SOME DIFFICULTIES IN THE LIFE OF AQUATIC INSECTS. 357 
of the surface film of water. You will readily believe that I have 
nothing new to communicate on this subject, and I venture to show 
you a few very simple experiments, merely because they are essential 
to the comprehension of what takes place in the gnat.* 
In any vessel of pure water, the particles at the surface, though not 
differing in composition from those beneath, are nevertheless in a 
peculiar state. 1 will not travel so far from the region of natural his- 
tory as to offer any theoretical explanation of this state, but will 
merely show you experimentally that there is a surface film which re- 
sists the passage of a solid body from beneath. |Mensbrugghe’s float 
shown.| You see (1) that the float is sufficiently buoyant to rise well 
out of the water; (2) that, when forcibly submerged, it rises with ease 
through the water as far as the surface film; (5) that it is detained by 
the surface film, and can not penetrate it. The wire pulls at the sur- 
face film and distorts it, but is unable to free itself. In the same way 
the surface film resists the passage of a solid body which attempts to 
penetrate it from above. This will be readily seen if we throw a loop 
of aluminium wire upon the surface of water. [Experiment shown. | 
The loop of wire floats about like a stick of wood. Aluminium is, of 
course, much lighter than iron, but the floating of this little bar does 
not mean that it has a lower density than that of water. If the bar is 
once wetted, it sinks to the bottom and remains there. Even a needle 
may, with a little care, be made to float upon the surface of perfectly 
pure water. Still more readily can a piece of metallic gauze be made 
to float on Water. {Experiment shown.] Air can pass through the 
meshes with perfect ease; water also can pass through the meshes with 
no visible obstruction. But the surface film, bounding the air and 
water, is entirely unable to traverse even meshes of appreciable size. 
These simple experimental results will enable us to appreciate certain 
facts of structure, which would otherwise be hard to understand, and 
which have been wrongly explained by naturalists of the greatest emi- 
nence, to whom the physical discoveries of this century were unknown. 
We imay now try to answer three questions about the larva of the 
enat, viz: 
(1) How is it able to break the surface film when it swims upwards? 
(2) How is it able to remain at the surface without muscular effort, 
though denser than water? 
(3) How is it able to leave the surface quickly and easily when 
alarmed? 
The tip of the respiratory siphon is provided with three flaps, two 
large and similar to one another, the third smaller and differently 
shaped. These flaps can be opened or closed by attached muscles. 
When open they form a minute basin, which, though not completely 
closed, does not allow the surface film of water to enter. When closed 
* A number of other experiments, illustrating the properties of the surface film of 
water, are described by Prof. Boys in his delightful book on ‘‘ Soap Bubbles.” 
