One way of thinking of the six hundred thousand kinds or species of insects—those tiny, ubiquitous fellow-creatures of ours which inhabit nearly every corner and cranny of the earth’s surface—is to associate them with the plants upon which, either for food or protection, the greater number of them are dependent. This makes them appear less overwhelming in their astonishing and, at first sight, meaningless variety, than when one calls them to mind pinned out in long lines in innumerable drawers and cases, or assorted, like with like, in the wonderfully accurate and interminable pictures of them produced by those patient benefactors of mankind the systematic entomologists. Every plant of any size has a number of insects associated with it, living more or less completely on its substance, or making its home in some part of the plant. Some trees are known to have more than a hundred and fifty kinds or species of insects thus dependent on them, those which are vegetarian serving in their turn as food to a variety of carnivorous insects.

The ways in which insects are associated with plants may be briefly stated. It must be remembered that often, though not always, one particular species of plant, and that only, is capable of serving the needs of a given species of insect. Thus, the leaves of a given plant are the necessary food of the grubs of one or more insects which bite their food; its internal juices serve others which suck; its roots others; its nectar in the flower others, which in return serve the plant by carrying away its pollen and fertilising the other plants of the same species which they visit. Protection is sought and obtained from the same plant by insects which burrow in its leaves, or roll them up, or cut them into slices and carry them away, or hide in its bark, or in the flowers, or in other parts—or burrow for food and shelter into its wood. Others lay their eggs in the soft buds, producing or not producing according to their kind distorted growths, known as “galls” (one plant is known to have as many as thirty species of gall-flies which make use of it). Other insects lay their eggs in the flower-buds and immature fruits, or place them on the plant so that the young grubs, when hatched, can at once eat into those soft parts. Others bore into the wood or into hard or fleshy fruits expressly to lay their eggs, or into the ripe seeds. Certain ants live in chambers specially provided by the woody parts of the plant for them, and benefit both themselves and the plant by devouring other insects which seek the plant in order to devour it. In a museum of natural history there should be exhibited at least one plant with specimens and enlarged models of all the insects which depend upon it for food, protection, or nursery, and with accompanying illustrations of the way in which those purposes are served.

A curious product of the relationship of an insect and a plant is the so-called “jumping bean,” which is brought to this country from Mexico, and may be purchased in some of the London shops which deal in “miscellaneous” articles. They have been known for some years, but are becoming now a regular article of commerce. As one buys them (Fig. 52) they are segments of a globular fruit which has divided into three, comparable to the familiar segments of an orange, but less numerous. They are about one-third of an inch long, light, quite dry, and apparently hollow, without any visible opening. Two sides of the little capsule are flat, and the third side is bulged and rounded, so that the capsule easily rocks when resting on that side. When these dry fruits or segments of a fruit are brought into a warm room or placed near a fire so as to make them as warm as the hand, they commence to rock and move with curious little jerks. They jump as much as one-eighth of an inch from the ground, and advance as much as a quarter of an inch at a time, though by rolling they may progress a good deal more. They will often move seven or eight times in the same direction so as to make a progress of a couple of inches on a flat surface, and I have found that if a cool surface or protection from warmth is within reach they will in the course of time arrive at that cool area and come to rest. When the plate on which they are placed becomes cool or the temperature of the room falls to what we should call “chilly,” they cease to move, but can be roused again by renewed warmth.

How and why do these “beans,” or, rather, fruit-segments (for they are not beans), move in this determined purposeful manner? The whole proceeding has a mysterious and uncanny aspect. They have no legs, no spring; they are simple little smooth capsules, and yet they jump and seemingly “walk” about. The explanation is that there is a grub inside each so-called “bean.” Cut one of the beans or capsules open, and you find that it is a very thin-walled and hollow case, but coiled on itself in the cavity you open, and about half filling it, is a yellowish white grub (Figs. 52 and 53). It is not a “maggot,” but a “caterpillar,” that is to say, it is not legless, but has eight pairs of legs—namely, three pairs of short walking legs in front, four pairs of sucker-like legs, and a hinder pair of larger size called “claspers.” It has a hard brown plate on its head, and possesses hard jaws. It refuses to leave the opened capsule, and crawls back again if forcibly removed, and in the course of a few hours spins a silken cover to replace the piece of “shell” you have cut away. Mr. Rollo has lately succeeded in getting the caterpillar to patch up its injured residence with a thin piece of glass, such as is used by microscopists, which he put in place of a side of the capsule removed by a knife. He was thus able subsequently to watch through the glass the movements of the little creature when it causes the mended capsule or “bean” to jump. It rears itself from the lower surface of the capsule, and gives a series of sharp blows to the roof, projecting its body with each blow, and thus overbalances the capsule, or, if the flat side is lying downwards, jerks it along much as one may sit with one’s feet on the rail of a chair and cause it to jerk along the floor by the swinging movements of the body. The caterpillar does not die at once when removed from the capsule; it has been kept alive in a glass tube for a month.

