THE TYMPANIC ORGANS OF THE BLOW-FLY. 623 



13. THE TYMPANIC ORGANS OF THE BLOW-FLY. 



a. Structure of the Tympanic Organs. 

 (Plate XLIIL, 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 Acrididse, which was described by Siebold [278] in the 

 following manner : 



' The thorax of all the Acrididas 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 coxae 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, 



