Title:                   Why there are no insects as large as many other animals?

Provided by:     Dr. Nan-Yao Su, University of Florida, Ft. Lauderdale, FL

Grade:           Middle school

Observations: 

Insects are generally small, and much smaller than most animals we are familiar with, including mammals, lizards, birds or fish. Put an elephant, the largest mammal on land, next to the Thorny Stick from Malaysia (Fig. 1, see below ), and you will find out the enormous size difference (Fig. 2).  Even a small dog is much larger than a large insect (Fig. 3).

Question:  Why are insects smaller than many other animals we see on earth?

Hints to form the hypothesis: 

Looking for differences in body structure between larger animals and smaller animals such as insects, and you will notice vertebrates (with solid bones inside the body) tend to be larger than invertebrates (with exoskeleton or bones on their body surface). Other invertebrates include shrimps or crabs, and they are also rather small.

Hypothesis: 

With the same amount of material available, a structure supported by solid bones inside is stronger than a structure supported by the outside shell (or exoskeleton-type structure).

Materials:

Tin foils.  Threads.  Small plastic sandwich bag or Ziploc® bag.  Two cardboard boxes of the same height.  Coins.  A digital camera to take pictures for presentations.

Experiment:  

1. Prepare four pieces of tin foil of the same size. 
2. Roll the tin foils into tubes of one, two, or three layers and a solid bar (Fig. 4). The solid bar represents a vertebrate bone and the hollow tubes represent the outer shell of an insect at different thickness.
3. Bridge the bar or tube between two boxes and hang a thread at the middle.  Attach a small plastic bag (a Ziploc® bag or sandwich bag) to the thread.
4. Place coins in the bag and count how many it takes to bend or break the bar or tubes.
5. Repeat the same experiments at least three times and take the average number of  coins it took to break the bar or tubes. 

Result:  Present the result in a bar chart (Fig. 5).

Discussion: 

Present your thoughts regarding the results and how do your thoughts confirm or reject your hypothesis, and why. Formulate the follow-up question and hypothesis if possible.

References:   (list, preferably with link to an internet resource)

Fig. 1.    Measuring at 14 cm (7”) long, the Thorny Stick from Malaysia is one of  the largest insects in the world.

Fig. 2.   If you compare with an elephant side-by-side, you’d realize how small the largest insect is.

Fig. 3.  In fact, even a small dog is much larger than a large insect.

Fig. 4.   Roll the tin foils of the same size into tubes of one, two, or three layers and a solid bar

Fig. 5.  A bar chart describing the numbers of coins it took to bend or break the solid bar or tubes of 1, 2, or 3 layers.