During my career, I attended many conferences on Aquatic
Biology and they always attracted exhibitors demonstrating nets, electronic
measuring equipment, collections of the latest books, etc.. Occasionally, there
would be something different and, at one conference, I came across a stand
where jewellery was for sale. At first, it was not easy to see the connection
to life in water, until it was explained that the various earrings and pendants
were made from cases that had been produced by caddis fly larvae. Partly
because of the novelty, and partly because they were attractive, I bought a
pair of earrings that had cases made from fragments of jasper and my wife was
pleased with them, and intrigued by the story of their manufacture.
Subsequently, I found that this type of jewellery is
available from several sources and, in addition to the use of fragments of
natural stone, caddis fly larvae are allowed to build cases with precious
stones, pearls and even gold fragments.
Many, but not all, caddis fly larvae build cases, and the
materials that they use vary from species to species. Some build cases of
vegetation and some of stones of a range of sizes, the common feature being
that all the separate pieces of material are cemented together with silk
produced by the insects from modified salivary glands, which exit near the
mouth. The silk is not only strong but highly adsorptive, and its “stickiness”
is not affected by it being produced under water. It was silk that bound the
stones of jasper in my wife’s earrings and, having produced a case, it was an
easy matter for the jewellers to remove the larva, pick up the cases, dry them
and then cover them with lacquer to ensure that they would not fall apart for
years. The larva could then start the process all over again, just as it would
do in natural conditions if, for some reason, it was separated from its case.
Caddis larvae use cases for protection, camouflage and
ballast, but some forms do not make cases, using silk to provide an enclosure
to which stones may be attached, or to make feeding nets. These vary from wide
aperture nets, useful for capturing materials from a fast current, through to
large sac-like nets in which the larva lives while grazing over the material
that has accumulated. Some caddis larvae are free-living predators and they
have neither cases nor silk nets. Other caddis larvae build cases entirely from
layers of silk. Whatever the adaptations shown by the larvae, it is important
to remember that the larval stage is but one part of their life cycle. Having
completed five larval stages, moulting between each to achieve larger size, the
final stage larva spins a cocoon in which to pupate. This cocoon is frequently
covered with stones or other materials and is cemented to the substratum using
silk. A complete transformation of the insect now occurs, with the adult biting
its way from the cocoon and emerging from the water to fly away and complete
the life cycle by mating and laying eggs back into the water.
For those who believe in the literal accounts of Creation in Holy Books,
the biology of these insects is easy to explain - a deity designed each
species, its life cycle and its choice of whether to build a case, or not. Furthermore,
the materials that each species used in case construction, and its preferred
habitat, were all designed and the insects put in place some time in the first
days of life on Earth. All very straightforward, but I am not a Creationist and
therefore face some challenging questions - questions that I cannot answer
readily (or at all):
1. Why the complex life cycle, with aquatic larval and pupal
stages and terrestrial adults?
2. Why the use of silk?
3. Why the choice of different case materials, or the
building of nets?
Here are some thoughts on each of these questions:
Why the complex life cycle, with aquatic larval and pupal
stages and terrestrial adults?
Primitive insects have larval stages that become
progressively larger and it is from the last larval stage that the flying adult
emerges. Evidence of this form of metamorphosis is seen by observation of
dragonfly larvae, where the growing wing buds are seen throughout larval life, and
especially in the last stage. Caddis larvae do not need to grow wing buds, or
other adult features, as the re-organisation of body form is completed within
the pupal skin. In both types of development, all mechanisms are under genetic
control, so the evolution of the genetic template is the basis of everything.
We can only speculate on how pupation developed, but it must have resulted from
mutations that, taken together, allowed the dramatic transformation from larva
to adult. How did it happen; and did it occur gradually, or in an explosion of
change? I don’t know.
We know that aquatic insects were originally terrestrial in
all their life stages and that they had originally evolved from an ancestral
form that was marine. Interestingly, there is evidence that the invasion of
fresh waters by different groups of aquatic insects has occurred at different
points in geological time, 1 from ca. 200,000,000 years ago through
to more recent times (perhaps just tens of millions of years ago). Interestingly,
while there are many types of aquatic insects in streams, rivers, ponds and
lakes, they are rare in marine habitats.
Why the use of silk?
The ancestors of caddis flies, and some other insects,
evolved the secretion of silk from the salivary glands, the original secretions
being use in feeding. Silk consist of a complex of materials, including very
strong protein filaments and, once evolved, the insects had an excellent mechanism
for joining components of cases or for making tent-like refuges. As silk is
equally effective under water, this feature was of advantage when the invasion
of flowing and still fresh waters occurred.
Why the choice of different case materials, or the
building of nets?
Having invaded water, ancestral caddis flies would need to
acquire food. The production of silk nets allowed the capture of particles from
flowing water but the water could also cause the displacement of larvae. This
may be the origin of stone cases as ballast, as well as for protection. If
larvae were protected from being swept away, and avoided predation by having
cases that were made of the materials that surrounded them, they would have had
an increased chance of survival. Thus, the genetic basis of the case-building
habit will have been passed to future generations. Different types of cases
show adaptation to different habitats and ponds with abundant vegetation will
provide different selection pressures than streams or lake shores with stony substrata.
As the caddis larvae spread throughout fresh waters, the formation of different
species resulted from the selection of characteristics that promoted separation
of ancestral breeding stocks and thus a diminished chance of inter-breeding,
something which also occurred with the geographical separation of adults,
together with changes in their breeding behaviour and apparatus. This process of
speciation explains the many different forms that we see today.
It is difficult to comprehend the time periods over which
evolution has occurred. However, thinking about the possible evolution of caddis
flies is, to use a modern term, awesome and, for me, much more compelling than
ideas based on Creation and a Designer. However, if it’s information you want
on the various events during evolution.......
1 Timothy M Bradley et al. (2009) Episodes in insect evolution. Integrative and Comparative Biology 49: 590-606.
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