THE INSECTS. 335 



Female. Wings paler than the male ; sometimes a spot 

 in the middle of the posterior wing. 1 



From a number of observations made by the river-side 

 by Mr. W. P. Crake and ourselves on the different genera 

 and species of the Ephemeridce taken in 1886 and 1887, we 

 found that all these insects, from the time they rose from 

 the surface of the water, remained in the sub-imago state 

 as near as possible forty-eight hours before changing to 

 the imago or perfect insect. In this latter state they lived 

 from six to eight days, and in some of the genera, as 

 Bae'tes and Ctoeon, as long as ten days. Whether they 

 would thus live in a state of nature is doubtful, as 

 some writers observe that probably the act of procreation 

 shortens the lives of most of the Ephemeridce, and, there- 

 fore, in a free state would not live so long as those kept in 

 confinement. 



The mode of transformation from the sub-imago to the 

 imago state is accomplished in the following manner : 

 Somewhere about the forty-eighth hour the insect com- 

 mences to flutter its wings very rapidly, so much so that 

 the movement could with difficulty be followed. This 

 rapid action continued from thirty to fifty seconds, then 

 ceased, and the wings, previously upright, became perfectly 

 flat and applied close to the body, with a very slight move- 

 ment of the head and thorax. These parts split open and 

 the perfect insect began to emerge, and rapidly divested 

 itself of its previous covering, the whole process taking 

 about a minute to complete. Sometimes the insect could 

 not get the setae, which appear folded up, clear of the old 

 covering ; and when this occurred it became very excited, 

 fluttering the wings and bending the body backwards and 

 forwards in its endeavours to release itself. The setae, 

 short in the sub-imago, become in many species of great 

 length in the perfect state. We found also that the colour 

 of many of these insects varied with their age. For 

 instance, on Wednesday, April 21, 1886, ten olive duns, 

 captured as they left the water, were put into a glass case, 



1 We have found the spots in the posterior wings of both male and 

 female, but they are more constant in the male. 



