1903] SNODGKASS:— THE TIPULID //VJ'OPVG/UM 1 89 



Turning now to the females we find less variety in the terminal parts of the 

 abdomen throughout the entire family than may characterize the males of one 

 genus. The female parts are constructed as follows: The eighth sternum is pro- 

 longed far back of the eighth tergum, and is terminaied by two elongate append- 

 ages set on edge beside each other. The ninth segment is much smaller in diam- 

 eter than the eighth, and consists of a narrow ring of chitin. The tenth segment 

 is generally elongate and terminates in two usually long and tapering cerci. The 

 dorsal side of the eighth sternum is concave and the concavity is covered above 

 by the ventral parts of the ninth and tenth segments. Rudimentaiy gonapophyses 

 are present in the form of small plates or bars of chitin on the upper side of the 

 eiglith sternum and on the opposed ventral side of the ninth. 



Within the group Tipulina the female genital parts differ almost exckisively in. 

 the form of the rudimentary gonapophyseal plates, and usually only the tips of 

 these project as free processes. On the other hand, the modifications of the male 

 parts within this group are excessive. Within the other groups the genera are 

 characterized by slightly varying shapes of the external female parts, but in none is 

 there any variety comparable with that existing between the males. 



In other words, it is impossible to point out any correlation between the great 

 variations of the male hypopygia and the extremely slight modifications of the 

 female genitalia. How, then, are we to explain the origin of the former ? If the 

 modifications in the structure of the body of the hypopygium, the varieties in size 

 and shape of the guard of the penis and the clasping apical appendages are not 

 adaptive, why do they exist .-' The conclusion seems to be unavoidable that some 

 other cause for specific variety must be operative here than that of adaptation to 

 the female organs. That the more fundamental modifications of the hypopygium 

 had an early origin in the evolution of the family is evinced by the fact that they 

 are characteristic of the larger groups into which the family has been separated on 

 other characters. 



It is such a common method of explanation in the case of specific variations 

 such as one meets with in these Tipulid hypopygia, to assume that they are adap- 

 tive, that it is interesting to see how such explanations may fail to be substantiated 

 when tests are actually made. It can, of course, be said that our knowledge of 

 the physiology of the parts is insufiicient, and that on this account nicely balanced 

 interrelations are not perceived. A failure to disprove, however, is not a proof of 

 the opposite, and no one can ever say of anything that his knowledge of it is 

 complete. 



