457 



nymphs of various sizes as well as adults), Gcrridae (Gcrris m-arginatus 

 adults)', and Alesoveliidae {Mcsoi'clia iintlsanti, adults; some of them 

 winged — a rather rare occurrence in aquatic lleteroptera and one most 

 frequently observed under conditions such as the present ones). The 

 abundance of these insects under pollutional conditions is scarcely signifi- 

 cant since they are all air-breathing forms tliat can come to the water- 

 surface to obtain their supply of oxygen. The smaller water-strtders 

 (Gerridae, Trcpobatcs pic Ins and Metrobatcs hcsperius; Velilidae, 

 RhagovcUa obcsa) ; the back-swimmers (Notonectidae, Notonecta vari- 

 abilis. Bnciwa platycncmis. B. tnargarilacca) ; the Nepidae (Ranafra 

 aincricana), and the Hydrometridae (Hydromctra martini) were all 

 closely associated with clean water in the present survey. A number of 

 genera aild species of Corixidae were taken but the names are not vailable 

 for consideration in this report. 



Neuroptera 



The lar\a of the orl-fly Sialis iiifiaiiata is to be considered as an 

 indicator of fairly clean water conditions, though it has been observed by 

 Needham to prefer trashy places, and one of the European species, 5'. 

 lutaria, was recorded by Kolkwitz and Marsson as living under strongly 

 pollutional conditions. Similarly the larvae of the other Nearctic mem- 

 bers of the family Sialidae, ChaitUodes spp. and Corydalis cornntus L., are 

 apparently confined to unpolluted waters. The larvae of the latter, the 

 familiar Dobson or hellgrammite of bass fishermen, was very numerous 

 in the lower portions of the Salt Fork and after the union with the Middle 

 Fork. 



Trichoptera 



Species of the genus Hydropsyche, together with their nets and cases, 

 were very common in the clean waters of the stream. None was observed 

 anywhere in the stream under conditions that could be considered as being 

 pollutional or even contaminate. 



COLEOPTERA 



At Station 8 in the most septic part of the Salt Fork, large swarms 

 of whirligig beetles, Dineufcs americanus, were noted. These, being air- 

 breathers, are quite independent of the poisonous conditions beneath them 

 and a careful search failed to reveal any sign of their larvae. 'J'here can 

 be no doubt that they breed in clean waters and later move to the 

 places where found. Whirligigs were common at intervals along the 

 stream and in most instances proved to belong to this same species. Water 

 scavenger-beetles ( Ilydrophilidae) were common at Stations 10. 13, 28, 

 and 20, in polluted and contaminated portions of the stream. These in- 

 cluded a rather considerable number of genera and species (Tropistrniiis, 

 Hclophorus. Berosus, Enochrus, and Paracyiuus). In clearer waters 

 members of this family was less abundant. The polluted waters included 

 not only the adult beetles but also a great abundrnice of larvae of the same. 



