XXX'ii, '21] ENTOMOLOGICAL NF\VS 261 



lateral and ventral to the eleventh tergite, ep, and consequently 

 the basal plates pr develop embryologically in a position some- 

 what lateral and ventral to the eleventh tergite ep, thus causing 

 Heymons to mistake these basal plates for the sternite of the 

 eleventh abdominal segment in insects, despite the fact that 

 they develop as two distinct plates, whereas a true sternite never 

 develops as two distinct plates in any insects of which I have 

 any knowledge. This suggests that Heymons is mistaken in 

 thinking that the cerci are appendages of the eleventh segment 

 (since he mistakes the basal plates bearing the cerci, for the 

 eleventh sternite) and it is quite possible that the basal plates 

 and cerci belong to the tenth segment instead of to the eleventh, 

 their position below and beside the eleventh tergite, ep, being 

 merely due to the fact that they are borne on the hinder margin 

 of the tenth segment; and Heymons himself admits that the 

 cerci, en, of Fig. 9 are appendages of the tenth, rather than the 

 eleventh segment from his study of the development of these 

 structures in immature damselflies. 



The median terminal gill // of the immature damselfly shown 

 in Fig. 9, apparently corresponds to the median terminal fila- 

 ment // of Apterygota (Figs. 2, and 6), mayflies (Fig. 3). etc. 

 When the immature damselfly becomes adult, this median 

 terminal gill // breaks off along the dotted line shown in Fig. 9. 

 though traces of the eleventh tergite ep of Fig. 9 may be 

 retained in adult damselflies, as may be seen by referring to 

 Fig. 6, Plate 2, of an article on the terminal structures of in- 

 sects, published in Vol. 13, of the Bulletin of the Brookl\n l-.u/ 

 Society for June 1918, where the structure in question bears 

 the label sa. In the article in question, the cercus, en, of Fig. 

 ( ) df this paper bears the label c, while the basal plate, pr, ot" 

 Fig. 9 of this paper bears the label pa. The parts of the adult 

 insect are figured and homologized in the article published in 

 the Bulletin, so that it is not necessarv to repeat the figures 

 here, since anyone interested in the subject may readilv refer 

 to the publication in question for further information on the 

 subject. \Yhen one compares the parts in a series of insects 

 and Crustacea, it seems almost incredible that the true signifi- 

 cance of the parts should have been so misunderstood, and this 



