MICROSCOPIC DRAWING AND ENGRAVING. 



and for the various organs supplied to it by nature for the gratifi- 

 cation of its ravenous propensities. It may be truly affirmed 

 that no similar creature is provided 

 with weapons of destruction so power- 

 ful, so numerous, and so perfectly 

 adapted to their end ; it is on this 

 account, that the insect, in this first 

 state of its existence, has been vulgarly 

 called the " water-devil." Its length, 

 when full grown, is about an inch and 

 a half, and the strength, courage, and 

 ferocity with which it attacks small 

 fish and other aquatic animals larger 

 than itself, are truly surprising. 



50. The representation of this crea- 

 ture, in its natural size, when young, and before it has reached 

 its full growth, is given in fig. 33. 



51. The magnified representation of it given in fig. 34, 

 has been engraved from Dr. Goring's drawing. 



52. In the first months of Spring, small nests con- 

 taining the eggs of these insects, may be seen floating 

 among the weeds, in stagnant pools ; they are formed 

 like balls, of a dusky-white colour, and silky texture ; 

 they are attached to the roots or stalks of weeds at the 

 bottom of the water by a thin stem of the same material 



as the nest, but stronger and more dense. Thus placed, they 

 remain during the winter preserved from the effects of cold, even 

 when the surface of the water is frozen over ; since by a natural 

 thermal law the temperature increases in going downwards.* 



Early in spring, the stem or thread by which they are attached to 

 the weeds, is broken by the winds, and the nest being detached and 

 lighter, bulk for bulk, than the water, rises by its buoyancy to the 

 surface, where being exposed to the warmth of the sun as the season 

 advances, the eggs are hatched. The larva, however, after breaking 

 the shell, is still confined in the bag- shaped nest ; it accomplishes 

 its liberation by gnawing a hole in it, from which escaping, it dives 

 immediately to the bottom, eagerly devouring all the small aquatic 

 insects that fall in its way. If, however, it should happen that 

 there is a short supply of this food, the voracity of these creatures 

 is such, that they fall upon and devour each other. 



53. "When the larva is very young, measuring not above a quarter 

 of an inch in length, it is sufficiently translucent to enable an 

 observer to see its internal structure with the microscope, by light 



* See Tract on " Terrestrial Heat," alsd Handbook of Natural Philosophy, 

 "Heat." 

 83 ' 



