186 Psyche [December 



For the normal, and primitive condition in the Lepidoptera 

 (except the very lowest Tineoids and Jugatse), we may take Apa- 

 telodes, a somewhat isolated genus of the Bombycid stem (Fig. 1). 

 Here the metathorax is moderately developed, and there are no 

 specializations which do not occur all the way from the Cossidae 

 to the Saturniidje. The scutum and scutellum are normal, the 

 cord connecting the scutellum with the inner margin of the wing 

 is somewhat wavy, but forms no distinct lobe, the epimeron is 

 V-shaped as in the mesothorax, and though too lightly chitinized 

 to be quite sure of its boundaries shows no sculpturing except a 

 slight articular surface where it meets the tergopleural line of the 

 abdomen. The postscutellum is not sharply set off from it, and 

 the entire structure is essentially as in the mesothorax. The first 

 segment of the abdomen is, as usual in Lepidoptera and related 

 insects, largely membranous; the tergum is a rectangle of mem- 

 brane, enclosed in an anterior band which articulates subdorsally 

 with the thorax, a posterior band, extended well down on the 

 sides, but not reaching the ventral region, and a pair of heavily 

 chitinized lateral grooves; — the latter seem to represent the ter- 

 gopleural suture, so that the narrow edge of chitin below them 

 should be considered the pleurite. The front end of this groove 

 runs to a shallow pocket between thorax and abdomen. The 

 lateral region is wholly membrane, containing a normal spiracle. 

 Ventrally the first two segments are not distinguished, and are 

 heavily chitinized only in front where they lie against, and are 



slightly grooved by, the posterior 

 coxse. The external modification to 

 form the first or Geometrid tym- 

 panum is merely the enlargement 

 of the coxal groove into an ample 

 cavity, and the distortion of other 

 structures to make room for it. 



The second type of tympanum oc- 

 curs in the Arctiidse, Lymantriidse, 

 Syntomidse, Pericopidse, etc. (Fig. 2.) 

 In this the thorax is undisturbed, 

 Fig. 2. Same view of Hemer- though in the forms I have studied 



ocamipa leucostigma (Liparidae), ^^^^^e is the usual lack of definiteness 

 showing second type ot tym- 

 panum. ' " of the epmieron; the abdomen dor- 



