Popular Science Monthly/Volume 57/May 1900/A Bubble-Blowing Insect
|A BUBBLE-BLOWING INSECT.|
By Prof. E. S. MORSE.
MANY years ago, while preparing an elementary book on zoölogy, I had occasion to make a drawing of the little insect which is found on grass and other plants immersed in flecks of froth. This substance is commonly known as frog spittle or cuckoo spit, and, being found in the spring, is known in France as "spring froth."
Works on entomology gave the general statement that this insect emitted the frothy mass from its body. Curious to ascertain what peculiar gas-secreting apparatus was contained within its anatomy, I dissected a number of specimens, without finding a trace of any structure that could produce from within the body a single bubble of air. On the contrary, I found that the little insect emitted a clear, somewhat viscid fluid, and by means of appendages at the extreme tip of its tail secured a moiety of air by grasping it, so to speak, and then instantly releasing it as a bubble in the fluid it had secreted. At the time of this observation—twenty-five years ago—I supposed that entomologists were familiar with this fact, but, on the appearance of my little book, I received a letter from the late Dr. Hermann Hagen, the distinguished entomologist, stating that he had ransacked his library and failed to find any reference of the nature of my statement. Doubtless the whole history of this insect has since been published, but a somewhat superficial survey of the literature has failed to reveal any reference to the matter. In this connection it is interesting to observe how often the more easily accessible facts of Nature escape the special student. The history of science is replete with such instances. One can hardly take up any subject connected with the life history of animals without finding lacunae which ought to have been filled long ago. The facts in regard to the ossification of the hyoid bones in man is a case in point. The persistence of these erroneous concepts or half-truths comes about by the acceptance at the outset of some fairly trustworthy account by an authority on the subject, and ever after the statements are copied without a doubt being expressed as to their accuracy.
If we look over the literature of the subject under discussion, we find that in nearly every case the statement in regard to the spit-insect conveys the idea that the creature secretes the froth in which it is immersed. Beginning with De Geer in the last century, we quote as follows: "One may see coming out of the hinder part of its body a little ball of liquid, which it causes to slip along, bending it under its body. Beginning again the same movements, it is not long in producing a second globule of liquid, filled with air like the first, which it places side by side with and close to the preceding one, and continues the same operation as long as there remains any sap in its body." Kirby and Spence, in their Entomology, describe "the white froth often observed on rose bushes and other shrubs and plants, called by the vulgar 'frog spittle,' but which if examined will be found to envelop the larva of a small hemipterous larva (Aphrophora spumaria), from whose anus it exudes." In Westwood's Insects we find the following statement: "One of the best-known insects in the family is the Aphrophora spumaria, a species of small size which frequents garden plants, the larva and pupa investing themselves with a frothy excrementitious secretion which has given rise to various fancies. A species of Aphrophora is also found in great quantities upon trees in Madagascar, the larva of which has the power of emitting a considerable quantity of clear water, especially in the middle of the day, when the heat is greatest." Here the statement is definitely made that the froth is excrementitious, and the Madagascar insect is shown to be different from Aphrophora in that it exudes a clear water. In Dr. Harris's Treatise on some of the Insects Injurious to Vegetation, of Massachusetts, we find a most definite statement as to the origin and nature of this froth. He says: "Here may be arranged the singular insects, called frog hoppers (Cercopididæ), which pass their whole lives on plants, on the stems of which their eggs are laid in the autumn. The following summer they are hatched, and the young immediately perforate the bark with their beaks and begin to imbibe the sap. They take in such quantities of this that it oozes out of their bodies continually in the form of little bubbles, which soon completely cover up the insects." In Dr. Packard's admirable Guide to the Study of Insects the statement is made that "Helochara and Aphrophora, while in the larva state, suck the sap of grasses and emit a great quantity of froth, or in some cases a clear liquid, which in the former case envelops the body and thus conceals it from sight. It is then vulgarly called 'toad's spittle.'"
In other accounts it is stated that the larvæ live covered by masses of froth which the insects produce by expelling from their beak the juices drawn out of the tree.
