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 lacunæ 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.
[Illustration: FIG. 1.--Grass stalk and leaves, showing appearance of
froth.]
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.
[Illustration: FIG. 2.--The insect first emitting a clear fluid which
fills up the interspaces between its body and the stem of grass upon
which it rests.]
[Illustration: FIG. 3.--After the lapse of some time it begins rapidly
to make bubbles.]
[Illustration: FIG. 4.--Entire fluid filled with bubbles, and the froth
thus made enveloping the stem.
MAGNIFIED FIGURES OF APHROPHORA, SHOWING SUCCESSIVE STAGES IN THE
PRODUCTION OF FROTH.]
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.
[Illustration: FIG. 5.--Showing underside of posterior extremity of
body with appearance of branchiæ.]
[Illustration: FIG. 6.--A single branchia under slight pressure.]
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 _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 larvæ
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.