Contributions to the Systematics and Ecology of Aquatic Diptera—A Tribute to Ole A. Sæther, 2007, T. Andersen (ed.),
pp. 73-79. © 2007 The Caddis Press.
Hydrobaenus saetheri Cranston, new species, an aestivating, winteremerging chironomid (Diptera: Chironomidae) from California
PETER S. CRANSTON1, GINA M. BENIGNO2 AND MARIANNE C. DOMINGUEZ1
1
Entomology Department, University of California Davis, Davis, CA 95616, USA
E-mail: pscranston@ucdavis.edu
2
California Department of Water Resources, 3251 S Street, Sacramento CA 95816 USA
Abstract. A new orthocladiine midge, Hydrobaenus saetheri Cranston, new species, is described
from larvae, pupae and adults, evidently belonging to the genus Hydrobaenus Fries. Larvae
aestivate over summer, and commence development as soon as wetted when winter-rains induce
flow in seasonal waterways. Larvae of this species dominate in the early flow, and adult numbers
can reach nuisance levels in mid-winter.
Key words: Chironomidae, Orthocladiinae, Hydrobaenus, new species, California
Introduction
adults. Life history details are provided here from
experimental and field observations. All stages are
identifiable readily as belonging to Hydrobaenus, but
no life history stage matches any previously
described taxon. Here the species is described as
Hydrobaenus saetheri new species, taking authorship
of the senior author alone. The epithet acknowledges
Professor Ole A. Sæther’s important resolution of
the generic boundaries in this area of previous
confusion, and in recognition of his many
contributions to systematics of Chironomidae, on the
occasion of his 70th birthday.
The chironomid genus Hydrobaenus Fries is
restricted to the northern hemisphere and includes
several species that emerge as adults in mid-winter
having developed from aestivating larval cocoons
(Sæther 1976; Grodhaus 1980; Kondo & Okuda
1990). Some species attain nuisance numbers such
as Hydrobaenus kondoi (Sæther) in Japan, described
by Kondo (1996) as “enormous swarms of midges
have brought serious nuisances: inhalation of dust
from dead adults, disturbance to drivers, and
invasions of factories and residences in the area”.
Other species have been alluded to as emerging in
large numbers elsewhere (e.g., for Germany,
Steinhart 2000). Amongst reports of aestivating
Chiro-nomidae, Grodhaus (1980) mentioned a
species of Hydrobaenus from California whose
identity has remained unknown. Recently, in a study
of fish and their diets in the Yolo Bypass, a winter
floodway of the waterways due west of Sacramento
(California), a dominant species of chironomid was
revealed to emerge abundantly in winter. Indeed, the
adult midges are abundant enough to cause nuisance
to the vehicle drivers on “the river of cars” (Gary
Snyder, unpublished) on the busy Interstate 80.
Rehydration of previously summer-dry sediments has
revealed early larval instars emerge post-inundation
and develop quite rapidly to mid-winter emergent
Material and methods
All specimens came from sites within the Yolo
Bypass waterway (Fig. 1), the primary floodplain for
the Sacramento River and associated waterways
(Sommer et al. 2001). This seasonally inundated
flood-control system extends some 61 kms from
north to south, is up to 16 km wide and covers some
24,000 hectares. The floodplain is typically
inundated at least once annually from the Sacramento
River or smaller tributaries during winter or spring
in all but the driest of years. All collections were
made between November 2004 and late January 2005
by G. M. Benigno. In November 2004 pre-inundation
sediment samples 45 x 30 x 10 cm were collected
73
from the perennial floodplain channel. Adult midges,
often with associated exuviae still attached, and
pharate adults especially were intercepted by such
drift nets. Samples were fixed in 10% buffered
formalin containing Rose Bengal dye and transferred
to 70% ethanol for preservation within one month
of collection.
Slides were made by clearing in KOH,
neutralization in Glacial Acetic Acid, dehydration
in Isopropanol and mounting in Euparal. Adults,
especially females, predominantly were teneral
with incompletely expanded wings, and were
collected into formalin which fails to clear and
make poor slides. Some larval mounts were made
in Hoyers Mountant and ringed with clear nail
varnish.
