Re: why don't ants grow as big as people

An interesting question for sure. But no ants will ever get to be as big 
as a person.  Maybe you saw some horror movies (THEM)or(Empire of the 
Ants) where giant ants are the stars.  Just a Hollywood production!  Thank 
goodness for that. 

There are large insects out there!  A few examples are: the Goliath 
beetle, Goliathus goliathus, from Africa weighing in at nearly 0.25 lbs, 
an Atlas Moth, Family Saturniidae, from India with a 10" wingspan and even 
a Mexican Owl Moth Family Noctuidae, which holds the record for the 
largest insect with a 12" wingspan and is recorded in the Guiness Book of 
World Records .  WOW!  

Why can't ants get as big as people?  There are many reasons! It is a 
complicated problem! One main reason is that as an animal (compare to a 
sphere) increases in size, its surface area only doubles [the surface area 
of a sphere is (4 pi)r2] while the mass triples [the volume of a sphere is 
(4 pi/3) r3].  The suface area/body mass ratio is 2 to 3, instead of 1 to 
1 which is the optimal.  

Giant insects would starve!  For example, that means the intestinal 
surface area is only twice as big, but needs to absorb nutrients for three 
time more insect tissue (Ratio 2 to 3).  Insects have evolved presumably 
so that the existing body mass to intestinal surface area is balanced
(Ratio 1 to 1).  Simply making the insect bigger without changing some of 
the physiology, would lead the insect to starvation.  

Giant insects would suffocate as the body mass to surface area ratio 
increases.  Insects breath by a system of trachea, which are small tubes 
that are found throughout the body. The oxygen necessary for cells to live 
pass through the walls of the trachea and dissolves in the insect blood to 
be passively transported (insects have an open circulatory system and 
don't have arteries and veins), and just like the intestines, the ratio of 
area of trachea to body tissue is 2 to 3. As a result, not enought oxygen 
would get to all the cells and the insect would suffocate.

Giant insects would poisin themselves.  Insects don't have kidneys like 
people do.  They have Malphigian tubules which are tubes attached to the 
hindgut and sweep the open circulatory system to extract any wastes in the 
blood.  The same problem with as with the intestines and the trachea.  The 
doubled surface area of the tubules needs to deal with the tripled body 
mass. The efficiency of the tubules would have to increase to remove 
toxins produced by the three-fold increase in body mass. Again, just by 
increasing the size of the ant without a change in physiology would result 
in the insect poisoning itself. 

Giant insects would break their legs with evey step. For example, as the 
insect enlarges the cross-sectional area of the leg would increase by the 
r2 while that of the weight supported by that leg would increase by r3! 
This means that the pressure(force divided by area over which the force is 
applied is called pressure)on the leg would increase. If the composition 
of the exoskeleton did not change, the insect exoskeleton would not be 
strong enough to support itself.

These are just a few of the problems for an ant becoming large like a 
person.  The body design for ants limits them to a small size on planet 
Earth. For ants to become much larger, many changes in their physiology 
and the composition of their exoskeleton would have to take place.

If you want to know more there is a good web page to visit.  http://entomology.unl.edu/ent806/lecture02-
03_insect_size.htm