STRUCTURE OF STIGMAL PLATE 667 



which lie on the basal layer and which shine through. If the entire 

 spiracle is examined after it has been cleared in caustic potash and 

 mounted on a slide, it will be seen to be a complex structure made 

 up of three superimposed layers, the relations of which are best studied 

 in transverse and longitudinal sections. The superficial layer has a 

 reticulated appearance, with circular meshes which represent the small 

 pores the openings of which are seen on the surface as clear spots. 

 Below the superficial layer there is the pedicel layer, which is traversed 

 by delicate rods, the pedicels, which arise from the basal layer of the plate. 

 The basal plate is composed of thick chitin and is pierced by minute 

 pores which correspond in position to the superficial pores, but which 

 are impervious to air. The pedicels themselves are triangular in section, 

 and are fused together at their upper ends on the ventral surface of the 

 superficial layer. 



That part of the stigmal plate which lies below the macula has a differ- 

 ent structure. It is formed of a columnar mass of connective tissue and 

 muscle fibres which extend upwards from the soft structure beneath the 

 spiracle. Nuttall and his collaborators have named this the columella. 

 Surrounding the columella there is a circular air space, the pericolutnellar 

 space, which communicates with the spaces between the pedicels in the 

 pedicel layer. The ostium or stigmal pore opens into the pericolumellar 

 space on the inner side of the columella, and that portion of the pericolu- 

 mellar space immediately below the ostium is continued downwards 

 as a large chamber, and has been named the atrium by the above observers. 

 The main tracheal trunks open separately into the cavity of the atrium, 

 which is elliptical in cross section. Extending up the columella there is 

 a muscular band which is inserted along the dorsal wall of the atrium, 

 some of the fibres being inserted also into the superficial layer just below 

 it. Nuttall and his collaborators believe that the contraction of this 

 muscle dilates the cavity of the atrium and at the same time probably 

 closes the ostium ; ' this causes the inspired air to filter through the 

 'external pores of the superficial layer'. They consider that the expul- 

 sion of the air is brought about by the contraction, of the dorso-ventral 

 body muscles, which express the contained air through the spiracle; 

 the air is drawn into the tracheal tubes when the action of the muscles 

 ceases. 



In her account of the spiracle of Ixodes ricinus, Samson describes g. 

 valve-like structure which she considers is brought into play in order to 

 prevent the air from entering the atrium ; ticks, it should be noted, may 

 remain a long time under water without being drowned. Opening 



