1918] Morphology of Genitalia of Insects 111 



Hemiptera. 

 1893 1894 1896 Vcrhoeff 11 abdominal somites 



Lepidoptera. 

 1832 Burmeister 9 abdominal somites 



1893 Kolbe 9 abdominal somites 



1894 Peytoureau 10 abdominal somites 



1903 Berlese At least 10 abdominal somites 



Trichoptera. 



1849-53 Lacaze-Duthiers, typical number in insects 11 abdominal somites 



1886 1888 Grassi, typical number in insects 10 abdominal somites 



1901 Zander 13 abdominal somites 



From investigations on Thysanura, according to Grassi, 

 there are 10 abdominal somites besides "ein Afterstiick, das 

 allerdings nicht mehr als Segment aufzufassen ist;" according 

 to C. Brunner von Wattenwyl, this is a terminal segment, 

 welches aus den drei Afterklappen und den seitlich inserirten 

 "Anhangseln (Cerci) gebildet wird." 



Berlese concludes his discussion of the matter by saying 

 that the entire number of abdominal somites varies from nine 

 to eleven, except for the Collembola, which he considered 

 aberrant, this reduction taking place from the lower to the 

 higher orders, and at the caudal as well as at the cephalic end 

 of the abdomen. He states emphatically that eleven is not 

 exceeded, aside from the telson with its rudiments, found only 

 in certain of the Thysanura and immature Orthoptera, and he 

 therefore would consider eleven as the typical number. 



Certain other investigators, represented mainly by Hey- 

 mons, uphold the view that the primitive number of abdominal 

 segments is twelve; the twelfth, just as much as the eleven 

 cephalad of it, containing a caelomatic cavity. Heymons calls 

 this twelfth somite the telson, and his evidence for it is found 

 mainly in certain Thysanura, where he identifies it with a small 

 sclerite, called by many the supra-anal plate, situated between 

 the eleventh tergite and the anus. He also describes it in 

 immature Acrididee, Locustid^, and Gryllidae as a triangular 

 plate separated from the eleventh tergite, to which it forms the 

 apex, by a suture. In the adults of some Acrididce this plate 

 becomes totally fused with the eleventh tergite. In this case 

 the terminal segment is commonly called the eleventh only, 

 instead of the eleventh and twelfth, according to Heymons. 

 Berlese gives another example of the twelfth tergite in Acrida 

 turrita L., a form not studied by Heymons. (Plate VI, Figs. 1, 

 10, 13, and 17 t 12.) 



