NO. 150-1. 



AMERICAN PAIIASITIC CO PEPODS— WILSON. 



671 



stout spines. This makes the segment nearly twice as wide across its 

 posterior margin as at the anterior end. 



In tig. 2 a short segment can be seen immediately behind the fourth 

 segment and in front of the genital segment proper. This short seg- 

 ment bears the rudiments of 

 a pair of legs at its posterior 

 corners, but it is not full}' 

 separated from the genital 

 segment. There is simply 

 the position of these rudi- 

 mentary legs and a deep 

 lateral incision on either 

 side just behind them to 

 indicate the posterior limit 

 of the segment. There is 

 no groove across the median 

 line on either the dorsal or 

 ventral surface. These rudi- 

 mentar}' tiftli legs subse- 

 quently disappear entirely 

 in the female, but are re- 

 tained in the male, and ap- 

 pear in the adult on the sides 

 of the genital segment two- 

 thirds of its length from the 

 anterior end. 



In other words, what is 

 ordinarily termed the geni- 

 tal segment is reall}" a fusion 

 of two segments, the tifth 

 and sixth, of which the fifth 

 forms more than half. 



The abdomen has length- 

 ened and become longer 

 than wide; it also is slightl}' 

 wider at its posterior end, 

 and the anal lamina? have be- 

 come twice as long as wide. 



All four pairs of legs are 

 now present and all are 

 biramose, but the rami have 

 only two joints instead of 

 three. 



The frontal plates have thickened considerably, but in the sinus 

 between them can still be seen the remnants of the frontal filament. 

 The antennae are relatively' much shorter and thicker than before, and 



OtSmm. 



Fig. 2. — Larva of Trebius exilis, 2.5 >nt. long. 



