvii. p. 187. 



