THE TRACHEAL SYSTEM OF THE BLOU'-FLY. 359 



or sometimes three pairs which open into the cloaca ; they 

 are situated in the corresponding region of the invaginated 

 cloacal segments. 



The abdominal spiracles are usually segmental, except the 

 first pair, which are most frequently placed on the syndesmosis 

 in front of the first abdominal segment. Dufour [19] con- 

 sidered the segmental or inter -segmental position of the 

 spiracles of the abdomen in the Diptera as of great import- 

 ance ; but it appears that the position of these spiracles not 

 only varies in different species of the same family, but also in 

 different individuals of the same species. All the abdominal 

 spiracles are probably inter-segmental at first, and they become 

 enclosed in the chitinized plates of the dorsal arches, more or 

 less completely, in relation to the size of these plates. In 

 other words, the plates only enclose the spiracles when they 

 are of large size, and the spiracles remain in the inter-segmental 

 syndesmoses when the plates are little developed. 



The second and fifth abdominal rings are always broader 

 than the others, and they always enclose the spiracles, whilst 

 the other plates either only just reach the spiracles, or the 

 latter remain in front of them in the soft syndesmotic integu- 

 ment. 



The Thoracic Spiracles. The anterior and posterior thoracic 

 spiracles differ in form, but exhibit the same valvular arrange- 

 ment, and are identical in all essential particulars in structure. 

 The posterior pair are larger, and are more easily examined, 

 especially in the living insect. I shall therefore describe them 

 and merely indicate the differences between them and the 

 anterior pair. 



The Posterior Thoracic spiracle is closed by an external valve, 

 the epitreme, within which there is a shallow cavity, the 

 vestibule ; the internal wall of the vestibule forms an internal 

 valve, beyond which the membranous tracheae commence. 



The external valve is composed of two valve-plates (PI. XXII., 

 Fig. 2). Each consists of a thin membrane, strengthened by 

 a number of dendriform tubes, and covered on either surface 

 by numerous diverging hollow setae, which communicate with 



