MICROSCOPIC OBJECTS. 77 



Fig. 153. Spiracle of Blow -Fly (Musca vomitoria), x 40. 



Other forms of spiracles are drawn in figs. 122, 139, &c. ; 

 the form is generally oval. To prepare this spiracle for 

 mounting, the thorax of the Fly must first be macerated in a 

 strong solution of potassa for two days, then cut open and 

 well washed. A spiracle may then be cut away, and mounted 

 separately ; or the entire thorax with its four spiracles may be 

 dried under pressure between two glass slips, soaked in tur- 

 pentine until transparent, and mounted in balsam. 



Fig. 154. Spiracle of the Breeze-Fly of the Horse ((Estrus 

 equi) , x 20. 



Another peculiar form of spiracle is drawn here ; it rather 

 represents a number of small spiracles massed into one. In 

 the centre of each ring which passes round the worm-like 

 bodies will be seen a small hole ; through these fine holes the 

 air passes into the tracheae ; and by the contraction of these 

 worm-like bodies no doubt the holes are closed at will. The 

 Breeze-Fly of the sheep ((Estrus ovis) has quite a differently 

 shaped spiracle, although the characteristics of the structure 

 are the same, viz. a number of minute holes closed at will by 

 an elastic or flexible tissue. 



The method of preparing this spiracle is the same as that 

 given in the description of the preceding figure. 



