1900] ENTOMOLOGICAL NEWS. 307" 



aquaria. On May 14th I found larvae of all sizes, from 3 mni. to 

 10 mm. long, and also a few just formed pupa?. The larger 

 larvae, 9 to 10 mm. long, which were full grown and ready to 

 pupate, had gathered together, forming conspicuous " patches ?> 

 on the rock bed. Each of these patches contained from one to 

 several hundred individuals. 



Most of these full-grown larvae were curiously covered dor- 

 sally with a close growth of diatoms. The most abundant 

 diatom in this growth was one of the stalked Gomphonenia. 

 The basis of this covering of the larva's back was the gelatin- 

 ous mass at the base of the stalked diatoms. Scattered upon, 

 and through this mass were individuals of Nitzscliia and sev- 

 eral other diatoniaceous genera. The covering had a soft, felt- 

 like appearance, grayish or brownish, and did not seem to 

 trouble the larva. In fact in the light of this condition it was 

 very interesting to me to discover that the principal part of 

 the contents of the alimentary canal of the larva was composed 

 of diatoms. Is there any connection between the food habits 

 of the larva and this convenient flourishing of diatoms on the 

 insect's back ? 



On May 17th there were many new pupa3, but also still many 

 larva and these of all sizes, from the 2 2 mm. fellows to the 

 full-grown ones. The pupae all lie with head down stream. 

 On May 20th the pupa? far outnumbered the larva?, and on 

 May 26th the larva? were scarce. There were no patches re- 

 maining ; the larva? were of various sizes. No adults had 

 issued ; this is certain from the fact that there were no empty 

 pupal cases to be seen in the patches of pupa?. (The pupal 

 skin always remains attached to the rock after the imago 

 issues.) On June 1st I found the midges issuing. A few were 

 seen flying, with slow, weak flight, over the stream, and I col- 

 lected a number as they issued from the pupal skins. Prof. 

 Comstock first observed, and described in the Manual, this in- 

 teresting escape of the delicate fly from the submerged pupal 

 skin. The swift water makes this matter a difficult one, and 

 a majority of the flies whose issuance I watched were carried 

 off before they could get above the water. By June 9th most of 

 the imagines had issued, although there were still pupa- and 

 even some larva? there, mostly old. Farther up the stream 



