Research
50 Million Years of Structural Stasis in
Water Striders (Hemiptera: Gerridae)
N.
M0LLER ANDERSEN, JOHN
R.
ABSTRACT Phylogenetic reconstruction is coupled with
information from the fossil record to infer timing of major
evolutionary events in the diversification of north temperate
water striders. We refer fossil gerrids from the Middle Eocene of
British Columbia to a modern species group and, along with other
Paleogene gerrine fossils, place them on a phylogenetic diagram.
Using the estimated ages of these fossils, we infer that two
monophyletic groups of Limnoporus Stal diverged >50 million
years ago, much longer ago than estimated using data from protein
electrophoresis and assumptions of a standard molecular clock.
The observed structural stasis in Tertiary fossils suggests that
gerrid specializations for life on freshwater surfaces have not been
associated with high rates of speciation or extinction.
SPENCE, AND
M.V.H.
WILSON
North Temperate Gerrid Fossils
In this article, we refer gerrine fossils from British Columbia,
Canada (Wilson 1977), to the modern genus Limnoporus. An
extremely well-preserved adult specimen from the Middle Eocene
( 50 million years ago) of Smithers, British Columbia (Fig. 2), is
probably a male. The L. rufoscutellatus group is held together by
two synapomorphies also shared by the fossil: (1) mesotarsal
segment 1 is more than half the length of the mesotibia, and (2) the
hind tibia is subequal in length to the middle tibia. The right
antenna of the fossil appears complete, and the second and third
elements taken together are clearly longer than the first segment, as
Limnoporus
W
ATER STRIDERS are common but specialized insects
found on water surfaces and, by virtue of associated
adaptations for surface life, are the only insects to have
invaded the open ocean (Andersen 1979,1982). Initial adaptations
for life at the air-water interface revolve around their completely
remodeled structure to facilitate effective movement on the water
surface (Darnhofer-Demar 1969a, b; Andersen 1976,1982). Subsequent adaptations associated with gerrid radiations in freshwater habitats have involved fine-scale adjustments to habitat
structure (Andersen 1979; Spence 1981, 1989) and use of ripple
signals to organize complex behavior (Wilcox 1972,1979; Spence
& Wilcox 1986; Wilcox & Spence 1986). Such conspicuous
specialization is supposed to result from high rates of speciation
and to lead to high risks of extinction for individual lineages
(Stanley 1979). In this report we show that neither of these
propositions holds for water striders.
Three genera dominate the gerrid fauna of north temperate
latitudes: representatives of Aquarius Schellenberg (15 species) are
large-bodied and occupy both lotic and lentic habitats I Andersen
1990); those of Limnoporus Stal (six species) and Gerris F. ( 40
species) live mainly on lakes and ponds. Phylogenetic relationships of these genera have been established recently (Fig. 1).
Andersen (unpublished), in a cladistic analysis of Gerris, has
established that Gerris and Aquarius are sister taxa and that
together they are most closely related to the gerrids of the genus
Limnoporus. Limnoporus has been resolved into two monophyletic groups (Andersen & Spence 1992): the L. canaliculatus (Say)
group composed of two small-bodied species and the L.
rufoscutellatus (Latreille) group including four large-bodied
species. Gerrid life-styles are conducive to fossilization, and excellently preserved gerrines have been found in beds dating back to
the Paleogene (Scudder 1890, Handlirsch 1910, Cockerell 1913,
Wilson 1977, Andersen 1982).
174
Brit. Columbia
Canada
Fur Formation
Denmark
Fig. 1. Phylogenetic diagram depicting relationships of extant gerrinine
genera (Andersen 1990) based on cladistic analyses of structural
characteristics. Dates based on published figures (Wilson 1977, Pedersen &c
Surlyk 1983). The fossils superimposed on the diagram (black dots) and
representing the extinct genus, the L. rufoscutellatus group, Aquarius, and
Gerris are described and figured elsewhere (Scudder 18 90, Handlirsch 1910,
Cockerell 1913, Wilson 1977, Andersen 1982). Fossils are placed on the
diagram in relation to age and horizon of fossilization.
AMERICAN ENTOMOLOGIST
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is diagnostic for Limnoporus (Andersen 1975, Andersen & Spence
1992). However, the fourth antennal segment is clearly shorter
than the third, a trait not found in any extant Limnoporus, but
which does occur in the more basal gerrinine lineage giving rise to
Gigantometra gigas (China). Thus we argue that the short fourth
antennomere is a primitive characteristic of the fossil and place the
specimen as representative of a sister taxon to the L. rufoscutellatus
group (Fig. 1).
A second specimen from Smithers, British Columbia (Wilson
1977, fig. 3C), is unquestionably a fifth-instar nymph. Although
most measurements of this fossil are similar to those for samples
of fifth instars of L. notabilis Drake & Hottes, the middle femur
and tibia of the fossil are distinctly longer than those in L.
notabilis. However, the relative shortness of the second and third
antennal segments as compared with the first, and the apparent
overall slimness of the body, allow us to exclude the juvenile fossil
from Aquarius (the only other north temperate genus to which a
fifth instar of this size could be referred).
