374 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



ences, and this notwithstanding the presence of the tympanum in the 

 noctuid and its absence in the saturniid. No single feature of the 

 noctuid musculature appears to be uniquely associated with the tym- 

 panic organ. Each of the muscles of the tympanic region has its 

 counterpart in Telea, and there seems no reason to suppose that the 

 function of any of them is drastically different in the two insects. 

 Muscle c?/ib (Eggers's median muscle) is so situated that its contrac- 

 tion could have but little if any effect upon the tympanic air sac. 

 More than likely it is a functionally unimportant vestige of a longi- 

 tudinal flight muscle. Muscle dha (Eggers's separate fan-shaped bun- 

 dle of the median muscle) might be capable of expanding the air sac 

 by direct action, though this effect would presuppose a firmer origin 

 than the prescutum appears to afford. Muscle d/2 (Eggers's middle 

 muscle, mM) runs almost tangent to the air sac from the scutum to the 

 anterior tendon plate {Muskelleiste of Eggers), which seems braced 

 to resist displacement in this direction. There is nothing to suggest 

 that this muscle could affect the tension of the tympanic membrane, 

 and in any event it has been shown (Roeder and Treat, 1957) that 

 tension in this structure is not essential to acoustic reception. From 

 its position, one would expect the action of dk to depress the antero- 

 lateral area of the scutum and thus to aid in elevating the wing. 

 Muscle dVi(^5) (Eggers's lateral muscle) could scarcely affect the tym- 

 panic region in any way ; it is probably effective in elevating the wing 

 and is classed as a tergal remotor of the coxa. Muscle pVi^s), by 

 virtue of its insertion on the subalare, might compress the lateral end 

 of the tympanic air sac indirectly as an incidental consequence of 

 wing movements. Snodgrass (1935, p. 206) describes this muscle in 

 the adult Dissosteira as a depressor-extensor of the hind wing ; it may 

 well have a similar action in Crymodes, though its effect on the tym- 

 panic air sac is hard to evaluate. It is noteworthy that the metascutum 

 is divided into anterior and posterior portions along a hingelike line 

 which follows the base of the scutal phragma. The anterior portion, 

 to which most of the vertical flight muscles are attached, can thus be 

 depressed without marked displacement of the posterior part which 

 roofs the tympanic air sac. 



Eggers thought that the structure of the tympanic organ was such 

 as to resist the deforming stresses that might result from muscular 

 activity.^ Detailed consideration of the metathoracic muscles tends to 



3 "Einigermassen fixiert ist die Form der Blase durch zwei kraftige, nach 

 innen gebogene Chitinleisten, die sie einfassen und einer Zerrung und De- 

 formierung durch die Tatigkeit der Muskulatur entgegenwirken." — Eggers, 1919, 