The above extracts are sufficient to indicate the common belief among entomologists that the insect in some way emits this froth from the body. A most cursory examination of the creature, however, shows that its only secretion is a clear fluid.
The so-called frog spittle or cuckoo spit (Fig. 1) appears as little flecks of froth on grass, buttercups, and many other plants during the early summer. These flecks of froth may be found very commonly at the junction of the leaf with the stem. Immersed in this froth is found a little green insect, sometimes two or three of them, concealed by the same moist covering. This little creature represents the early stage of an insect which in its full growth still lives upon grass, and is easily recognized by its triangular shape and its ability of jumping like a grasshopper. There are a number of species; the one living on grass apparently confines itself to the grass alone, though I have seen one species that frequents a number of different plants. A species found on the white pine is dark brown in color, and the froth in which it is found not only hangs pendent from the branch, but the lower portion appears as a large drop of clear water.
Let one provide himself with a good hand lens, a bit of glass (a watch crystal is especially suitable for this purpose), and a common camel's hair brush, and he is ready to make a preliminary study of Aphrophora. The brush is convenient for easily removing the insect from the froth which invests it. If the insect is cleared from the mass of froth, it will crawl quite rapidly along the stem of the plant, stopping at times to pierce the stem for the purpose of sucking the juices within, and finally settling down in earnest, evidently exerting some force in thrusting its piercing apparatus through the outer layers, as shown by the firm way in which it clutches the stem with its legs. After sucking for some time, a clear fluid is seen to slowly exude from the posterior end of the abdomen, flowing over the body first and gradually filling up the spaces between the legs and the lower part of the body and the stem upon which it rests (Fig. 2). During all this time not a trace of an air bubble appears; simply a clear, slightly viscid fluid is exuded, and this is the only matter that escapes from the insect. In other words, its secretion of clear fluid is precisely like that of the Madagascar species referred to by Westwood and others.
This state of partial immersion continues for half an hour or more. During this time, and even when the insect is roaming up and down the grass or twig, the posterior segments of the abdomen are extended at intervals, the abdomen turning upward at the same time. It is a kind of reaching-up movement, but whether this action accompanies a discharge of fluid or is an attempt at reaching for air I have not ascertained. Suddenly the insect begins to make bubbles by turning its tail out of the fluid, opening the posterior segment, which appears like claspers, and grasping a moiety of air, then turning the tail down into the fluid and instantly allowing the inclosed air to escape (Fig. 3). These movements go on at the rate of seventy or eighty times a minute. At the outset the tail is moved alternately to the right and left in perfect rhythm, so that the bubbles are distributed on both sides of the body, and these are crowded toward the head till the entire fluid is filled with bubbles, and the froth thus made runs over the back and around the stem (Fig. 4).
Even when partially buried in these bubbles the tail is oscillated to the right and left, though when completely immersed the tail is only occasionally thrust out for air which is allowed to escape in the mass apparently without the right-and left movement, though of this I am not sure. It is interesting to observe that in half a minute some thirty or forty bubbles are made in this way—a bulk of air two or three times exceeding the bulk of the body—without the slightest diminution in the size of the body.
If the insect is allowed to become dry, by resting it upon a piece of blotting paper, and is then placed upon a piece of glass and a drop of clear saliva be allowed to fall upon it, it proceeds to fill up this fluid with bubbles in precisely the same manner as it did with its own watery secretion. It is quite difficult to divest the creature of the bubbles of air which adhere to the spaces between the legs and the segments on the underside of the body. It may be readily done, however, by immersing it in clear water and manipulating it with a brush. If now it is again dried and placed on the glass it will slowly secrete what spare fluid it has in its body, but not the minutest bubble of air is seen to escape. These experiments should be made on glass, for then one may get transmitted light, and the highly refractive outlines of the air bubbles are more quickly detected. Using a higher power with a live cell, new features may be observed. Confining the insect in this way, inclosed in a drop of water, a very clear proof is offered that it gets all the air for its froth in the way I have described. So long as the insect remains surrounded by water not the minutest bubble of air is seen to escape from the body. During this immersion the creature is incessantly struggling to reach the edge of the drop, and no sooner has this been accomplished than it thrusts out its tail and begins the clutching of air and the making of bubbles. The bubbles, however, disappear as soon as made, as the clear water will not preserve them. As the water becomes slightly viscid from the insect's own secretions, the bubbles remain for a longer time. A bubble will be partially released and then held, or even partially withdrawn, between the claspers.
