LEE BARKER WALTON 165 



of the caudal segments, seems better in accord with the 

 facts as I have already suggested (1913). Furthermore, 

 on such a basis one might explain the paired eversible 

 sacks of the abdominal coxae in many Thysanurans as 

 modified integumentary acicular sacks of the notopodium 

 and neuropodium which in most forms had become re- 

 duced to a single sack through fusion or suppression. In 

 the single case known where there are three sacks on a 

 "coxa," some of the sacks if actually separate would have 

 to be accounted for as secondary. These suggestions, 

 however, relate to problems which are beyond the scope 

 of the present paper and for their solution demand a larger 

 background of embryology and morphology than exists 

 at the present time. 



The dorsal appendages of the segment as represented 

 in Fig. 3 consist of an evaginated sack forming the wing 

 here designated as the pteron, and the area anterior to its 

 base, which from its constancy throughout the insects 

 both on the mesothorax and on the metathorax is in- 

 cluded as a part of the typical segment and termed from 

 its position the propteron. Like the wing it is a hollow 

 segment where structurally large enough to examine and 

 in all probability has similarly developed as an integu- 

 mentary outpushing. Various names such as pterygo- 

 dum, parapteron, tegula, squamula, etc., have been ap- 

 plied to it on the different segments in various orders of 

 insects, and it might seem that less confusion would re- 

 sult from the retention of some of these names. While 

 the term pterygodum of Latreille (1820-22) has priority, 

 it was used in a restricted sense for the part at the base 

 of the mesothoracic wing in the Lepidoptera and is rarely 

 noted in the literature relating to the insects. The more 

 familiar term, parapteron, was first used by Audouin 

 (1824) to designate a supposed sclerite which he believed 

 to exist on the anterior margin of the mesepisternum of 

 Dytiscus circumflexus Fab. This was merely an area 

 bounded posteriorly by the articulatory ridge at the point 

 of junction with the prothorax, extending in many Cole- 

 optera entirely across the sternum. The transfer of the 

 term to the pieces in front of the wing (propteron) arose 



