60 NEW YORK STATE MUSEUM 



account of that interesting species, which differs in some respects 

 from the well-known P o 1 y m i t a r c y s virgo Oliv : 



Polymitarcys albus Say 



BY W. E. HOWARD 



This description was undertaken at the request of Prof. Need- 

 ham, who identified the imago for me. Without his kind assist- 

 ance in this respect, as well as in many others, it would not have 

 been prepared. 



Nymphs of P. a 1 b u s are abundant in both the Illinois and 

 Fox rivers at Ottawa. These rivers flow at this place over bot- 

 toms of solid sandstone, with bars of loose sand accumulated in 

 the eddies. The streams are swift in the main currents, and the 

 nymphs of this species are to be found under flat stones at the 

 edge of swift water when about ready to transform. It was 

 from two such situations that most of my collections were made, 

 from which I succeeded in breeding a single specimen. I have 

 seen the subimagos emerge and arise from the surface of the 

 water in great numbers, but always just far enough out from the 

 shore, so that the nymph skins were immediately swept into the 

 current, where they disappeared before they could be procured. 

 The difficulty in collecting 'the skins from the natural breeding- 

 places is further heightened by the emergence occurring during 

 the evening twilight. 



According to my observation, not only this species but all 

 others observed invariably emerge from the nymph skin at the 

 surface of the water and leave the skin afloat. This makes the 

 collecting of the sloughs a much more difficult task than in the 

 case of stoneflies and dragonflies. < 



My collections indicate that this is a midsummer species in 

 northern Illinois. My bred specimen is dated June 22. None of 

 the imagos in my collections shows an earlier date than this, but 

 I have nymphs which are evidently near to transforming which 

 were collected the first week of June. Imagos and subimagos of 

 the collections are scattered all through July, but August 5th 

 shows them most abundant. At about this date they were ob- 

 served in swarms. By the end of August they are much less 

 numerous, and I have no collections which are as late as Sep- 

 tember. 