Morphological terminology follows Sæther (1980)
and Cranston (1994) except for using taeniate for
filamentous.
Institutional abbreviations
BMNH – British Museum (Natural History),
Cromwell Rd, London SW7 5BD, UK.
USNM – United States National Museum,
Washington, D.C. 20560, USA.
ZMB – The Natural History Collections, Bergen
Museum, Bergen, Norway.
ZSM – Zoologische Staatssammlung, Munchen,
Germany.
Hydrobaenus saetheri Cranston, new species
Type material. Holotype male, slide mounted in
Euparal. USA: California, Yolo Co., Causeway,
wildlife area, North of I80 freeway, 38°31’27”N
121°35’21”W, 9.xi.2004, G. M. Benigno (USNM).
Paratypes: same locality as holotype; 24 L (10 slides,
8 Hoyers), 6 Pe/male, 1 Pe/female, 9 males, 6 females
(BMNH, USNM, ZMB, ZSM).
FIG. 1. Map of the Yolo Bypass and lower Sacramento
River, Sacramento, California, from Schemel et al.
(2001).
from summer-dried areas of the Yolo Bypass.
Samples were placed in clear plastic bins covered
with a 1 mm mesh and flooded to a minimum depth
of 10 cm on November 5, 2004. Flooded sediments
were exposed to ambient photoperiod and water
temperature, which ranged in the bins from 9–14°C.
For three weeks after inundation, samples were
collected twice weekly using a 100 μm mesh hand
net and preserved in 70% ethanol.
One paratype comprises the voucher for the
species (and hence the genus) in an ongoing
molecular phylogeny project (Morse & Cranston
in prep).
Male (n = 10)
Body length 3.8–5.4 mm, wing length 2.6–3.2 mm.
Thorax, legs and abdomen brown-black.
Head. Antennal segments 1–12: 185–210 μm
long, 13: 300–350 μm long. AR 1.57–1.89. Head
with 8–11 uniserial outer verticals / postorbitals, 9–
12 clypeals. Palp segment lengths (in μm): 2–5, 45–
60, 120–150, 90–160, 100–120.
Initial inundation of the floodplain occurred in late
December, 2004. During the first month of
inundation, weekly drift net collections were made
74
Hydrobaenus saetheri Cranston, new species
Hypopygium, as in Figure 2A. Gonocoxite 240–
260 μm long. Gonostylus 110–118 μm long, with 3–
7 apical / subapical megasetae (Fig. 2B)
Thorax. Antepronotal lobes broadly connected
medially, with weak notch. Thoracic setation: 4–6
lateral antepronotals, ? 0–3 minute acrostichals, 4–9
dorsocentrals, 4–5 prealars, 0 supraalars, 7–9
uniserial scutellars.
Wing. Anal lobe slightly protruding, costa very
weakly extended. Squama with 14–20 setae, R with
4–5 setae. V.R. c. 1.10.
Legs. Midleg with pseudospurs on ta 1 . LR 1
0.73–0.96, LR 2 0.54–0.59, LR 3 0.50–0.53.
Female (n = 4)
Body length 3.0–3.8 mm, wing length 2.2–2.5 mm.
H e a d . Te r m i n a l ( 5 t h ) a n t e n n a l s e g m e n t
darkened, 120–145 μm long. AR 0.61–0.74. Head
with 2–3 frontals, 2–3 strong postorbitals, 9–13
clypeals.
FIG. 2. Hydrobaenus saetheri Cranston, new species, adult. A. – Male hypopygium. B. – Apex of gonostylus. C.
– Female genitalia, lateral view. D. – Female genitalia, ventral view, left side.
75
Cranston, Benigno, and Dominguez
Seminal capsules bicolored (dark in oral 2/3), 135–
155 μm long, 62–75 μm wide, elongate-ovoid,
without distinct neck; seminal duct with 10–15 μm
long microtrichia orally, with strong loop.
Thorax. Setation: 4–6 lateral antepronotals, many
small hooked acrostichals, 5–8 dorsocentrals, 3–4
prealars, 0 supraalars, 9–10 uniserial scutellars.
