Replies:
They are so light that they can support themselves by spreading their weight
on the surface tension of the water.
J. Elliott
Some insects have a waxy coating on the body and/or feet. The surface
tension of this coating [~20 to 30 ergs/cm^2] is much less than water [ 72
ergs/cm^2]. The water then tends to bond to itself rather than "wetting" the
bug's feet and unless the bug it too heavy it can remain on top of the
water's surface.
Ants, especially carpenter ants, are a good example of such insects. An
effective safe insecticide is to surround the base of a house with a surface
active agent [surfactant] that adheres to the ants feet. There are a number
of non-ionic suitable waxy materials. The surfactant sticks to the ant's
feet. The ant returns to the nest and further spreads the surfactant to
other workers. When the ants then come into contact with water they will no
longer be buoyant and will drown.
You can demonstrate this effect by sprinkling some black pepper on the
surface of some pure water in a glass. The pepper floats. Add a drop of
liquid dish soap to the surface and the pepper grains will rush away form
the soap droplet. Stir gently once or twice and the pepper sinks.
Vince Calder
Hi Stargirl!
It is a property of liquids called surface tension that
enables some insects to walk across a liquid surface,
usually of a pond or lake.Also a needle can float over
the surface of a glass filled with water...
Surface tension is a special force that resists to be
disrupted by the penetration of a needle (horizontally!)
or by a water bug, for example.
These surface forces can be explained by intermolecular forces.
A molecule in the center of a liquid is pulled equally in all
directions by the molecules surrounding it.
But if the molecule is on the surface, it is attracted
by molecules at its sides and below it only. There is no
corresponding upward attraction. These unequal forces tend to
pull inward at the surface of the liquid and cause it to contract.
That is why a drop is spherical, for example( to minimize its
surface area.)
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