THE TRACHEAL SYSTEM OF THE BLOW-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 Thoracie 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 tracheze commence. 
The external valve is composed of two valve-plates (Pl. 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 sete, which communicate with 
