88 THE ANIMALS AND MAN 



and a pair of tufts or brushes of hair on the head which 

 vibrate rapidly and constantly. These brushes by their 

 vibration create currents in the water setting toward the 

 mouth, which lies between them, and thus bring food to it. 

 This food consists of any tiny animalcules and microscopic 

 bits of organic matter in the water. Are there any legs or 

 wings? Examine the posterior end of the body and note 

 its division into two parts one the end of the hind body 

 or abdomen, the other a breathing-tube projecting from 

 the next to last body-ring. Make a drawing of the larva, 

 showing and naming all these parts. 



Observe again the larvae in the jar. When they hang 

 from the surface note that only the tip of the breathing- 

 tube reaches it. Note the vibration of the mouth-brushes. 

 The larvae feed busily for most of the time. If they sink 

 in the water when they stop "wriggling," i.e., swimming, 

 how is it that they can rest quietly at the surface? For 

 this reason: the tip of the stem-like breathing-tube pro- 

 jects slightly above the surface when the wriggler comes up 

 to breathe, so that the expanded edges of its mouth are 

 caught by the tense surface film and the wriggler's body 

 being but slightly heavier than water, is thus supported or 

 suspended by the film. It is easier to prove the existence 

 of this film than to explain it. If you carefully lay a clean 

 needle on the surface of the water it will not sink, although 

 much denser, i.e., heavier than water, but will be supported 

 by the surface film. If you fill a tumbler to its brim you 

 can still add more water carefully and so heap it up above 

 the level of the brim. This is because the surface film ex- 

 tending over the water from edge to edge holds it in place. 

 If you dip your finger in and then lift it up the water does 

 not all run off, but a large drop will remain hanging to your 

 finger. The tense surface film holds the little mass to- 

 gether in the form of a drop. The mosquito larva takes 

 advantage of the surface film and is able to keep itself at 



