Thu, Aug 15, 2013 10:14 PM
The Outside Story
On the lower levels of the food chain, danger is rarely out of
spitting distance. Risk from predators has spurred the evolution of
many clever adaptations - camouflage coloring, speedy retreat,
distasteful secretions, and armor plating among them. Small jumping
insects known as froghoppers approach concealment in a unique way:
their developing nymphs cover themselves in a bubble bath. From
this trick they derive their common name, "spittle bug."
If you investigate the clumps of white froth, sometimes referred
to as 'cow spit' or 'frog spit,' that appear on plant stems this
time of year, you'll find that each dollop of foam envelops a soft,
greenish insect. Who would have thought that froth, so soft and
insubstantial, could be protective? Yet predators can't see the bug
for the bubbles, and if they probe the foam, they soon find that it
has an acrid taste. The spittle bug's foam is also a good insulator
against heat and cold. And it is a great moisturizer, without which
the soft-bodied nymph would soon dry up.
Despite its name, spittle bug "spit" isn't spit at all, but comes
from the other end of the bug. Like all insects in the order
Hemiptera (true bugs, not to be confused with the common usage of
the word "bug" for insect), spittle bugs feed on plant sap through
piercing mouthparts. Worldwide, there are a wide variety of
Spittlebugs suck sap from the xylem, the long tubular cells that
transport water in plant stems; most other sucking insects feed on
the phloem that lies close beneath a plant's outer sheath. Since
the xylem sap contains fewer nutrients than the phloem, the spittle
bug has to process a lot of fluid to grow.
The bug feeds standing on its head and excretes excess fluid from
its anus. This fluid runs down and coats the spittle bug's body.
Specialized glands mix in mucilaginous compounds that increase the
viscosity of the fluid and also stabilize the bubbles. The nymph
sucks air into its abdominal breathing tube and then forces it out
to blow bubbles while pumping its abdomen up and down. As bubbles
form, it uses its legs to pull the froth over its body. Safe within
this foamy bath, the nymph grows and molts a few times, finally
emerging as an adult.
The adult spittlebug, or froghopper, is a rather drab, stubby
creature in shades of brown to gray. However, it too has a claim to
fame. Although it is a fraction of an inch in length (the common
Meadow Spittlebug averages less than a quarter of an inch), this
bug can jump more than two feet in the air. This is equivalent to a
man leaping over the Gateway Arch in St. Louis. This revelation has
established the spittle bug as one of nature's most powerful
The froghopper doesn't have enormous hind legs, so how does it do
it? The mechanism can best be compared to a crossbow. Large
internal muscles stretch an elastic structure composed of a
substance dubbed resilin. This appears to be the perfect rubber -
it can remain stretched to over twice its length for months, and
still rebound to its original dimensions. The stretched resilin
exerts tension and flexes the bug's stiff exoskeleton, like flexing
a bow. When this tension is released, it powers explosive extension
of the rear legs. When not jumping, the bug keeps its rear legs
poised for takeoff and only uses its four front legs to walk
A jumping frog hopper moves ten times faster than a flea. In fact,
the acceleration is so great that the bug is subjected to a force
equivalent to 400 times that of gravity. Compare that to a diving
fighter plane, which reaches a force equivalent of about ten G.
That's an impressive feat for a little bug, and another great
example of an outstanding adaptation to escape predators.
Li Shen is an adjunct professor at the Dartmouth Medical School
and the chair of the Thetford, Vermont, Conservation Commission.
The illustration for this column was drawn by Adelaide Tyrol. The
Outside Story is assigned and edited by Northern Woodlands magazine
and sponsored by the Wellborn Ecology Fund of New Hampshire
Charitable Foundation: firstname.lastname@example.org