Wing. Squama with 18–24 setae, R with 8–10,
R1 with 4–6, and R4+5 with 1–2 setae. V.R. 1.01–1.05.
Legs. LR1 0.69–0.72, LR2 0.50–0.51, LR3 0.50–
0.53.
Genitalia (Figs. 2C, 2D). Gonocoxite IX
protruding, microtrichiose, with 12–19 setae. Tergite
X with 15–21 long setae on each of 2 protrusions.
Pupa (n = 7)
Length 4.4–5.1 mm.
Cephalothorax 1.06–1.22 mm long. Frons with
paired warts, frontal setae c. 120 μm long. Thoracic
horn (Fig. 3D) spinose throughout, 350–400 μm
FIG. 3. Hydrobaenus saetheri Cranston, new species, larva (A–C), pupa (D–E). A. – Mentum. B. – Antenna. C. –
Mandible. D. – Thoracic horn. E. – Abdominal tergites.
76
Hydrobaenus saetheri Cranston, new species
long, maximum width (subapical) 54–68 μm.
Antepronotals: 2 dorsal, 90–120 μm long; 1 lateral,
45–55 μm long; 3 pc, 1 thicker and longer, 120–140
μm long; 2 short and thin, 35–45 μm long;
dorsocentrals evenly spaced on mid-thorax, dc2 10–
20 μm long, dc1,3,4 50–55 μm long.
Abdomen. Tergites as in Figure 3E. L-setae of T
IV and V non-taeniate, VI with 3 taeniate L-setae,
VIII with 4 (rarely 3) taeniate L-setae; L4 located
very posteriorly on III–VII. Anal lobe fringe with
24–30 setae; 3 macrosetae of maximum length 325–
400 μm. Genital sacs extending slightly but
distinctly beyond anal lobe.
Based on the normal palp and antenna, non-triangular
gonostylus without crista dorsalis, weak anal point,
and measures / counts (based on essentially
overlapping numbers) the adult male keys to the
pilipes group. Based on distribution and length of
L-setae, smooth genitalic sheath without terminal
papilla and mensural features the pupa keys to the
rufus / lugubris group, conforming to the latter except
for the longest anal lobe fringe seta being longer and
posterior-most amongst the macrosetae. Keys to
larval species groups are inadequate, but the coarsely
plumose SI seta may be indicative of the pilipes
group. That H. saetheri does not fit comfortably into
any one species group in all life history stages may
be unsurprising given the inability to recover these
groups in numerical cladistic analyses (Cranston &
Humphries 1988).
Larva (n = 14)
Body length 5.9–8.5 mm, color uncertain (preserved
stained pink).
Head capsule. Length 400–550 μm, yellowbrown with occipital margin dark, mental,
mandibular and premandibular teeth brown-black.
Mentum (Fig. 3A) 133–188 μm wide, ventromental
plate maximum 15–20 μm wide. Antenna (Fig. 3B)
segment lengths (in μm): 60–77.5, 12.5–18.75, 5–8,
5, 3.75–5. AR 1.85–2.95. Blade 25–35 μm long.
Lauterborn organs subequal to segment 3 in length.
Labrum with S I seta c. 50 μm long, with 6–8 coarsely
plumose branches; S II stout, 75 μm long, S III
minute, S IV short. Premandible with 2 dark apical
teeth, 172–205 μm long. Mandible (Fig. 3C) 140–
175 μm long.
Abdomen. Procercus, brown, darker posteriorly,
60–80 μm high, 48–52 μm wide, bearing 7 anal setae
of maximum lengths 600–660 μm. Supraanal setae
270–340 μm long. Posterior parapods about 250 μm
long, with 17–22 simple, dark brown claws. Anal
tubules subconical, about half length of posterior
parapods.
Ecology
Hydrobaenus saetheri appears to be a dominant
species in the seasonally-inundated floodplains of
the Sacramento River. In Yolo Bypass, sampling
during 1998–2005 revealed that Hydrobaenus
dominates the chironomid fauna during flood.
Larval H. saetheri appear rapidly after inundation
and are maintained at relatively high levels
throughout flood events (Sommer et al. 2004).
