I 



INSECUTOR INSCITI^ MENSTRUUS 107 



istic. The larvae also show good characters. The basis of 

 classification adopted is the male hypopygium. The larvae 

 show a different series of adaptations. The present study is 

 confined to the American forms, leaving the European ones 

 aside, both as to their relations to each other and to the 

 American ones. 



The adults divide into three series: 1. excrucians; 2. stimu- 

 lans; 3. Htchii. Series 1 contains the species excrucians and 

 ificrepitus ; series 2, fletcheri, stimulans, and cantator; and 

 series 3, fitchii, riparius, grosshecki, and squamiger. 



The larvae of series 2 have the air-tube stout and fusiform 

 with broad tracheae, as is usual in the genus, elongated only in 

 fletcheri; figured, fletcheri, Monog., ii, pi. 117, fig. 403, 1912; 

 stimulans, Monog., ii, pi. 63, 1912 ; cantator, Monog., ii, pi. 67, 

 1912. 



Series 1 has the air-tube of the larva of this same type as 

 to increpitus (Ins. Ins., Mens., v, pi. ii, "sansoni," 1917) ; but 

 excrucians has the "long-tubed" type, in which the outer half 

 of the air-tube is narrowed and the tracheae reduced in width 

 (Monog., ii, pi. 65, "ab fitchii," 1912). 



In series 3 most of the species have the "long-tubed" type 

 of larva {fitchii, Monog., ii, pi. 64, 1912; riparius, unknown; 

 grossbccki, Monog., ii, pi. 69, 1912) ; but squamiger has re- 

 verted to the "short-tubed" type (Monog., pi. 122, fig. 422, 

 1912). 



The peculiarities of the air-tube are an adaptation to habits, 

 the long-tubed type being adapted to pools containing vegeta- 

 tion of a semipermanent character, the short-tubed type to 

 open, transient pools. Since differences in habit are one of 

 the most potent factors and conditions of specific differentia- 

 tion, these larval characters are excellent indices of specific 

 validity ; but being essentially adaptational, they do not form 

 a guide to phylogenetic relationships, for which the hypo- 

 pygium seems well adapted. 



As to coloration, the primitive marking has been retained in 

 some members of all the series, the name-species, excrucians, 

 stimulans, and fitchii being practically indistinguishable. In 



