Science from an Easy Chair

by Sir E. Ray Lankester

[XXXII]
THE JUMPING BEAN

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.

Fig. 52.—On the right two jumping beans; on the left the caterpillar removed from a jumping bean. The figures are a little larger than life-size, as is shown by the line drawn near the caterpillar giving its actual length. The shape of the “beans,” as segments of a tripartite sphere, is seen. One shows a round hole, with a lid-like piece marked a, removed from the hole. This hole did not exist when the bean first came into my possession in November 1908. At that time the caterpillar within was active, and the bean or fruit-segment often jumped. In April the caterpillar cut this round hole from within, leaving the circular lid in place, and became a chrysalis. The lid was pushed out, as shown in the drawing, by the moth when it escaped from the chrysalis in July. (Drawn from nature for this work.)

[Transcriber’s Note: The line drawn near the caterpillar is approximately ½ inch (1cm) long in the original.]

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.