1918] Morphology of Genitalia of Insects 119 



anlages of the appendages of an embryonic somite. In fact, 

 Heymons has called the cerci " modifizierte Extremitaten," and 

 he states very emphatically that the cerci are homologues of the 

 above-mentioned styles of some Thysanura, just as those styles 

 themselves are descended from ancestral abdominal appendages. 



6. Acknowledgments. — It is with the foregoing points 

 of theoretical importance in mind that the following investiga- 

 tion is presented. At this point it would seem fitting to state 

 that the investigation was begun at the suggestion of Dr. Alex. 

 D. MacGillivray, of the Department of Entomology, University 

 of Illinois, and I wish to express my gratitude to him for that 

 and for countless timely suggestions and criticisms. I am 

 indebted to Professor S. A. Forbes for many courtesies and for 

 the use of material from the Illinois State Laboratory of Natural 

 History, and also to the staff of that institution. I also desire 

 to thank Dr. Harris H. Wilder, of the Department of Zoology 

 of Smith College, for many important suggestions and criticisms. 

 Dr. H. S. Pratt, of the Department of Zoology of Haverford 

 College, very kindly collected for me specimens of Mantis sp. 

 from a colony introduced on nursery stock, near Haverford, and 

 I would at this point express to him my thanks. For consid- 

 erable mechanical help with the drawings and their arrangement 

 in the plates, I am very grateful to Miss N. B. Lewis, of Smith 

 College. 



7. Material. — The records upon which we must rely in 

 determining the origin of the genitalia are the anatomy of 

 generalized adult forms, a comparison of these with more 

 specialized species and earlier developmental stages. Therefore, 

 this paper consists of a comparative study of the caudal seg- 

 ments and appendages of adults of the following species: 

 Lepisma saccharina and Machilis sp. of the Thysanura; Peri- 

 planeta orientalis, Melanoplus differentialis, Dissosteira Carolina, 

 Xiphidium ensiferum and Gryllus pennsylvanicus of the Orthop- 

 tera; Tibicen septemdecim, Notonecta u?idulata, Benacus griseiis, 

 Corixa sp., Anasa tristis and Euschistus variolarius of the 

 Hemiptera; Corydalis corniita of the Neuroptera; Panorpa 

 luguhris of the Mecoptera; Nerophilus calif ornicus of the 

 Trichoptera ; Harpalus caliginosus, Cybister fimbriolatus, Hydro- 

 philus triangularis and Doryphora decemlineata of the Coleop- 

 tera; Hepialus hiimidi, Prionoxystus robinice, A gratis ypsilon, 

 Anosia plexippus and Phlegethontius sexta of the Lepidoptera; 



