MadSci Network: General Biology

Re: cicadas

Area: General Biology
Posted By: Justin Remais, Student and Engineer Asst., University of California at Berkeley/Lawrence Berkeley National Laboratory
Date: Thu Apr 10 16:12:24 1997
Message ID: 857847510.Gb

What an interesting phenomena you witnessed!

As you may know, cicadas are born as grub-like insects that subsist on
juices drawn from the roots of deciduous trees. As breeding time approaches,
they molt to a more mobile form of the nymph, which eventually burrows upward,
using its saliva to soften the soil as it goes, stopping just below the surface.
There the nymph waits until warming temperatures signal that it is the right
time to emerge. It then makes its way to the surface, and crawls to the nearest
tree. Using its hooked and spiny forelegs, it climbs upward, then firmly grips
the bark, and begins the final molt to the adult stage. The exoskeleton splits
along one medial, longitudinal fissure, and the animal emerges slowly over
a period of about one-half hour. The back arches upward, until the head is free.
Finally, with what looks like a great effort, the insect strains forward and
emerges completely, seizing the shell or the bark with its legs and crawling
free. At first it is yellowish white except for two dark spots which look like
eyes, but are not: perhaps they serve to frighten away casual predators. The
wings must then be "inflated": the veins are pumped with fluid which hydraulically
extends them to their full length and form. As the minutes pass, the exoskeleton
hardens and turns almost black.

A successfully emerged adult, its new exoskeleton and wings adequately seasoned
and hardened, will then crawl upward toward the canopy of the tree. Eventually
it make take off in flight, and within a few days the males among them will
begin their characteristic song.

During adult life, the cicada is able to draw nourishment from plants using its
beak. It is this feeding which results in the phenomena that you described.
Cicadas feed on plant sap from the so called tracheal system of plants (xylem),
which is a very poor food source. This fluid consists of mostly water, with
some sugar and small amounts of other compounds (e.g. nitrogen, amino acids,
proteins, salts, etc.) which are essential constituents of food. Therefore,
cicadas have to suck a lot of this fluid to get enough of these essential substances.
This leaves them with a tremendous excess of water and sugar, which is passed
through their digestive system, and periodically ejected from their bodies,
out their anuses.

This is the case for many related "true bugs" (the suborder Homoptera) which
feed on plants (e.g. aphids), often explaining why ants enjoy their company
(lots of sugar for the taking!).

Don't leave anything under the trees for long unless you want it to have a

These insects are harmless to humans. They cannot bite, and may readily be
handled. Cicadas only very rarely cause any serious permanent damage to trees.

For further information on Cicadas, check out the following web page:
Cicada Mania!

Keep on exploring!

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