Science of Water : Concepts and Applications

190 The Science of Water: Concepts and Applications
FIGURE 6.21 Caddis larvae, Hydropsyche spp.
Mostly herbivorous, most caddisfl ies feed on decaying plant tissue, small bits of plant material,
and algae. Some species build nets that they use to catch drifting food. Their favorite algae are
diatoms, which they scrape off rocks. Some of them, though, are predaceous.
Caddisfl y larvae can take a year or two to change into adults. They then change into pupae
(the inactive stage in the metamorphosis of many insects, following the larval stage and preceding
the adult form) while still inside their cases for their metamorphosis. It is interesting to note that
caddisfl ies, unlike stonefl ies and mayfl ies, go through a “complete” metamorphosis.
Caddisfl ies remain as pupae for 2–3 weeks, then emerge as adults. When they leave their pupae,
splitting their case, they must swim to the surface of the water to escape it. The winged adults fl y
during evenings and nights, and some are known to feed on plant nectar. Most of them will live less
than a month: like many other winged stream insects, their adult lives are brief compared to the
time they spend in the water as larvae.
Caddisfl ies are sometimes grouped by the kinds of cases they make into fi ve main groups:
free-living forms that do not make cases, saddle-case makers, purse-case makers, net-spinners and
retreat-makers, and tube-case makers.
As mentioned, caddisfl ies demonstrate their “architectural” talents in the cases they design and
make. For example, a caddisfl y might make a perfect, four-sided box case of bits of leaves and bark
or tiny bits of twigs. It may make a clumsy dome of large pebbles. Others make rounded tubes out of
twigs or very small pebbles. In our experience in gathering caddisfl ies, we have come to appreciate
not only their architectural ability but also their fl are in the selection of construction materials. For
example, we have found many caddisfl y cases constructed of silk, emitted through an opening at
the tip of the labium, used together with bits of ordinary rock mixed with sparkling quartz and red
garnet, green peridot, and bright fool’s gold.
Besides the protection their cases provide them, it also provides another advantage. The cases
actually help caddisfl ies breathe. They move their bodies up and down, back and forth inside their
cases, and this creates a current that brings them fresh oxygen. The less oxygen there is in the water,
the faster they have to move. It has been seen that caddisfl ies inside their cases get more oxygen
than those that are outside of their cases—and this is why stream ecologists think that caddisfl ies
can often be found even in still waters, where dissolved oxygen is low, in contrast to stonefl ies and
mayfl ies.
(4) TRUE FLIES (ORDER: DIPTERA)
True or two- (Di-) winged (ptera) fl ies not only include the fl ies that we are most familiar
with, like fruitfl ies and housefl ies, but also midges (see Figure 6.22), mosquitoes, cranefl ies
(see Figure 6.23), and others. Housefl ies and fruitfl ies live only on land, and we do not concern
ourselves with them. Some, however, spend nearly their whole lives in water; they contribute to
the ecology of streams.