THE MOSQUITO. 15 



You can see that a little stem-like process (fig. 5 b.t.) 

 projects from very near the posterior tip of the body and the end 

 of this process reaches the water's surface. This process is a 

 breathing tube. (I shall describe it later, more in detail.) The 

 mosquito does not breathe thru its mouth, nor thru any organs on 

 its head, but thru this tube on the tail end of the body. It is 

 necessary for the wriggler to come to the surface of the water to 

 breathe. If the wriggler is prevented by any means from coming 

 to the surface, it soon drowns. Many wrigglers too weak to swim 

 to the top of the water drown. 



But how can the wriggler remain thus at the surface without 

 sinking if it is heavier than water and always sinks when it stops 

 wriggling? It holds on to the tense surface film of the 

 water. The tip of the breathing tube projects slightly above the 

 surface when the wriggler comes up to breathe. The slightly 

 expanded edges of the mouth of the tube are caught by the sur- 

 face film, and the body of the wriggler supported at the surface. 

 This tense surface film exists because the molecules of 

 water which constitute the surface layer of the water are more 

 strongly attracted laterally by each other and downward by the 

 water molecules beneath than by the molecules of air which lie 

 directly over them. It is easier, however, to prove the existence 

 of this tense surface film than to explain it. If you carefully lay 

 a clean needle on the surface of water it will not sink altho much 

 denser than water, but will be supported by the surface film. If 

 you fill a tumbler to its brim you can still add more water (doing 

 it carefully) and so heap up the water above the level of the tum- 

 bler's brim. You can do this because the surface film extending 

 over the water from edge to edge holds it in position. If you dip 

 your finger into the water and lift it up all the water does not run 

 off" but a large drop will remain hanging to your finger. The 

 tense surface film keeps this little mass of water together in the 

 form of a drop. Many aquatic insects and other animals take ad- 

 vantage of the presence of this surface film on water. The water 



