THE TYMPANIC ORGANS OF THE BLOW-FLY. 623 
13. THE TYMPANIC ORGANS OF THE BLOW-FLY. 
a. Structure of the Tympanic Organs. 
(Plate XLIII., Fig. 3, and Plate XLIV.) 
The tympanic organs of the Blow-fly have apparently 
hitherto been entirely overlooked. Their external parts are 
revealed by an examination of the tympanic fissure, when the 
wings are drawn as far forward as possible. A small pit is 
then seen about ‘7 mm. in diameter immediately behind the 
wing-root. The inner wall of the pit is formed by a circular 
tympanic membrane supported by a marginal ring-like sclerite. 
This membrane is very similar to the tympanic membrane 
of the Acridide, which was described by Siebold [278] in the 
following manner : 
‘The thorax of all the Acrididz examined exhibits three 
pairs of spiracles, of which the first pair is situated between 
the pro- and meso-thorax, and the second between the meso- 
and meta-thorax. The anterior pair lie entirely behind the 
prothorax in the soft syndesmosis. The posterior spiracles 
are close to the articulation of the intermediate coxe with the 
thorax, in the seam between the meso- and meta-thorax. 
These four stigmata have horny lips. The third pair of 
thoracic spiracles are behind the thorax, in relation with the 
tympana. 
‘The dorsal half of the first abdominal segment has an oval 
emargination on each side in front, in which the tympanum is 
situated. The edge is a thickened horny rim, which more or 
less completely surrounds an extremely thin membrane, by 
which the emargination is closed. The horny margin of this 
membrane is triangular in front and below, and surrounds the 
third thoracic spiracle. This spiracle differs from all the other 
spiracles in having no lips, so that it remains permanently 
open. The oval tympanic membrane is darkly punctate; the 
punctation is augmented in places, so that the membrane 
assumes a brown colour. 
‘ The tympanic membrane examined from its inner surface, 
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