528 



W. L. TOWER, 



The wings of all Coleoptera develop in exactly the same way 

 up to and through the formation of the first invagination, but from 

 that point until the pupal stage is reached the type of wing develop- 

 ment varies in different families and species. Three types of imaginai 

 discs were recognized by Gonin (1894) and were called by him the 

 Corethra^ Pieris and Musca types. From my observations upon Co- 

 leoptera and several Holometabolous insects and Pratt's (1900) upon 

 Melophagus^ the list of types of imaginai discs is extended to at least 

 five with numerous gradations between the first four. 



Fig. A. Diagrammatic representation of the rise and development of the "simple 

 type" of imaginai disc. 



1) Simple type. The fundament of the imaginai organ not 

 sharply marked off from the body hypodermis and usually directly 

 evaginated to form the imaginai organs. The evagination is usually 



preceded by a slight invagin- 

 ation. Characteristic of all 

 known Heterometabola and 

 many families of Coleoptera. 

 It is the dominant type of 

 wing development in beetles. 

 2) Recessed type = 

 Corethra type of Gonin. The 

 fundament is invaginated 



Fig. B. Diagrammatic representation of the and then evaginated and SO 

 rise and development of the "recessed type" of Jigg jj, ^ shalloW niche in 



imagina isc. ^^^ hypodcrmal wall, but 



opens broadly against the cuticula.' Scarahaeidae in the Coleoptera 

 and Nematocerous Diptera. 