So far so good. The next questions are: What Mexican plant is it that forms the capsule or tripartite fruit in which the caterpillar is found? How did the caterpillar get there? What kind of an insect does it turn into, and when? I will answer the last question first. The caterpillar turns into a chrysalis in the early part of the year, having first cut a perfectly circular ring in the shell of the capsule. The circular plate thus within the ring is not disturbed, and cannot be observed without very close inspection. The making of this perfectly circular cut without removing the piece marked out must be effected by a rotation of the caterpillar’s head and jaws as a centre-bit—an astonishing performance. But when the moth emerges from the chrysalis, a gentle push is enough to cause the little circular plate to fall out, and the moth creeps through the hole to the outer world. The moth, which comes out of the chrysalis-coat, is a very pretty little creature (see Fig. 54), measuring two-thirds of an inch across the opened wings, which are marked with dark and reddish-brown-coloured bands. It is a close ally of the British codling moth, the caterpillar of which eats its way into the core of apples, and is familiar to all growers and eaters of that fruit. The codling moth and the Mexican “jumper” belong to a group of small moths called TortricinÆ, and they are named respectively Carpocapsa saltitans (the one whose grub or caterpillar inhabits the “jumping bean”) and Carpocapsa pomonana, the codling moth. There are other British species of Carpocapsa, the grubs of which eat into the acorn, the walnut, the chestnut, and the beechnut—a distinct kind or species for each. None of these grubs cause the nuts they attack to “jump.”

The “jumping bean” of Mexico is a segment of the triply divided fruit of a large spurge, which is called Sebastiana palmeri. The spurges are known in England as little green-leaved annuals, with yellow-green flowers and a milky juice. Botanists call them the EuphorbiaceÆ, and in that “natural order” are included the boxwood tree and some tropical trees of great value and importance. None other than the Brazilian indiarubber tree, Hevea, of which we hear so much nowadays, its rubber to the value of £14,000,000 being exported every year from Brazil, is one of them. So also is the Chinese candle-tree, which furnishes a tallow-like fat, made into candles in China. Others are the croton oil and the castor oil shrubs, natives of India, and the manihot or tapioca plant. The fruits of Sebastiana (the jumping bean) are very much like those of the croton; and as there are crotons (though not the one of the purgative oil) in abundance in Mexico, it has taken some time to make sure that the “jumping bean” is not the fruit of a croton, but that of the allied plant Sebastiana. It appears that there is no commercial value for this plant, and that those capsules which happen to contain a grub and move are collected from the ground by the native Mexican boys and sold as curiosities.

The moth (Carpocapsa saltitans) lays its eggs on the Sebastian shrub, and the young grub, on hatching, eats its way into the young fruit when the latter is still quite soft and the seed unformed, and so leaves no hole to mark its entrance. As the fruit swells the grub eats out the seed and surrounding pulp of the segment of the fruit into which it entered early in life. By the time the fruits are dry and fall to the ground the caterpillar is fully grown. Of course, it is only a very few of the capsules which are thus invaded by a grub.

The question very naturally arises, “Why should the caterpillar put itself to the great muscular effort of making the little capsule in which it is contained jump and move over the ground?” It seems probable that these movements are made in order to bring the capsule from an exposed position when it falls on to the ground —where it might be crushed or eaten by some animal—into a position of shelter, either into a hole, or under some stone or fallen wood. The warmth of the sun in an exposed position excites the caterpillar to activity, which ceases when it has reached the shade offered by some protecting cranny. In the same way I have applied artificial heat and, alternatively, shelter from heat, so as to cause the movements or the resting of the jumping bean in a London sitting-room.

These things and others of absorbing interest may be seen in the truly wonderful museum of Kew Gardens, where perhaps the visitor will be disposed to spend more time in cold weather than in the summer. The park at Kew Gardens, with its splendid forest and lakes, and its Italian tower, is one of the beautiful things of England, and it has a special quality even in this season of mist and veiled sunshine. I found there recently, under the trees, as I did fifty years ago, a rare and strange-looking fungus, the Phallus impudicus of botanists,—a furtive denizen of the glades which in late spring are purple with wild hyacinths. The same spot in June presents within a few minutes’ journey from the smoke and smell and noise of Piccadilly a perfect sample of what is, perhaps, the most beautiful sight in Nature—bright sunlight breaking through the young green leaves of a forest on to green herbage. And close by are the azaleas!