624 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxix. 



THE SPECIALIZED TENTHREDINOIDEA. 



Beginning with the families here included, there is found the lirst 

 marked departure from the typical hymenopterous wing. This 

 group is differentiated by the almost entire loss of the base of sub- 

 costa. The onl}' exception is found in certain species of Siricid» (tig. 

 87), in which a subcosta of the type found in Neurotoma (tig. 36) per- 

 sists. It can be traced as a pale, indistinct line through the middle of 

 the area between costa and R+M in wings which have been cleared 

 and mounted in Canada balsam. In addition to the loss of the base of 

 subcosta, there is a decided shortening of cell R, due to the coalescence 

 of radius and media to near the base of the stigma. The w4ngs are 

 longer, narrower, and more efficient organs of flight. The base of the 

 radial sector has lost its prominent transverse curve, and measured 

 along R., extends to the margin of the wing in a regular curve. The 

 stigma has lost its broad quadrate outline, and, except in the genus 

 Blast icotoma (fig. 44), it is narrow and diamond shaped. 



The specialized Tenthredinoidea are divisible into two distinct phyl- 

 logenetic groups on the position of the posterior end of the radial 

 cross-vein, in one ending in the cell R^,, in the other in the cell R^. 

 The position of this cross-vein, together with the position of themedio- 

 cubital cross-vein and the direction of the base of media, mark these 

 groups as very different lines of development. 



The determination of the sequence of these groups in a lineal 

 arrangement has been a difficult one. In the answering of questions 

 of this nature, the rule laid down by Comstock" seems the most avail- 

 able one: 



It seems to me that the most piactictable way of meeting tliiw difiiculty is to l)egiii 

 with the deserij>tion of the most generaUzed form known, and to follow this with 

 descriptions of forms representing a single line of development, passing successively 

 to more and more specialized forms included in this line. When the treatment of 

 one line of development has been completed take up another line, beginning with the 

 most generalized member of that line and clearly indicating in the text that a new 

 start has been made. 



This shows clearly the method of procedure so far as the components 

 of each line of development are concerned, but the difficulty here to be 

 met is the determination of the sequence of the lines of development 

 themselves. For the sake of brevity and convenience in referring to 

 these lines of development, they may be known as the cell Rr, group 

 and the cell R^ group. As is indicated above, in the former the radial 

 cross-vein ends in the cell Rj and in the latter in the cell R^. Both lines 

 contain families that are very generalized and are consequently near 

 the stem form. In the arrangement here adopted, it has been assumed 

 that the group that departs farthest from the condition of the original 

 progenitor of the group should be given the highest rank, because 



« J. H. Comstock, Wilder Quarter-Century Book, y>. 42. 



