NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS lOI 



the result of migrations either forward or rearward into the segmental 

 regions of the body. 



The abdominal spiracles are situated, with few exx:eptions, on the 

 anterior lateral parts of the abdominal segments, where they lie 

 in the tergal plates, between the terga and the sterna, or in the edges 

 of the sterna. There are usually eight pairs of abdominal spiracles 

 in adult and larval insects, though the number may be variously 

 reduced. There is evidence, however, of more than eight spiracles 

 having been present on the abdomen of primitive insects. Cholod- 

 kowsky ( 1 891 ) reports the existence of a pair of tracheal invaginations 

 on the first nine abdominal segments of Blattclla,2ind Heymons (1897) 

 says there are apparent rudiments of spiracles on the tenth abdominal 

 segment of Lepisma. In certain insects the spiracles of the first 

 abdominal pair are situated very close to the base of the metathorax, 

 and long discussions recur as to whether these spiracles belong to 

 the thorax or to the abdomen. In all cases, however, it will be found 

 that the spiracles in question lie posterior to the third phragma, which 

 marks the intersegmental line between the metathorax and the first 

 abdominal segment, or behind the lateral extensions of the postnotal 

 plate in the metathorax. The spiracles are therefore abdominal, as 

 is shown also by the destination of their tracheae. 



The external aperture of a spiracle may be a simple opening leading 

 directly from the exterior into the trachea. In most cases, however, 

 there is a pre-tracheal chamber, or atrium (fig. 53 A, B, Atr), formed 

 by an inflection of the body wall, from the inner end of which arises 

 the trachea (Tra). The atrium of the spiracle, therefore, appears 

 to be a secondary invagination of the body wall, which has carried 

 the mouth of the original tracheal invagination to a more protected 

 position beneath the surface. In some cases the edges of the atrial 

 orifice are elevated to form a pair of protruding lips guarding the 

 entrance (fig. 53 A, c, d), in others the opening is fringed with 

 opposing brushes of hairs, usually thickly branched, or it is itself 

 reduced to a very small diameter. 



Spiracles are usually provided with a closing apparatus. In the 

 Apterygota the spiracles are said to lack an occlusor mechanism 

 (Du Buisson, 1926; Davies, 1927), and the thoracic spiracles of 

 Plecoptera are simple apertures giving open passages into the tracheae. 

 In general, however, the spiracles have either a device for closing 

 the outer lips of the atrial chamber, or an apparatus for blocking the 

 passage from the atrium into the tracheae. The occlusor mechanism 

 of the abdominal spiracles is of the second type ; that of the thoracic 



