1916] 



Forbes — On the Tympanum of Certain Lepidoptera 



185 



T3*A1 



Fig. 6. Tympanum with its principal muscles and nerve connections, of the 

 Geometridse (drawn from Biston ursaria and Pkigalia fitea). Only two of the 

 numerous air-sacs are indicated, the one lying ventrad of the nerve, and along 

 which the nerve runs from the tympanum to the spiracle, and the large double one 

 connecting the tympana. The latter appears ventral in the dissection, which was 

 opened out on the middorsal line, but in fact lies above the digestive system. 

 When the dissection is first opened it is normally filled with air and may We the 

 most conspicuous object in the field. 



rid?e, but appears to be omitted in Prout's consideration of the 

 (Enochrominse (Genera Insectorum, fasc. 104). A superficial 

 study of the single dry specimen available shows that there is no 

 true tympanum, either of Geometrid or any other type, but there 

 is a slight thickened bar that may represent the bar across the 

 drum-head of the tympanum, lying in the membrane of the ven- 

 tral surface. There is no definite suggestion of Notodontid struc- 

 ture; and I believe that the genus represents a new family type, 

 leading up to the Geometridtie from something intermediate be- 

 tween the Dioptidse and Notodontidse, — ^probably direct from the 

 unknown Geometrid-Bombycid ancestral form. There seems 

 to be nothing in venation or mouth-parts to contradict this refer- 



