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Aspects of the taxonomy, ecology and hydrodynamics of the Australian Psephenidae (Coleoptera)

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Davis, JA (1982) Aspects of the taxonomy, ecology and hydrodynamics of the Australian Psephenidae (Coleoptera). PhD thesis, University of Tasmania.

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Abstract

Psephenid larvae (waterpennies) are well known members of the
freshwater benthos, possessing a characteristic dorso-ventrally flattened
shield. They are found clinging to rocks in rivers and streams
that experience fast and turbulent flow regimes for at least part of
the year. However, despite the fact that they occur in many of the rivers and streams of eastern Australia, and throughout Tasmania, very
little study has been made of the family here.
Sclerocyphon is the only Australian genus of the Psephenidae.
A brief review of the history of the Psephenidae, with particular emphasis
on this genus and the subfamily encompassing it, the Eubriinae, is given.
Sclerocyphon comprises 12 species, six of which are described as
new. Descriptions of six larval types representing discrete species
but not, as yet, associated with adults, are also given. Re-descriptions
of the genus and all previously described species are included as earlier
workers possessed only limited material and features now considered to
be of taxonomic importance, in particular, the male and female genitalia,
were not described. Keys for the identification of both adults and
larvae are given.
A major emphasis of the taxonomic study was on the association of
adults and larvae and this was achieved mainly by laboratory breeding.
Larvae are the dominant life history phase and fairly common while adults
are much rarer.
The relative apomorphy or plesiomorphy of a number of larval characters is discussed and a phylogenetic classification of Sclerocyphon,
based on larval characters, is proposed. Hinton's (1966) view of the
Eubriinae as a subfamily (encompassing Sclerocyphon) of the Psephenidae,
rather than Bertrand's (1972) concept of the Eubriinae as a separate family,
is supported.
The distribution of each species and larval type is described
and discussed with respect to present and some past environments in
Australia. The complete endemism of the Tasmanian species of Sclerocyphon is considered to be a reflection of the low vagility of the genus
and appears to be best explained by the vicariance model of historical
biogeography.
A Gondwanaland origin of Sclerocyphon is suggested as it (or its
nearest relative) occurs in Chile, as well as Australia, but nowhere
else in the world. This disjunct southern distribution in a genus
with low vagility is considered to be best explained by the vicariance
of an ancestral Gondwanaland fauna.
Multivariate statistical techniques were used to investigate the
shape of larvae of the Tasmanian species of Sclerocyphon. The most
common Tasmanian species, S. secretus, was found to be the most variable,
exhibiting a west-east trend in larval shape. Variation in larval shape
was continuous with the widest, almost circular forms occurring on the
west coast and the narrowest,most elongate forms on the east coast.
Larval shape was found to be correlated with rainfall distribution, substrate
and stream order. Contrary to expectations the narrowest, most
elongate larvae were found to occur in streams of low mean velocity while
wider forms occurred in streams of higher velocity. Three possible explanations for this paradox are given.
The second river and stream dwelling species in Tasmania, S. aquaticus, was found to be less variable in shape with larvae being predominantly wide and almost circular in form.
The occurrence, and therefore the shape, of S. aquaticus larvae was found to be highly correlated
with high stream order.
Flow visualisation techniques were used to investigate the hydrodynamics
of larval Sclerocyphon. Dye trails revealed that all larvae
are streamlined and inhabit a region of reduced flow immediately adjacent to t.he substrate known as the laminar, or viscous, sublayer. The
viscous sublayer exists within a thicker region of reduced flow assoc~
iated with the substrate, the boundary layer, which, under most stream
conditions, is turbulent.
Drag forces upon the larva are minimal while it remains within
the viscous sublayer however respiratory exchange across this
vii
is limited to ·the slow rate of molecular diffusion. Larval Sclerocyphon
overcome the cons.traints of this situation by creating their own respiratory
current with a pair of anal, retractible, tracheal gills.
During ventilation actively pumping gills create a turbulent area at ·the
rear of the body which greatly enhances respiratory processes and waste
removal. The gills also act as vortex generators, decreasing drag
forces by controlling the larva 1 s wake.
At high Reynolds number the thickness of the sublayer is reduced
and larvae appear to employ a method of boundary layer control known,
in fluid mechanics theory, as suction. The spaces between the lateral
laminae act as slots through which a small amount of boundary layer fluid
passes, creat.ing a thinner but. more stable boundary aver the larva..
This delays separation and therefore decreases the likelihood of the
larva being swept off the substrate. For larvae of the Psepheninae and
Eubrianacinae, which possess passive ventral tracheal gills, suction
through slots between the lateral laminae may be the only means of main-taining
adequate water: flow over the gills.
Streamlining and associated boundary layer control allow larvae to
move across substra·tes in high energy flows and to exploit the food
sources there that are unavailable to other benthic invertebrates not possessing these hydrodynamic advantages.

Item Type: Thesis (PhD)
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Date Deposited: 25 Nov 2013 21:35
Last Modified: 15 Sep 2017 00:59
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