64 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.39. 



secutive elements, as so often described, but consists of one principal 

 wing-bearing plate and of a secondary postalar plate, the first of which 

 becomes differentiated into the secondary regions termed prescutum, 

 scutum, and scutellum; and, second, to show a logical reason for this 

 structure, based on the necessity for it, arising when the primitive 

 segmental muscles had to take on them the newly acquired duties of 

 moving the wings. 



4. STRUCTURE OF THE IIYMENOPTERAN THORAX. 



This paper is designed especially to elucidate the external mor- 

 phology of the thorax of the nonaculeate Hymenoptera. Therefore 

 the Aculeata have been illustrated by only three forms selected from 

 three representative families. The writer has, furthermore, made no 

 attempt to apply the facts of anatomy to the classification of the 

 families. This must be done by systematists who are widely ac- 

 quainted with the comparative structure of all the different parts of 

 the body. Writers who become intimately acquainted with one set 

 of characters are ever prone to reconstruct classifications on a basis 

 of their specialty and are as often misled by the narrowness of their 

 horizon. Any system of taxinomy or phylogeny must be founded on 

 a consideration of all the characters of all the forms concerned. 



The following is a list of the species studied, arranged according to 

 the present classification by Hymenopteran systematists : 



I. TENTHREDINOIDEA. 



Pamphilid^. 



Bactroceros pallimacula (Norton). — fig. 17. 

 Tenthredinid^ . 

 Argin^. 



Arge, species. — figs. 10-12, 14, 15. 

 Nematin^. 



Lygsenematis ericJisoni (Hartig). — fig. 18. 



DOLERIN^. 



Dolerus aprilis Norton. — ^fig. 13. 



ClMBICIN^. 



Trichiosoma lanuginosa Kirby. — figs. 16, 19. 

 II. SIRICOIDEA. 



SlRICID^. 



Tremex columha (Linnaeus). — figs. 1-9. 

 III. ICHNEUMONOIDEA. 



Braconid^ (Of the numerous subfamilies of this group the 

 following two have been selected as the ones 

 most likely to show the extremes of variation). 

 Braconin^. 



Euurohracon penetrator (Smith). — fig. 20. 

 Aphidiin^. 



Diseretus ficeus (Cresson). 



