Corethrellidae (Diptera), Vectors of Present and Perhaps Some
of the Earliest Anuran Trypanosomes
Sturgis McKeever and Frank E. French
Department of Biology Georgia Southern University, Statesboro, GA
30460-8042 USA
Anura are hosts of a large number of trypanosomes, but little is
known of the biology of most species. Of the 26 trypanosome species listed by Diamond
(1965), the vectors, leeches, were known for only three. A dipteran, Phlebotomus vexator
occidentalis, the vector of a trypanosome that infects the toad, Bufo boreas halophilis, acquires the trypanosome by feeding on blood of infected toads. Toads
are infected by eating infected Phlebotomus (Anderson & Ayala 1968). The
bullfrog, Rana catesbeiana, is infected by Trypanosoma rotatorium. The infected bullfrogs exhibited high peripheral parasitemia during periods of high
ambient temperature. The vector was unknown, but a leech was the main suspect along with
hematophagus insects as probable vectors (Bardsley & Harmsen 1969).
Following the report by McKeever (1977) that Corethrella (Diptera: Corethrellidae) were attracted to calls of and fed upon frogs, Johnson et al. (1993) found that 72% of 215 male green frogs, Hyla cinerea, collected in
Florida were infected with an undescribed species of trypanosome. Female Corethrella wirthi collected on or near the male frogs in the field had conspecific
trypanosomes in their mid and hind gut. Trypanosomes in the frogs exhibited nocturnal
peripheral parasitemia which was synchronized with activity and calling of H. cinerea. Female frogs do not call and, therefore, do not attract Corethrella; none of 31
female frogs collected at the same time and area as the males was infected. Female frogs
were susceptible to infection by inoculation. Of 19 female Corethrella brakeleyi collected near infected frogs, six were collected before and 13 were collected after
feeding; none was infected. Trypanosomes were found in all 109 male H. cinerea collected between June and the end of the breeding season. The infected frogs retained
parasitemia from October through the following March.
Other than Corethrella, the only other parasite, an argasid
tick (Acari), Ornithodorus concanensis, is known to be attracted to calls of
its vertebrate host. Ticks are attracted to calls of and feed upon nesting cliff swallows Petrochelidon pyrrhonoto (Webb et al. 1977).
Corethrella were first observed to be attracted to calls of
and feed upon three species of tree frogs: Hyla avivoca, H. cinerea and H. gratiosa (McKeever 1977). Subsequently, C. brakeleyi and C. wirthi were found to be attracted to calls of four additional eastern
species of Hyla and one species of Bufo; C. laneana, at
Saratoga Springs in Death Valley, California, was attracted to calls of the eastern
species, H. gratiosa (McKeever & French 1991a).
As a result of their being attracted to the calls of frogs, female Corethrella are easily collected by placing a Center for Disease Control miniature
mosquito trap in front of a cassette player and playing calls of Hyla (McKeever
& Hartberg 1980). In one 45 minute trapping period, with calls of H. gratiosa as an attractant, 566 Corethrella were collected with one trap.
Corethrella, the lone genus of Corethrellidae, has a
worldwide distribution and consists of 62 extant and two fossil species (Borkent 1993). Most species are restricted to tropical or subtropical climates (Borkent 1989), but some
species are found between approximately 45 degrees north, and 45 degrees south latitude
(McKeever & French 1991b). With the transfer of Lutzomiops kerrvillensis to Corethrella (Borkent 1993), five species are indigenous to the United States.
Larval Corethrella are found in small pools at the edges of
bogs, streams or small lakes (Borkent 1993), in leaf axils of epiphytic plants and in tree
holes (Frank et al. 1988), and in shallow woodland pools (personal observation). In the
laboratory, C. brakeleyi larvae consumed an average of 3.0 Anopheles quadrimaculatus during a 19-day larval period and were observed feeding upon Anopheles crucians as well as A. quadrimaculatus in rice fields in Louisiana (McLaughlin 1990).
In Singapore, a study of dipteran larvae that are able to
withstand the power of digestive enzymes of the carnivorous pitcher plant, Nepenthes ampullaria, demonstrated that 1st and 3rd instar Corethrella larvae attack
larvae of Dasyhela (Ceratopogonidae) and Tripteroides tenax (Culicidae). In the absence of Dasyhela larvae, Corethrella larvae became
cannibalistic and only 2% pupated (Mogi & Chan 1996).
Females of all species of Corethrella have sclerotized
mouthparts with one or all structures toothed (McKeever 1986), with one exception, an
Australian species C. mckeeveri (Colless 1994), in which the females have
toothless mouthparts that resemble the unarmed mouthparts of males (McKeever & Colless
1991). Toothed mouthparts enable females to feed upon blood of their host Anura. They
almost always feed upon the posterior appendages of the host (Fig. 1); most feed to
repletion and are unable to fly immediately after their bloodmeal and simply walk off the
frog onto the substrate (personal observations, Fig. 2).
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| Figure 1. Two female Corethrella wirthi feeding on the right hind leg of Hyla versicolor |
 |
| Figure 2. Replete Corethrella wirthi walking away from host, Hyla versicolor |
In many hours of observation, we never saw a frog attempt to eat a Corethrella. Johnson et al. (1993) found trypanosomes in the mid and hind
gut of Corethrella, and they postulated that this indicated posterior station
transmission. We were unable to get Corethrella to feed upon captive Hyla so
the precise method of transmission of trypanosomes from the flies to their host is unknown
to us.
Male Corethrella have toothless mouthparts and are thought
to feed upon nectar and/or honeydew (aphid excrement). They are not attracted to the calls
of anurans.
Corethrellidae is considered to be the sister group of the
Chaoboridae plus Culicidae (Wood & Borkent 1989; Miller et al. 1997). Pawlowski
et al. (1996) found that Corethrellidae is indeed a sister group of the Chaoboridae, but
not the Culicidae; they believed that the taxonomic status of Corethrellidae needs
revision.
According to Borkent & Szadziewski (1992), the oldest known
Chaoboridae are fossils from the Jurassic of Siberia and, they believe, Corethrellidae
should be of equal or greater age. However, they reported that the first Corethrellidae
were found in amber of Miocene Age from Germany and from amber 15 B 40 million years old
from Dominic Republic. Later Szadziewski (1995) found a new fossil species, C. cretacea from Lower Cretaceaous Lebanese amber.
True frogs also are known from the Late Jurassic of South America
and Europe (Romer 1966). Borkent & Szadziewski (1992) state that the earliest lineage
of known extant frogs, Leiopelma, as well as the earliest lineage of Corethrella, C. novaezealandia, are found in New Zealand. They found that females of this
species of Corethrella have biting mouthparts and likely feed upon Leiopelma,
the only endemic frogs in New Zealand. This frog lacks a true calling voice, but produces
chirping calls during the mating period. These calls may attract female Corethrella and enable the flies to serve as vectors of trypanosomes in the frog population. Further,
a host parasite relationship may have existed between true frogs and Corethrella since the Jurassic, and the flies may have served as vectors, since the earliest origin of
anuran trypanosomes.
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