BRN 9-2 (uncompressed) - Flipbook - Page 74
ÒThe cocoon-like egg case of a giant
water scavenger beetle, such as
Hydrophilus triangularis, is constructed from silk and attached to
ßoating vegetation. It is not a cocoon
in the traditional sense, which is built
by a larva to pupate within.
Construction process
Oviposition and case formation: In
early summer, the female beetle
deposits up to 140 eggs within a
silken, waterproof case.
Case type: The female can either
attach the egg case to underwater
plants, leave it ßoating on the water's
surface like a raft, or carry it on her
abdomen, depending on the species.
Mast creation: In many species, a
vertical, air-Þlled "mast" is built into
the egg case that extends above the
water's surface.
Hatching structure: The larvae hatch
from the egg case by chewing their
way out. The mast may serve as a
potential escape hatch for the larvae,
which are cannibalistic and will
consume their siblings if they remain
in the case. The mast is also likely
involved in respiration for the
developing eggs.
Life cycle
Hatching: The larvae hatch from the
egg case and disperse. Due to high
rates of cannibalism, not all larvae will
survive to exit the case.
Pupation: The mature larva leaves the
water and burrows into moist soil to
pupate.
Adulthood: The adult beetle emerges
from the soil after a few weeks and
returns to the water.Ó
The Giant Water Scavenger Beetle is a
Colorado Species of Interest: https://
webdoc.agsci.colostate.edu/bspm/
Hexapoda%20(Insects)/
Giant%20Water%20Scavenger%20Be
etle.pdf in which the 'Life History and
Habits' section states:
"Giant scavenger beetles winter in the
adult stage, buried in the mud at the
bottom of water bodies. They emerge
in spring and eggs are laid in late
spring or early summer. The female
produces eggs in the form of a mass
that is laid in a silken case that is
attached to ßoating debris. The
immature stage is a predator, working
by ambush to lie in wait, seizing and
crushing prey that comes within
reach. Most of their diet is made up
of small insects and other aquatic
invertebrates. However, their jaws
are quite powerful allowing them to
consume snails whole as well as catch
larger prey such as tadpoles and small
Þsh. After completing development it
leaves the water and pupates within a
soil chamber produced a few inches
deep in moist soil. Adults emerge in
about 2-3 weeks.
ÒAdults also are predators but will
scavenge dead animal matter in the
water. Water scavenger beetles are
excellent swimmers, with large hind
legs feathered to propel them quickly.
They are also able to stay under water
for extended periods (including all of
winter) by carrying with them air
bubbles under the wing covers and
along the underside, trapped by small
hairs of the body. As the oxygen is
depleted within the bubbles it is
replenished from the oxygen in the
water. This air supply is also
periodically renewed by a brief visit
to the water surface. When surfacing
they go head Þrst, extending the
antennae to break the surface which
allows fresh air to move over the body
and form a new bubble."
Additionally, on August 24, 2025, I
observed a Giant Water Scavenger
Beetle (in iNaturalist, it was identiÞed
to the subgenus Hydrophilus by the
contributor, ÒrzeczpospolitaÓ)
repeatedly swim head-Þrst into the
edges of a ßoating mat of algae,
which I initially interpreted as
foraging/feeding behavior. (See
photographs on following two
pages.) When I later processed the
images, there were bright white ovoid
forms in the water near its abdomen
tip, and I considered they might be
new eggs emerging from this beetle.
There were no observable attached or
ßoating cocoons (egg cases) as shown
on the previous page, connected to
the algal mat.
James Von Loh iNaturalist observation of 04 August 2024. His note: ÒLarge larvae hunting and foraging on a spadefoot
toad tadpole/larva within the retention pond at Dripping Springs Visitor Center, Organ Mountains-Desert Peaks National
Monument. Once captured, as many as 1-6 additional large larvae would join the tussle to consume the tadpole. The
morning was warm, sunny, and under clear sky.Ó Shown here under a creative commons license. (IdentiÞed as
Hydrophilus triangularis.)
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