Abundance trends correlate closely with flow,
probably as a result of increased seasonal
inundation of terrestrial habitats at higher flows.
H. saetheri larvae emerged from 18 of 24 (75%)
rehydrated floodplain sediment samples from
t h r o u g h o u t t h e Yo l o B y p a s s . C o - o c c u r r i n g
Chironomidae, one species each of Limnophyes,
Goeldichironomus and Paratanytarsus, were
uncommon. Mandible length measurements
indicate that larval H. saetheri collected four days
after experimental rehydration were second and
third instar larvae and fourteen days after
rehydration all larvae collected were in fourth
i n s t a r. T h i s s u g g e s t s e m e rg e n c e f r o m a
desiccation-resistant resting stage (likely a
cocoon) upon rehydration is followed by rapid
maturation.
Systematics
Adult males broadly conform to generic diagnoses
(Sæther 1976; Cranston et al. 1989) with
additional species (Sæther 1989), except for the
virtual absence of acrostichals (a pale elongate
oval area in the mid-third of the thorax may
enclose few very short, perhaps curved, setae), the
virga is indistinct, and the gonostylus, uniquely
in the genus, bears several megasetae (Fig. 2B).
The female adult conforms to Sæther’s (1976)
description. The pupa and larvae both key to
Hydrobaenus and match completely the diagnoses
of Sæther (1976), Coffman et al. (1986, pupa) and
Cranston et al. (1983, larva).
Emergence of H. saetheri larva from floodplain
sediments was observed also in the field. In
shallow, slow-flowing floodplain waters, drift net
samples two and three weeks after initial
inundation of the floodplain collected 183 and 320
larvae per m 3 , respectively. In the faster flowing
main drainage channel of the floodplain, very high
densities (up to 550 per m 3 ) of incompletelyemerged adults were collected from drift nets.
Drifting larvae are a major prey item for native fishes,
which rely on Yolo Bypass as nursery habitat (Moyle
Six species groups are recognized for adults (Sæther
1976; Cranston et al. 1989), with fewer and less
clearly delimited groups for the immature stages.
77
Cranston, Benigno, and Dominguez
et al. 2004; Sommer et al. 2001). As a consequence
of the exceptional production of Hydrobaenus in the
Yolo Bypass, feeding success of young fish is
superior to that of adjacent perennial rivers.
Hatching of pupae from the drift creates dense
swarms of adults, providing additional prey for
aquatic and terrestrial food webs. During the drier
summer and autumn months, the species evidently
is sustained by aestivating larval cocoons in
floodplain soils.
References
Coffman, W.P., Cranston, P.S., Oliver, D.R. &
Sæther, O.A. (1986) The pupae of Orthocladiinae (Diptera: Chironomidae) of the
Holarctic region - Keys and diagnoses. In:
Wiederholm, T. (Ed.), Chironomidae of the
Holarctic region - Keys and diagnoses. Part 2.
Pupae. Entomologica scandinavica, Supplement
28: 147–296.
Cranston, P.S. (1994) 2. Morphology. In: Armitage,
P.D., Cranston, P.S. & Pinder, L.C.V. (Eds.), The
Chironomidae: Biology and ecology of nonbiting midges. Chapman and Hall, London, pp.
11–30.
Cranston, P.S. & Humphries, C.J. (1988) Cladistics
and computers: a chironomid conundrum?
Cladistics 4: 72–92.
Cranston, P.S., Oliver, D.R. & Sæther, O.A. (1983)
The larvae of Orthocladiinae (Diptera:
Chironomidae) of the Holarctic region - Keys
and diagnoses. In: Wiederholm, T. (Ed.),
Chironomidae of the Holarctic region - Keys and
diagnoses. Part 1. Larvae. Entomologica
scandinavica, Supplement 19: 149–291.
Cranston, P.S., Oliver, D.R. & Sæther, O.A. (1989)
The adult males of Orthocladiinae (Diptera:
Chironomidae) of the Holarctic region - Keys
and diagnoses. In: Wiederholm, T. (Ed.),
Chironomidae of the Holarctic region - Keys and
diagnoses. Part 3. Adult males. Entomologica
scandinavica, Supplement 34: 165–352.
