You have but few singing-birds in cities, and for that reason caterpillars become very troublesome, and, unless checked by some Ichneumon, or parasite insect, go on increasing rapidly until famine does its work.

I have known some kinds of caterpillars in such numbers upon trees as to consume the foliage before they were fully grown, and in their attempts to reach other trees to be entirely at a loss as soon as they reached the ground - their instinct, so Surprising on other occasions, now entirely at fault. Their helplessness on the pavement was pitiable, and they soon perished.

In this city (New York) you have a. caterpillar of this class, the little span-worm, or Geometer. It is unnecessary to describe its personal appearance. This little worm, like many others, is something of a philosopher; it knows that if the wind should throw it from a high tree, and it had no means of breaking the force of the fall, it would be hurt. Now, to guard against any thing so unpleasant, it lays along its path, wherever it goes, a silken cable; push it overboard any where, and it will lower itself down deliberately and safely. It has also a kind of handover-hand way of climbing back again by the same cord. You may have noticed, also, that, when descending, they drop themselves a little way at a time, and then stop. These silken threads are spun from a fluid, and require some time to dry, otherwise they would not bear the strain.

Some of us in the rural districts of Jersey read the papers. Once last summer I noticed that the authorities were about to cut down the trees of this city to get clear of the worms. Now, nothing in your newspapers perplexes us so much to understand as the accounts of your local government. At one time your aldermen are called the "forty thieves;" then they appear to have nothing to do but to seek out the nativity of a fat policeman; and lately they seem to have been mixed up with the Japanese ambassadors in some very queer operations; but when your city governors do order your shade-trees cut down to get rid of the little caterpillars, we shall know exactly what manner of men aldermen are; then we, who pay some attention to classifying the orders of nature, will know precisely where to place them.

These little Geometers of your city, like most other caterpillars, feed upon leaves; they can not live without them. They are born in the trees; leaves are plenty, they are all around them every where, and they feel no kind of hesitation about taking all they want. And why should they? Their mothers selected these trees for them the year before; hand this selection is the result of an instinct the most wonderful of the world of wonders in which we live. She selects from the many those few trees to deposit her eggs upon whose leaves will put forth the next spring just at the right time to afford food for her little ones when they come out of the eggs. In other words, she knows how much caloric is required to burst the buds of trees and the shells of her eggs, and she puts always the right eggs on the right trees.

If you will now search the bodies of certain kinds of trees in your city, you will find thousands of clusters of little eggs. . Examine the elms, just under the large branches, where they go off from the main trunks, and in some places you may see the bark almost covered with them ; if you let these eggs alone, you will find, in midsummer, when you want the shade, these trees will be almost as leafless as they are now, and the span-worms will be every where acting out Mohammed's coffin.

Man was created with dominion; but, if he does not choose to exert it, he should not blame the little insects. Had I a favorite shade-tree coated over with these eggs, I would, within a month of this time, do something to prevent those eggs from becoming caterpillars. To tap each cluster with a hammer would do it - to take them off with a gouge or small adze, or to daub each cluster with paint, or varnish, or tar, would probably save the foliage of that beautiful tree. But nothing of this kind will be done here; and I shall have the opportunity next summer of seeing lots of these span-worms, and shall come on purpose.

The insectiverous birds in the country attend to the caterpillar business for us; but these birds will not stay with you. They do not like the noise, the smoke; and especially the boys of the cities.

These little span-worms, next summer, when they have eaten all the leaves they want, will choose others to make their houses of; for they belong to a large class called leaf-curlers; and this leaf-curling process, with some of them, is a very strange one. One much less .than your span-worm, that lives on the plum-trees, and wraps a leaf around it, so as to resemble a well-formed cigar, I have watched throughout the process with surpassing interest. I have seen this little speck of a worm take a calm survey of a leaf, then fix her cord on one side, then cross over, and then again and again, and then at different angles; and when some twelve or fifteen of these cords were arranged, she would go backwards to some distance, and with a single cord act upon all these other cords, as if by a combination of leverage, operating with a power utterly above and beyond any thing so minute a creature could accomplish unaided by such a combination of mechanical forces; and this will be repeated again and again, gaining a little every time, till the whole is completed. Sometimes her strength or her cordage is unequal to the work; then she will cut away the obstructing part, and try again.

In a few hours the leaf will be curled; the overlapping parts will be neatly sewed together; the inside will be lined with silk, composed, in a great measure, of the cordage she had used as an engineer. In a few days her limbs will have fallen off, and she will have assumed the pupa or chrysalis state, and appear dead; and in a few more days she will be a butterfly that flies at night - one that your lamp-light "leads to bewilder".

Once last summer, in crossing from Jersey to this city, a lady came on board the boat, with one of these little Geometers on her bonnet. The little thing seemed busy in measuring that bonnet; but it did not take long, and then it quietly perched itself upon the highest point. It was what is called a "love of a bonnet" - very beautiful, but very small; the lady herself was very beautiful also, but very large - a perfect Juno in her style, and most elaborately clothed in silks. She cast a hasty glance at her fellow-passengers; but the silk-worms had done much more for her than for any of the rest of us. Thousands upon thousands had spent their entire lives in her service. She evidently felt how magnificent she was, but probably did not ascribe it to the little caterpillars that had made her silks.

[The above we take from a lecture recently delivered in New York by Dr. Trimble, the subject being, " The Wonders of Creation as exemplified in Insect Life." He succeeded in interesting his audience for nearly two hours. There is something fascinating in the wonders of insect life which never fails to arrest the attention of both young and old; and the Doctor's characteristic humor took nothing- from the interest of the occasion. Lectures on this and kindred subjects, in our rural towns, would be more useful and entertaining than many which are selected for the purpose. - Ed].