230 THE BUTTERFLY VIVARIUM. 



rise towards the surface in proportion to the in- 

 creasing warmth, so that they may he made, hy a 

 course of accurate ohservations, a kind of living ther- 

 mometer. On warm summer evenings they creep to 

 the edge of the water, and emerging from what 

 seems (judging hy the nature of their fin-like 

 and ciliated swimming-legs) their native element, 

 they spread their wings and ascend almost perpen- 

 dicularly into the air to such a height as to he no 

 longer distinguishable. After a time they descend 

 as vertically, dropping with considerable force into 

 the water, towards which it would seem that they 

 are guided by the light reflected from its surface ; 

 for it not unfrequently happens that they drop in a 

 similar manner upon cucumber or melon frames, 

 the light reflected from the bright surface of which 

 they have evidently mistaken for that of water. 

 Some of them are known to be attracted by the 

 light of a candle at night ; and when an intruder 

 of this kind appears he may be easily captured 

 and consigned to the tank of the Vivarium. 



Among the prettiest of the Water Beetles may 

 rank the Gyrmidee, popularly called Whirligigs, 

 and in French Tourniquets, from their curious 

 spinning motion, in which action their bright wing- 

 cases glitter like burnished metal. When they dive 



