52 Annals Entomological Society of America [Vol. XV, 



accessory genitalia is an exceedingly difficult one but is outside 

 the scope of the present paper. Their structure has been ably 

 dealt with by Schmidt ('15). 



The 10th segment has a remarkably well developed sternum, 

 which, however, often shows a tendency to fuse with the 

 tergum, although its boundaries may be clearly defined. 



Behind the 10th segment are two paired processes and an 

 unpaired one, which upon anatomical grounds alone would be 

 unhesitatingly interpreted as the cerci, paraprocts and supra- 

 anal plate. In the Anisoptera what appear to be the cerci 

 and supra-anal plate are the structures termed by systematists 

 the superior and inferior appendages, these structures serving 

 as claspers in copulation. In the Zygoptera the homologue of 

 the inferior appendage is rudimentary in the adult, but bears 

 the median caudal gill in the nymph, while the structures called 

 the inferior appendages in the adults and which bear the lateral 

 caudal gills in the nymph are apparently the paraprocts. 

 In the adults of Anisoptera they are inconspicuous and do not 

 function as claspers as they do in the Zygoptera. 



This interpretation of these parts, which seems obvious to 

 the comparative anatomist, is denied by Heymons and others 

 on embryological grounds, the apparent -paraprocts being 

 interpreted as the true cerci, while the apparent cerci are 

 regarded as secondary structures, termed "cercoids." Three 

 small papillae, a dorsal and two ventral, which surround the 

 anus in the nymphs of some dragonflies, are considered to be 

 the true supra-anal plate (telson) and subanal plates (para- 

 procts) ; the apparent supra-anal plate being the 11th tergite. 

 I believe that further investigations will show that the former, 

 more obvious interpretation is the correct one. It has full 

 support from the standpoint of comparative myology, as 

 determined by Miss Norma Ford, whose results, however, 

 are not yet published. Moreover, if we adopt Heymons' 

 view we are forced the improbable conclusion that the 11th 

 segment is an embryonic structure in all insects except the 

 Odonata, while in this order it is highly developed and 

 morphologically exactly replaces the 12th segment of other 

 insects. 



