415 



The larvae of the species of Chironomus present a great diversity 

 of structure, and, as far as I am aware, possess no characters by 

 which they may be readily separated generically from other Chiro- 

 noinimr. Ventral blood-gills on the eleventh segment can not be used 

 even as a subgeneric character, since ferrugincovittatus and plumosus, 

 which have almost identical imagines, represent both types of larvae, 

 the latter possessing and the former lacking ventral blood-gills. I have 

 little hope that a better knowledge of the larval forms of this genus 

 will enable us to separate them into subgenera in agreement with the 

 subgeneric divisions proposed for the imagines by Kieffer. Biological 

 data are given in notes on viridicollis, species 39. 



The pupae of such species as are known to me have the thoracic 

 respiratory organs ending in many threadlike filaments, and the apical 

 abdominal appendages usually broad, rounded apically, and fringed 

 with numerous flattened hairs. 



I have figured the hypopygia of many species of Chironomus in 

 order to give an indication of the great variation in structure that 

 exists within the genus. Sometimes, as in the case of modestus and 

 tenuicaiidatus, species of very similar appearance have very different 

 hypopygia, while in other cases species with a very different general 

 appearance have hypopygia of very similar structure. 



With the exception of flaz'icingida, the description of which is un- 

 mistakable, none of Walker's species have been identified by the 

 writer. Coquillett has recorded some of these species, and Johannsen 

 besides accepting these has recorded the occurrence of some others. 

 In view of the extreme brevity of Walker's descriptions and his use 

 of color characters alone, I consider it unwise to adept the hazardous 

 course of the writers mentioned, and prefer, like Verrall, in his list of 

 British species, to consider Walker's species as "unrecognizable" in 

 their present condition. Many of Walker's types are lost, some of 

 them probably destroyed, and in such cases the very inadequate de- 

 scriptions of the species should be entirely disregarded. 



The key given herewith is, for convenience, divided into 

 "groups". These are largely artificial, and are not in any \yay in- 

 tended to indicate a generic or subgeneric division of the species. 



I have included in the key those species that are recorded from 

 Illinois or represented in the collection of the State Laboratory of 

 Natural History. 



