Biotic Inventory:
Documenting Diversity at the Katharine Ordway Natural History Study Area
Dineutus hornii, the whirligig beetle
Taxonomy
Kingdom Animalia
Phylum Arthropoda
Class Insecta
Order Coleoptera
Suborder Adephaga
Family Gyrinidae
Genus Dineutus
Species hornii
Common Name: The whirligig beetle and the apple beetle
The term whirligig beetle comes from the swimming pattern of adults
across the water’s surface, and the name apple beetle is due to the
unpleasant secretions of adults to ward off predators, an odor similar
to that of ripe apples1,4.
Diagnostic Characteristics
Aquatic beetles can be identified by their hardened exoskeleton and
veinless sclerotized forewings called elytra that protect their
membranous hind wings. They also contain three pairs of
segmented legs4. The whirligig beetle (family Gyrinidae) is a
type of aquatic beetle that is found in calm bodies of water such as in
some streams, rivers, lakes, and ponds. There are three genera of
whirligig beetle and around 50 species in North America3.
Adult whirligig beetles are usually seen on the surface. The
adult beetles are able to float on top of the water because air is
taken in under the elytra near the tip of the abdomen and is stored in
a dorsal reservoir for buoyancy, and they have a water repellent body
surface (which can be seen in the top right photograph above) and a set
of bristles around the head and antenna, both of which aid in
maintaining the beetle above the water. However, they are also
able to go below the surface as long as they are rapidly swimming or
grasping underwater vegetation2. Whirligig beetles are able to
stay underwater for extended periods of time because they have
abdominal spiracles within their dorsal reservoir which allow them to
store air. Because water beetles swim both above and below the
water’s surface, they have compound eyes which appear to be two pairs
of eyes—one to see below the surface of the water and one to see above
it2.
Whirligig beetle larvae, on the other hand, are found underwater.
Dineutus larvae are creamy white with the exception of their dark brown
head and dorsal plates of the prothorax. The larvae grow to be 25
to 30mm long. They breathe using ten pairs of lateral tracheal
gills, and swim using these densely fringed gills and dorsoventral
movements of their abdomen2.
An adult beetle belonging to Gyrinidae is elongate-oval in shape, from
about 3 to 15 mm long. Short, clubbed antennae extend from the
head. The first ventral segment of the body, or sternite, is
divided by the hind coxae, which are the segments of leg that attach to
the body. Each tarsus, or final segment of leg, contains five
joints, a characteristic which distinguishes the insect from many other
types of beetles. As shown in the picture above, only the front
legs are visible from a dorsal view because these forelegs are long and
thin, whereas the middle and hind legs are rather short and paddlelike
and cannot be seen from this view3.
This specimen, collected at Katherine Ordway Natural History Study
Area, belongs to Dineutus because of its size, 10.5mm, which is in the
length range of this genus (9-15mm) and places it within the category
of the large whirligig beetles. This, along with a concealed
scutellum (posterior portion of the thorax), distinguishes it from the
small whirligig beetles of genus Gyrinus, which are typically only
5-7mm long and have an obvious scutellum. Dineutus beetles are
also characterized by their broad oval shape, and black color which can
be either dull or shiny or have a bronze tint. Also, nine
indented lines are found on the elytra5.
To differentiate between common species of Dineutus in the Midwest,
color patterns and the morphology of the elytra and anterior legs are
examined2. In this case, the specimen obtained is most likely
Dineutus hornii because of the distinct reddish-brown color of its
epipleura (the bent under portion of the elytra), and because its body
length is 10.5mm long which is between the length range of 9.5-11mm for
the species. The specimen is black with a dorsal bronzy sheen,
and its ventral side is dark reddish-brown. Its legs are
brownish-yellow1. Although Dineutus discolor is also
reddish-brown on its ventral surface, it is only found in running
water, and the specimen collected was found in a stagnant pond2.
Dineutus americanus can be ruled out because the adult body length of
the species is 8.0-9.0 mm, which is too small to be the specimen
found. Neither can the specimen be Dineutus carolinus, which has
a much rounder, less elongate body shape. The specimen found is
most similar to the Dineutus species, Dineutus nigrior, because both
species are oval, shiny black on their dorsal side, with brownish
elytra, and brownish yellow legs. However, Dineutus nigrior is
black ventrally, whereas Dineutus hornii is a dark reddish brown on its
ventral surface1. Also, another source indicates that the
specimen is slightly too small to be Dineutus nigriori because males
and females of the species range from 10.8-12.1mm, and the species that
was found is 10.5mm long2.