To further clarify relationships of the fossil taxon to extant
members of the L. rufoscutellatus group, we undertook a principal
components analysis (PCA) using 19 linear measurements that
could be made on the fossilized adult specimen (Fig. 3). Most
measurements of the fossil are correlated with its large size, and
so the first principal component score of the fossil compares
favorably with those for male L. notabilis, especially those from
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Fig. 3. Results of a principal components analysis on measurements of
males of all extant L. rufoscutellatus-group species. Scores for the fossil were
calculated from equations obtained for extant species. Lengths of the
following characters were used: body; head; antennal segments 1-4; femur
and tibia of pro-, meso-, and metathoracic legs, tarsomeres 1 and 2 of
prothoracic leg, first tarsomere of mesothoracic leg; intercoxal distances for
coxae 1 and 2, and coxae 2 and 3, and distance from coxa 3 to end of
abdomen. Polygons representing three species based on the following
samples: L. rufoscutellatus, 10 males each from Alaska, Denmark, and the
far east of the former USSR; L. genitalis, 10 males each from Kunashir,
USSR and Asahikawa, Japan; L. dissortis, 10 males each from Alberta and
Ontario, Canada. Data for L. notabilis based on 10 males each from the
following populations: (British Columbia, Canada) VT, Valemont.; SM,
Smithers; CO, Coombs; QC, Queen Charlotte Islands; (United States) MT,
Montana; CA, California.
-1.5
western British Columbia. In particular, measurements for the
middle and hind legs match almost perfectly those for extant L.
notabilis. Length of the fourth antennal segment loaded most
heavily on the second PCA axis. Although the second axis explains
only about 6% of the variation in the data set, the score for the
fossil clearly separates it from any of the four modern species.
Several other gerrine fossils are known from the Fur Formation
(Late Paleocene) of northern Jutland, Denmark (Andersen 1982,
Pedersen & Surlyk 1983). Among these is an unusually large adult
specimen similar to modern Gigantometra gigas, the largestbodied of extant gerrids, which is presently known only from
Hainan Island, China, and northern Vietnam. Other fossils from
the Fur Formation resemble closely modern species of Aquarius
and Limnoporus. Gerris defuncta Handlirsch (Handlirsch 1910,
figs. 34-35) from the Middle Eocene of western Canada might also
be referred to Aquarius by virtue of its length (15 mm), stout
structure, and relatively long first antennal segment. Additional
material from western Canada (Scudder 1890, Wilson 1977)
clearly falls within the structural limits for extant Gerris species.
Divergence Times for Gerrid Lineages
All fossils discussed above are members of a monophyletic
group and may be placed on the phylogenetic diagram (Fig. 1).
This permits inferences about the minimum times of divergence
Fig. 2. Adult males of Limnoporus collected from near Smithers, British for these genera. The Danish fossils suggest that the Gigantometra,
Columbia (Wilson 1977).The upperspecimen represents L. notabilis and the Limnoporus, and Aquarius + Gerris lineages had already diverged
some 55 million years ago. The records from western Canada
lower specimen is a fossil dated at 50 million years ago. The body length
confirm that Limnoporus, Aquarius, and Gerris were distinct by
of the fossil is 1.93 cm.
Fall 1993
175
the Middle Eocene. Our assignment of western Canadian fossils to
the L. rufoscutellatus group requires a minimum time of 50
million years ago for divergence of the two extant species groups
of Limnoporus.
This estimate of minimum divergence time is much earlier than
those based on assumptions of a standard molecular clock
(Sperling & Spence 1990), suggesting that the molecular clock
runs far slower than normal in water striders. An alternative but,
in our opinion, less probable scenario is that the western Canadian
fossils should be placed on the ancestral lineage of all Limnoporus.
This ignores the shared synapomorphies between the fossils and
the L. rufoscutellatus group, and leaves us unable to specify
minimum divergence time of the two Limnoporus species groups.
However, even this most conservative interpretation demonstrates that gerrine genera, and Limnoporus in particular, are
ancient lineages and that they have persisted with little change for
50 million years despite their conspicuous specialization.
The 7-fold range of species diversity across the three genera
considered above makes us doubtful that the observed structural
stasis is related to a low rate of speciation in the North temperate
fauna. On the other hand, a recently discovered and highly
specialized gerrid from Dominican amber clearly represents an
extinct subfamily (Andersen & Poinar 1992). Because the presentday fauna of Hispafiola is composed mainly of widely distributed
Central and South American genera, this new fossil suggests that
the Oligocene fauna included specialized clades that have gone
extinct. However, because there is evidence for long and conservative persistence of species groups for several other modern,
diverse insect genera (Askevold 1990, Rayner & Waters 1990), we
suggest that in specialized insect taxa there is no general or
necessary reciprocal relationship between rate of speciation and
either morphological or molecular evolution.
Acknowledgments
We thank J. Waddington (Royal Ontario Museum) for loan of
specimens and G. E. Ball, B. S. Heming, J. Kukalova-Peck, and
F.A.H. Sperling for helpful discussions and comments about the
manuscript. This study was supported by operating grants to
N.M.A. from the Danish Natural Science Research Council and
to J.R.S. and M.V.H.W. from the Natural Sciences and Engineering Research Council of Canada.
Andersen, N. Mailer & J. R. Spence. 1992. Classification and phylogeny of
the Hplarctic water strider genus Limnoporus Stal (Hemiptera, Gerridae).
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Received for publication 13 August 1992; accepted 4 January 1993.
•
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176
N. Moller Andersen is with the Zoological Museum, University
of Copenhagen, Universitetsparken IS, DK-2100 Copenhagen.
John R. Spence is with the Department of Entomology, University
of Alberta, Edmonton, Alberta, Canada T6G 2E3. M.V.H. Wilson
is with Department of Zoology, University of Alberta, Edmonton,
Alberta, Canada T6G 2E9.
AMERICAN ENTOMOLOGIST