1916] 



Forbes — On the Tympanum of Certain Lepidoptera 



189 



cial sclerites, and in this case the hol- 

 low area which normally exists dor- 

 sally between thorax and abdomen is 

 very much enlarged, extending back 

 into the second segment, as a thin- 

 walled balloon. In all variants of 

 type three this fold seems much bet- 

 ter developed than in the primitive 

 condition. 



As to the evolution of these forms I 

 have little doubt that I am correct as 

 to the primitive condition, since it 

 occurs generally in both Micro- and 

 Macrolepidoptera ; from it I should 

 derive type three (in its simpler No- 

 todontid subtype) directly, and from 

 that the more specialized subtypes 

 and type two. Type one seems, how- 

 ever, independent. Working this out 

 as a genealogy we will have a first 

 dichotomy between the primitive con- 

 dition (plus type one) and types two plus three. This corresponds 

 (so far as I recall, exactly) with the division between upright and 

 flat egg, and between the caterpillars with hooks in a single length 

 and those alternately of two lengths, and in those two particulars 

 would dispose of the anomalous position of the Notodontid?e (ex- 

 cluding Apatelodes). It would leave, however, the Notodontidse 

 as a solitary trifid family in a mass of Quadrifidse, and would simi- 

 larly maroon the Lasiocampidiie in a wilderness of trifida\ Possibly 

 the importance of the position of M- in the fore wing is less than 

 it has been considered. Already we give little weight to it in the 

 hind wing, and in certain Hydriomenid Geometridse, such as 

 Eudule. 



Families Examined. 



Primitive type. 

 Microlepidoptera. " Bombycidse. 



Cossidie. Saturniidiie. 



Eucleidse, etc. Sphingida?. 



Eupterotidse (Apatelodes). Lasiocampida?. 



Fig. 4. Same view of Alypia 

 ocfomacidata (Agaristidse). The 

 internal expansion of the fold 

 between thorax and abdomen 

 is indicated by the dotted lines. 

 A strong suggestion of this 

 structure occurs in some Noc- 

 tuidfe. 



