PROC. ENT. SOC. WASH., VOL. 20, X;). :i, MAR., 1918 57 



study of the intersegmental region has been made, it will be im- 

 possible to interpret the different intersegmental plates with 

 any degree of certainty or satisfaction. 



Relationship Indicated. 



As is apparently true of many features which are of no vital 

 importance to the organism (and would not therefore be greatly 

 modified by natural selection or by use and disuse) the thoracic 

 sclerites furnish many valuable clues as to the relationships of 

 the different groups of insects. In the case of larvae, as is true of 

 the adult forms, it is necessary to examine the most primitive 

 insects, and these are usually too rare to be readily accessible for 

 study. Furthermore, an insect which has preserved certain other 

 features in a comparatively primitive condition will frequently 

 be quite highly specialized in the particular feature one wishes 

 to study; so that it is largely a matter of chance whether one ob- 

 tains the proper material for his purpose, or not. Some of the con- 

 clusions to be drawn from an examination of the larvae available 

 for study, despite the incompleteness of this material, are in 

 harmony with the results obtained from other sources, and may 

 therefore be regarded as of general application. 



Among the most interesting of the results here obtained, are the 

 evidences of relationship obtained from a comparison of the scler- 

 ites of the nymphal Plecopteron Perla with those of the Euplexop- 

 teron Arixenia. Thus, in comparing the sclerites of the nymphal 

 Perla (fig. 15), part for part, with those of Arixenia (fig. 13), 

 it is at once apparent that there is a remarkable similarity be- 

 tween the two a similarity which is all the more striking when one 

 compares the sclerites of Perla with those of other insects such as 

 those shown in figures 10, 18, 19, 23, etc., all of which differ 

 markedly from the Plecopteron in question. The relationships 

 indicated by the sclerites are borne out by a comparative study 

 of the head region in various Plecoptera and Euplexoptera, 

 and by a comparison of the segmented cerci of certain Euplexop- 

 tera, such as nymphs of Di-[>l<tttjs (in which segmented cerci pre- 

 cede the forceps-like structures of the adults) with the segmented 

 cerci of certain Plecoptera, etc. Since the Plecoptera are the 

 more primitive of the two groups of insecfs, the probabilities 

 are that (he Kuplexoptera arc descended from ancestors not very 

 different from modern Plecoptera. 



The sclerites of an Ephemerid nymph (fig. 2) bear a slight 

 resemblance to those of a nyinphal Plecopteron (fig. 15), but are 

 more similar to those of a larval Neuropteron (fig. 18). The 

 sclerites of the Ephemerid (fig. 2) differ more from those of a 

 nymphal Odonatan (fig. 11) than one would be led to expect 



