142 HOW BUTTERFLIES 



they resumed their activity. The one in the open 

 air had experienced a temperature of at least 

 -5°C., and Goossens discovered that numbness 

 only comes on at -2°C., for when it was warmer 

 their position showed that they appreciated the 

 difference between day and night. At such a 

 time the hind wings are kept motionless, but in 

 the daytime the fore wings are advanced, so that 

 the inner margin is at right angles to the body ; 

 at dusk, the fore wings creep backward and finally 

 pass partly behind the hind wings, nearly filling 

 the empty space between the two wings in this 

 genus, due to their great excision. The process 

 is again reversed in the morning, showing that 

 the butterflies are not completely benumbed and, 

 however quiet, recognize the distinction between 

 night and day. 



One November day a Mourning Cloak (Euvan- 

 essa antiopa) flew into my cellar and took up its 

 position on one of the risers of the stairway just 

 beneath the projecting edge of the tread above, 

 the extremity of its wings projecting beyond the 

 tread. Here it remained for three months with- 

 out moving, except that it shifted its position six 

 or eight times in the course of the winter, some- 



