AQUATIC MIDGES AND SOME RELATED INSECTS. 49 



the ledges. They also seemed to select those parts of the rocks which were free from 

 any vegetable growth, a selection probably closely correlated with their method of 

 locomotion. » 



That the larvae are unable to live on the surface of any other than perfectly quiet 

 water was discovered by repeated attempts, as above stated, to carry living larvae 

 home in bottles half filled with water. The result was nearly ioo per cent fatalities. 

 This does not mean that they can not move over quiet water, for they are very much at 

 home in such conditions ; but as soon as the surface film rises above the ventral third of 

 the body, which is distinguished by being somewhat flattened and white in color, the 

 result is total submergence. Total submergence is, of course, only fatal in larvae 

 which breathe by means of tracheae and have no special means of escaping from the 

 water. The larvae of Orphnephila do breathe by tracheae, and, while they are very well 

 adapted to move rapidly on the surface of moist ledges, their very peculiar sidewise move- 

 ment of the body is not at all suited to locomotion beneath the surface film. In fact, 

 they depend very largely upon the surface film which holds them so closely in contact 

 with the rock's surface that with the claws of their anterior and posterior prolegs they 

 are able to anchor one end while the other is being swung around in a horizontal plane. 

 This zigzag sidewise movement is sufficiently rapid to enable them to move four or five 

 times as fast as a chironomid can crawl and gives them the appearance of being very 

 sprightly. 



This poor adaption to an aquatic environment is one of the factors that doubtless 

 makes for their infrequent occurrence, next to the nature of their habitat, which is of 

 itself rather unique. In several laboratory experiments in which the author attempted 

 to duplicate natural conditions the larvae were observed leaving the moist stones upon 

 which they had been placed and voluntarily subjecting themselves to the current which 

 swept them over the edge of the dish. So far as it is possible to judge, the same thing is 

 liable to occur in nature, and the results are doubtless fatal, for the bigger streams are 

 constantly agitated by swirls and cross currents which would submerge and drown the 

 larvae. 



In order to try to eliminate the nonessentials in the environment of the larvae, the 

 author began searching for suitable methods of rearing them in captivity. At first an 

 experiment, referred to above, was set up in which rocks taken from the natural habitat 

 were placed in a tray through which a stream of gently flowing water was maintained. 

 The result was that the larvae allowed themselves to be carried over the edge and were 

 lost down the sink spout. At Dr. Johannsen's suggestion a cheesecloth pocket was used 

 and resulted in the successful completion of the transformation of some five or six adults. 



This pocket was made by placing a double thickness of cheesecloth over the top of a 

 wide lamp chimney and pressing it down so that it would hang in a conical sort of a 

 pocket. This was covered with two thicknesses of cheesecloth after the larvae and a 

 fair supply of food had been placed within. Then both the cover and the pocket were 

 made fast by successive coils of white thread, which were wound about so tightly that the 

 larvae were unable to creep out between the layers of the cheesecloth. The pocket was 

 moistened by water which was kept dripping slowly over its surface. To insure the 

 more uniform distribution of the water to all parts of the receptacle, a mass of cotton 

 wool was placed on the cheesecloth cover. That this sort of an artificial environment 

 seemed to meet their every need was demonstrated by the fact that the larvae lived under 



