180 H. R. SNODGRASS. 



paper. The word has generally been used to signify the entire pos- 

 terior enlargement of the male abdomen. Since, however, this is 

 an extremely variable structure, and since, also, in many insects 

 there is no enlargement of the abdomen, the word used in this fash- 

 ion must have a very indefinite meaning, or none at all when the 

 male genital parts do not form an enlargement. "The hypopygium" 

 would, within the same genus, in some cases mean one segment and 

 in other cases mean several segments. Within the same family some 

 genera would have large hypopygiums and others none at all. 

 Hence, it seems best, in order to give the term a definite significance, 

 to make it mean the genital segment of the male, i. e., the segment 

 that carries the intromittent and clasping organs. This is, in all 

 insects, the ninth segment of the abdomen. The etymology of the 

 word is such that the derivational meaning may be disregarded. 



The general shape of the hypopygium in the family Tipulidse 

 is that of a cup opening posteriorly. The cavity of the cup is 

 the genital chamber. It is produced simply by the invagination 

 of the posterior face of the segment. This carries into the depres- 

 sion the tenth segment, which morphologically terminates the abdo- 

 men. The tenth segment, bearing the anus at its end, is reduced to 

 a small membranous tube, or to a, simple prominence, arising from 

 the upper part of the anterior wall of the genital chamber. It sel 

 dom projects much beyond the hypopygium. 



Attached to the posterior rim of the hypopygium are from one to 

 three lobe-like appendages on each side. These are called the apical 

 appendages. The body of the segment is made up of four plates, 

 one dorsal, two lateral and one ventral. It is evident that the dor 

 sal plate is simply the tergum and that the ventral plate is the ster- 

 num of the segment. The lateral plates vary greatly in position, 

 but in two groups of genera, from each of which a line of more spec- 

 ialized genera may be derived, these plates occupy a typically pleu 

 ral position. That is, they extend along the entire length of the 

 segment, one on each side, between the tergum and the sternum. On 

 this account they will be given here the name of pleura. This name 

 is adopted, however, simply on a basis of analogy. But probably 

 many names applied to different parts of the external anatomy of 

 insects have no basis in homology. In our present ignorance of the 

 external homologies of insects, there is no need to make a new word 

 when " pleural " may refer to any plate intervening on the side of 

 the segment between the tergum and the sternum. 