The claspers seem to be the tergal portions of the ninth segment. On the sides of the seventh and eighth segments may be clearly seen leaflike appendages, which are possibly branchial in their nature (Figs. 5 and 6). They are extremely tenuous, and appear like clusters of filaments, slightly adhering together and forming lamellate appendages similar to the gill-like appendages seen in the early stages of Potamanthus, a neuropterous insect, not, however, having the definiteness of these structures. While in
|Fig. 5.—Showing underside of posterior extremity of body with appearance of branchiæ.||Fig. 6.—A single branchia under slight pressure.|
Potamanthus one may easily trace the ramifications of the tracheal system, I have not been able to detect a similar connection with these appendages in Aphrophora. Certainly the insect does not depend upon these structures for respiration, as when the creature is perfectly dry it seems to suffer no immediate inconvenience, but will crawl about the table or even on the dusty floor and live for an hour or two in this condition. The usually glabrous surface of the body, however, becomes shriveled after a while. On the other hand, it immediately sinks in water, and will live for some time immersed in this way, and this leads me to believe that the appendages above described may perform a slight respiratory function. The fact that the insect immediately sinks may be cited as an additional evidence that it does not emit air.
It is interesting to observe that regarding this stage of Aphrophora as an aquatic stage, since it lives immersed in fluid, we have the same behavior that we observe in the aquatic stages of other Hemiptera, as well as in insects of other orders. The great water beetle Hydrophilus has an aquatic larva. Myall, quoting Lyonet, says: "They never remain long at the bottom of the water; air is necessary for them, and this they take in by the tail, which they raise from time to time to the surface of the water." In the larva of Dysticus, another water beetle, the only functional spiracles are the last pair, opening at the tail. The little oval beetles, known as whirligigs, from their rapid whirling motion, when swimming on the surface of the water carry down a bubble of air on the end of the abdomen, and when this has been exhausted in the process of respiration rise to the surface for a fresh bubble. The larvæ of some forms are furnished on each side with long respiratory filaments.
A number of neuropterous insects whose early stages are passed in the water are furnished with branchial tracheæ or false gills. These consist of filaments springing from the sides of the abdominal segments. In the early stages of certain dragon flies the rectum supports epithelial folds which are filled with fine tubes from the tracheal system. Among certain aquatic insects belonging to the order of Hemiptera the creatures reach out the hinder portion of the body to secure air. Dr. Myall, in his very interesting book on the Natural History of Aquatic Insects, says: "A Nepa or a Ranatra may sometimes be seen to creep backward along a submerged weed until the tip of its breathing tube breaks the surface of the water."
The Aphrophora while immersed in the watery fluid, whether secreted by itself or consisting of clear water which has been supplied to it, reaches out for air in a precisely similar manner. Primarily the froth made by this insect not only keeps the body moist, but acts as a protection against its enemies.
A number of individuals may often be found in one fleck of froth, and they are entirely hidden from sight while immersed in this way. The viscid character of the fluid secreted insures the retention of the air the insect collects in the form of little bubbles. This peculiar feature must have been a secondary acquisition. The bubbles not only surround the insect and the stem upon which it rests, but flows in a continuous sheet between the ventral plates of the abdomen, and the insect probably utilizes this air in the manner of other air-breathing aquatic larvæ—namely, through its spiracles. As many aquatic larvae respire in two ways, either inhaling air through the spiracles or by means of branchial leaflets, so Aphrophora may likewise utilize its branchial tufts for the same purpose. For this reason we can understand how each fresh bubble added to the mass may aërate the fluid, so to speak, and thus insure at intervals a fresh supply of oxygen.