624 THE SENSES AND SENSORY ORGANS. 
after the removal of the soft parts, exhibits a small triangular 
and a large somewhat complex chitinous piece attached to the 
membrane; these sclerites can be seen from without through 
the thin membrane. The larger sclerite has two unequal 
arms united at an obtuse angle; the shorter is directed up- 
wards, and the longer backwards, and from the angle of junction 
a third short process projects inwards into the body cavity, 
this process is connected with Miiller’s ganglion. The horny 
ring beneath the stigma exhibits a process projecting into the 
body cavity, which has already been observed by Burmeister.’ 
The Tympanic Membrane of the Blow-fly is about *5 mm. in 
diameter; it is nearly circular, and is surrounded by a circular 
sclerite—the tympanic ring. It’ has a triradiate sclerite 
attached to its central portion, with two processes in the plane, 
and a third at right angles to the plane of the membrane. 
The upper and posterior part of the membrane is thicker than 
the lower and anterior part, and bears a group of sete. The 
triradiate sclerite is connected with the attached margin of the 
great wing-scale by two folds (Pl. XLIII., Fig. 3). 
A Minute Spiracle, °o5 mm. in diameter, surrounded by a hard 
margin, remains permanently open at the deepest part of the 
tympanic pit below, and in front of the tympanic membrane. 
This spiracle, although its orifice is permanently open, is closed 
by a valvular arrangement, a short distance within the external 
spiracular opening. The valve is similar to the valves of the 
abdominal spiracles. 
There is no structure in the Blow-fly which has given me 
DESCRIPTION OF PLATE XLIV. 
The tympanic apparatus of the Blow-fly. 
Fic. 1.—A semi-diagrammatic section of the tympanic apparatus, at right angles to 
the tympanic membrane in a vertical plane. s¢, supra-tympanic organ; g, 
ganglion cells ; 4, hypodermic cells ; # 6, membrana basilaris ; # g, Miillerian 
ganglion ; #, nerve to Miillerian ganglion ; ¢, tympanic membrane ; ¢r ¢7, tracheal 
sacs; f; folds of membrane uniting the tympanic membrane with the outer wall 
of the tympanic fossa. The dotted parts of the diagram are blood sinuses. 
Fic. 2.—A and B, actual sections through the supratympanic organ. 4, bulb of the 
crista ; ¢/, chitinous integument ; c/o, chordotonal organs ; ¢, epithelium form- 
ing the roof of the organ; the other letters as in Fig. 1. 
