Lesson LXXVII

Leaf-miners

Leading thought—The serpent-like markings and the blister-like blotches which we often see on leaves are made by the larvæ of insects which complete their growth by feeding upon the inner living substance of the leaf.

Method—The nasturtium leaf-miner is perhaps the most available for this lesson since it may be found in its mine in early September. However, the pupils should bring to the schoolroom all the leaves with mines in them, that they can find and study the different forms.

Observations—1. Sketch the leaf with the mine in it, showing the shape of the mine. What is the name of the plant on which the leaf grew?

2. Hold the leaf up to the light, can you see the insect within the mine? What is it doing? Are there more than one insect in the mine? Open the mine and see how the miner looks.

3. There are three general types of mines: Those that are long, curving lines called serpentine mines; those that begin small and flare out, called trumpet mines; and those that are blister-like called blotch mines. Which of these is the mine you are studying?

4. Study a serpentine mine. Note that where the little insect began to eat, the mine is small. Why does it widen from this point? What happened in the part which we call the serpent’s head?

5. Look closely with a lens and find if there is a break above the mine in the upper surface of the leaf or below the mine in the lower surface of the leaf. If the insect is no longer in the mine can you find where it escaped? Can you find a shed pupa-skin in the “serpent’s head?”

6. Why does an insect mine in a leaf? What does it find to eat? How is it protected from the birds or insects of prey while it is getting its growth?

7. Look on leaves of nasturtium, columbine, lamb’s quarters, dock and burdock, for serpentine mines. Are the mines on these different plants alike? Do you suppose they are made by the same insect?

8. Look on leaves of dock, burdock, beet and spinach for blotch mines. Are there more than one insect in these mines? If the insects are present, hold the leaf out to the light and watch them eat.

9. Look in the leaves of pitch or other thick leaved pines (not white pine), for pine needles which are yellow at the tip. Examine these for miners. If the miner is not within, can you find the little circular door by which it escaped? Would you think there was enough substance in a half a pine needle to support a little creature while it grew up?

10. If you find leaf-miners at work, do not pluck off the leaves being mined but cover each with a little bag of swiss muslin tied close about the petiole and thus capture the winged insect.