Objects for the Microscope. 121 



The female Gnat is that blood-thirsty little creature 

 whose shrill clarion sounds an attack upon man and beast 

 throughout the warm summer's day and night. She flies 

 silently in the spring, and seldom thirsts for blood until she 

 begins laying eggs ; of these she produces about 300 in one 

 season in stagnant water, and their transformations occupy 

 about a month ; so there are several generations in one 

 year, which accounts for the swarms which occasionally 

 trouble us. 



A few of the Iarva3 of the Gnat are amusing in the 

 aquarium, and being, when young, very transparent, give 

 excellent observation of the circulation of blood and of the 

 tracheal organs. 



The little Gnat lays her eggs in the form of a boat, which 

 floats upon our waterbutts or ponds for about a week, when 

 it sinks down to the bottom, and the young larva escapes 

 from each egg. This rises to the surface for air, which 

 it breathes through a most curious organ placed at the 

 extremity of its body, and it hangs, therefore, head down- 

 wards whilst breathing. This organ is a tube which 

 springs from the last segment but one of its abdomen, and 

 terminates in five points like a star. In the interior of 

 this tube a tracheal vessel runs which supplies the body 

 with air, and carries down a little bright globule when the 

 larva descends to the bottom of the water ; this renders it 

 so buoyant that its greatest effort is required to descend, 

 and when the insect wishes to rise again, it has only to 

 unclose its tube, and it ascends without any exertion to 

 the surface, remains suspended there, drinking in the sur- 

 rounding element, and swallowing shoals of little " living 

 things," invisible except under the microscope. A tumbler 

 of stagnant water will not only show the merry evolutions 

 of these larvae, but an abundant variety of the Infusoria, 

 upon which they feed, Monads, Paramecium, Rotifers, 

 together with the Desmidiacese, which abound in such 

 water. 



In the next change the Gnat larva becomes a pupa, with 

 quite a different appearance and respiratory organ ; for now 

 it has two horn-like appendages on the upper side of the 