Grodhaus, G. (1980) Aestivating chironomid larvae
associated with vernal pools. In: Murray, D.A.
(Ed.), Chironomidae. Ecology, systematics,
cytology and physiology. Pergamon Press, New
York, pp. 315–322.
Kondo, S. (1996) Life cycle of Hydrobaenus kondoi
Sæther (Chironomidae) at the middle reaches of
the Kiso River, Japan. Hydrobiologia 318: 79–84.
Kondo, S. & Okuda, T. (1990) The larval cocoon of
Hydrobaenus kondoi Sæther (Diptera:
Chironomidae) for aestivation in the Kiso River,
Japan. In: Abstracts, V International Congress
of Ecology. Yokohama, Japan, pp. 359.
Moyle, P.B., Baxter, R.D., Sommer, T.R., Foin,
T.C. & Matern, S.A. (2004) Biology and
population dynamics of Sacramento splittail
(Pogonichthys macrolepidotus) in the San
Francisco Estuary: a review. San Francisco
Estuary and Watershed Science 2(2) (May
2004), Ar ticle 3. Available from: http://
repositories.cdlib.org/jmie/sfews/vol2/iss2/
art3 (11 February 2006).
Sæther, O.A. (1976) Revision of Hydrobaenus,
Trissocladius, Zalutschia, Paratrissocladius, and
Several other species of Hydrobaenus can take
advantage of temporary aquatic conditions by
producing desiccation-resistant cocoons as 2nd or
3rd instar larvae (Sæther 1976; Grodhaus 1980;
Kondo & Okuda 1990). An unidentified Hydrobaenus species was reported from cocoons in dried
sediments from Northern California vernal pools
(Grodhaus 1980). Larvae removed from cocoons
were 2nd instar, and laboratory-reared larvae built
cocoons when they reached second instar
regardless of moisture conditions, suggesting an
obligatory diapause period regardless of
hydrologic conditions (Grodhaus 1980). This
material appears lost, since it is not in the modest
Grodhaus collection housed in the Essig Museum,
Berkeley. Two visits to a vernal pool Grodhaus
studied (CA: Siskiyou Co., Murphy Well, 41°40’N
125°07’W) in mid-summer 2005 revealed no
cocoons in apparently suitable (winter-wet vernal)
soil, no larvae developed on inundation, and
neither were Hydrobaenus present in the farm
pond created at the site (apparently since
Grodhaus’ visits). Thus, we cannot equate the new
taxon described here with that studied by
Grodhaus.
Acknowledgements
We thank Ted Sommer (California Department of
Water Resources) for drawing our attention to this
p h e n o m e n o n , f o r f u n d i n g G i n a B e n i g n o ’s
ecological studies and for permission to include
otherwise unpublished data on the Yolo Causeway
ecosystem. Marianne Dominguez’s examination of
specimens and excellent drawings were funded
from the Evert Schlinger endowment for
systematic entomology to the senior author.
Finally, it gives us pleasure to dedicate this
contribution to Professor O.A. Sæther for
collaboration,
intellectual
debate
on
phylogenetics, and above all for providing the
scientific community with a phylogenetic and
taxonomic framework within which to make so
many observations on Chironomidae.
78
Hydrobaenus saetheri Cranston, new species
some related genera (Diptera: Chironomidae).
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Sæther, O. A. (1980) A glossary of chironomid
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14: 1–51.
Sæther, O. A. (1989) Two new species of
Hydrobaenus Fries from Massachusetts, U.S.A.,
and Japan (Diptera: Chironomidae). Entomologica scandinavica 20: 55–63.
Schemel, L.E., Sommer, T.R., Muller-Solger, A.B.
& Harrell, W.C. (2001) Hydrologic variability,
water chemistry, and phytoplankton biomass in
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Sommer, T.R., Nobriga, M.L., Harrell, W.C., Batham,
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lugubris Fries (1803), a chironomid (Diptera)
species dwelling in temporary waters.
Internationale Vereinigung für Theoretische und
Angewandte Limnologie Verhandlungen 27:
2392–2395.
Accepted: 8 February 2006.
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