Ecology
Throughout North America, the habitat of Dineutus hornii includes slow
running bodies of water such as in some ponds, lakes and, streams5.
Larvae are below-the-water predators of aquatic insects, preying on
mites, snails, and small aquatic insects such as Odonata and
Ephemeroptera nymphs and larvae of Dipterans2,5. The larvae suck
out the body fluids of their prey using their hollow mandibles, and the
exoskeleton of their prey is left behind as waste.
The adults inhabit the surface of the water and are scavengers, feeding
on dead and dying insects on top of the water3. Adults do not
have many predators because they produce putrid secretions to ward off
enemies4. To avoid obstacles and find prey, adult D. hornii use
their antennae to detect slight water movements on the water’s surface
and they use their long forelegs and short paddlelike hind legs to
swim4. Oftentimes, adults come together to swim in clusters that
can be a few feet across4,5. If threatened by a predator, a few
individuals of the gathered group may dive below the surface while the
rest of the group swims rapidly above the surface to escape5.
Life History
Gyrinids begin copulating in the spring, and the copulation period may
last until August for some species. The eggs are laid in
bunches or rows on underwater plants shortly after mating occurs.
The genus Dineutus lays more eggs than the genus Gyrinus—a total of
around 20 to 50 eggs2. The adults die almost immediately after
the eggs are laid, so life expectancy depends on when the adult
whirligig beetle mates. Usually there are two generations
produced per year, one in the spring from April to June, and one in the
summer from July to August6. Egg incubation lasts around 5 to 17
days, and the wide variation is due to the environmental conditions and
interspecific differences within the family Gyrinidae2.
Once the larvae hatch, they move to the bottom of their water habitat
and begin feeding. At the end of the larval stage, the larvae
leave the water to build a pupal case on plants emerging from the water
from materials such as mud or other debris2,6. Finally, the
adults emerge after being in the pupal stage for eight to ten
days. The time from copulation until the new generation of
whirligig beetles is produced is approximately six weeks.
Many adults spend winter in the mud or sediment on the bottom floor of
their water habitat or cling to underwater vegetation during the cold
months, however some species of Gyrinids have been known to stay active
all year long. Most species of Gyrinids are diurnal, but some
have been found to become nocturnal in response to adverse
environmental conditions2.
Distribution
D. hornii is found throughout North America, predominantly in the
eastern half of the United States from Minnesota to New
York1,2,5. Conservation efforts are not necessary because this
whirligig beetle is “widespread, common, and not globally threatened.�1
Voucher Information
This specimen was collected on the surface of the small, calm pond
located next to the tall grass prairie at Ordway Natural History Study
Area in Inver Grove Heights, Minnesota.
The stagnant water in which the specimen was found was full of aquatic vegetation.
References
1. Discover Life in America. 2009.
http://www.dlia.org/atbi/species/Animalia/Arthropoda/Insecta/Coleoptera/Gyrinidae/index.shtml
2. Ferkinhoff, W.D., Gundersen R.W. 1983. A Key To
the Whirligig Beetles of Minnesota and Adjacent States and Canadian
Provinces (Coleoptera: Gyrinidae). Scientific Publications of the
Science Museum of Minnesota, St. Paul, MN.
3. Foltz, J.L. 2001. ENY 3005 Family Identification (Coleoptera: Gyrinidae)
http://entomology.ifas.ufl.edu/foltz/eny3005/lab1/coleoptera/gyrinid.htm
4. Giude to Aquatic Invertebrates of the Upper Midwest. 2004.
http://wrc.umn.edu/prod/groups/cfans/@pub/@cfans/@wrc/documents/asset/cfans_asset_115813.pdf
5. Milne, M., Milne, L. 1980. The Audubon Society Field
Guide to North American Insects and Spiders. Alfred A. Knopf,
Inc., New York, NY. Pp.543-544.
6. Van der Eijk, R.H. 1986. Population Dynamics of Gyrinid Beetles II. Reproduction. Oecologia. 69: 31-40.
Compiled by Holly Schiedermayer
Biodiversity & Evolution (BIOL 270) Professr Sarah Boyer. Spring 2010
Specimen collected at Macalester College’s Katharine Ordway Natural History Study Area on April 15, 2010.