Distribution of Pheromermis pachysoma (Mermithidae)
Determined by Paratenic Invertebrate Hosts
George O. Poinar, Jr. 1
Poinar et al. (6) described the unusual
life cycle of the yellowjacketmermithid
parasite, Pheromermis pachysoma, which
included a paratenic host. Instead of hatch-
ing in their aquatic environment, as do most
other mermithid species, the infective stages
of P. pachysoma remain inside the eggs
until they are ingested by aquatic inverte-
brates. After ingestion the infective second
stage juveniles (one molt occurs inside the
egg) hatch and penetrate through the in-
testinal wall, entering and coiling up in
various tissues of the host. The nematodes
remain viable throughout tile life of the
host but do not develop. Growth is finally
initiated when these paratenic hosts are
captured and used as nourishment for yel-
lowjacket (Vespula pennsylvanica) larvae.
The cycle is completed in the fall of the year
when the parasitized adult yellowjackets
emerge and visit a water source, allowing
the nematodes to escape. The study by
Poinar et al. (6) in California was the first
record of P. pachysoma in the Americas;
previous findings were restricted to England
Received for publication 2 January 1981.
1Division of Entomology and Parasitology, University of
California, Berkeley, CA 94720.
Appreciation is expressed to Nancy Erman, C. R. Parker,
R. S. Jacobson, and G. Cosgrove for collecting the paratenic
and definitive hosts of Pheromermis tmchysoma.
and Germany. Since those findings more
North American sites containing P. pachy-
soma have been found. This paper lists
these localities and illustrates how new in-
festations can be determined by recognizing
tile juveniles in various aquatic inverte-
Adults of the caddis fly, Limnephilus
peItus (Limnephilidae: Trichoptera), con-
taining black spots on the body wall were
collected at the Sagehen Creek field station
located near Truckee, California (elevation
approx. 2,000 meters). Adults and larvae of
Hesperophylax sp. (Limnephilidae: Tri-
choptera) containing small dark circles were
collected from Saskatchewan, Ontario, and
British Columbia in Canada and from Wy-
oming, Arizona, Nevada, Oregon, South
Dakota, Colorado, New Mexico, Utah, and
California. Postparasitic mermithids were
collected from parasitized yellowjackets in
tile Calderwood Reserve, North Carolina,
and along the Hiwassee River near Wart-
burg, Tennessee. Similar mermithids were
collected from PoIistes annularis in Louisi-
ana and from Dolichovespula arenaria in
Coiled juveniles of P. pachysoma ap-
peared as black spots on L. peltus adults
(Fig. 1) and as small dark circles on larvae
422 Journal o/Nematology, Volume 13, No. 3, July 1981
~ 3 m,.,rn ........
Fig. 1. Adult of Limnephilus peltus containing black spots representing the infective stages of
Fig. 2. A single infective stage of Pheromermis pachysoma coiled in the body wall of an adult ttespero.
Distribution of Pherornermis pachysoma: Poinar 423
and adults of HesperophyIax sp. (Fig. 2).
Adult and postparasitic juveniles of P.
pachysoma were also collected from the site
that contained L. peltus. Size of the juvenile
and shape of the stylet were used to identify
the mermithid species.
Up to 200 tightly coiled infective stage
juveniles of P. pachysoma occurred in each
adult caddis fly; the coils measured 60-100
t~ttl in diameter and occurred primarily in
the abdominal wall with some in the wall of
tile thorax and legs. The center of the coil
was dark; a short cylindrical tnbe-like struc-
ture sometimes could be seen arising from
the center of this ntass. The significance of
this remains unknown. The postparasitic
juveniles collected from yellowjackets in
Tennessee, North Carolina, Louisiana, and
California were also P. pachysoma, but no
paratenic hosts were collected in those areas.
These findings demonstrate that the
nematotte, P. pachysoma, not only occurs in
its original location at Hopland, California
(about 700 meters elevation), but also at
higher elevations (2,000 meters) in the
Sierra Nevada mountains. It also occurs as
far north as Saskatchewan, Canada, and as
far south as Louisiana. Therefore, it can be
surmised that P. pachysoma is probably dis-
tributed throughout North America, possi-
bly wherever the yellowjacket hosts occur.
Caddis fly larvae that feed on detritus,
such as members of the Limnephilidae,
make good paratenic hosts, but many other
invertebrates are also suitable. Poinar et al.
(6) found cranefly larvae (Tipulidae), beetle
larvae, and ephemerid nymphs naturally in-
fected and experimentally infected mos-
quito larvae (Culex pipiens) and mayfly
nymphs (Callibaetis pictus) with P. pachy-
soma. In Europe, juveniles of P. pachysoma
were recovered from trichoptera larvae in
Austria (Kaiser, personal communication).
Invertebrates other than insects may also
be infected with juveniles of P. pachysoma.
The mermithid juvenile reported in the
oligochete, Lirnnodrilus silvana, (7) came
from the Sagehen Creek area in California.
This area is now known to contain P.
pachysoma, and a re-examination of the
specimens confirmed their similarity to
juveniles of the wasp mermithid.
Predaceous nematodes may also serve as
paratenic hosts to mermithid juveniles by
ingesting mermithid eggs during feeding.
Nanomermis nemicola was described from
the body cavity of Monochus subtenuis in
Virginia. From the description and habits
of this nematode (3,4), it appears to be a
mermithid juvenile using M. subtenuis as a
paratenic host. Small coiled nematodes in
the body cavity of Tripyla setifera and
Bathyodontus mirus were observed (1) and
snbsequently named Nanornermis tripylae
and Idioblaptus elachistus (2). The true
identity of these mermithids remains un-
known. There are probably a number of
mermithids using paratenic hosts to reach
their developmental host. Pheromermis
myopis, a parasite of Tabanus larvae, ap-
pears to have this type of cycle (8).
In tile development of P. pachysoma, a
paratenic host is an obligatory step in the
life cycle, since the eggs do not hatch in the
gnt of adult yellowjackets (6). This makes
further characterization of the type of para-
tenic host difficult in light of the compli-
cated terminology discussed by Odening (5).
Invertebrate hosts carrying the infective
stages of P. pachysoma and other mermith-
ids might be called euparatenic hosts, since
there is no growth or development of the
parasite as occurs in metaparatenic or para-
paratenic hosts. However, euparatenic hosts
are generally considered to be additional
and not obligatory hosts. The exact termi-
nology describing these hosts must await
further clarification of these terms.
1. Andrassy, I. 1971. A remarkable phenomenon:
parasitic nematodes in nematodes. Allattani Kozle-
menyek 58:15fi-159. In Hungarian.
2. Andrassy, I. 1976. Evolution as a basis for the
systematization of nematodes. London: Pitman Pub-
3. Cobb, N. A. 1924. A nema parasitic in a nema.
J. Parasitology 11:120-121.
4. Cobb, N. A. 1925. A new mermithid infesting
another nema. J. Parasitology 11:217-218.
5. Odening, K. 1976. Conception and terminol-
ogy of hosts in parasitology. Pp. 1-93 in B. Dawes,
ed. Advances in parasitology. 14. New York: Aca-
6. Poinar, G. O., Jr., R. S. Lane, and G. M.
Thomas. 1976. Biology
Pheromermis pachysoma (V. Linstow) n. gen, n.
comb. (Nematoda: Mermithidae), a parasite of
yellow jackets (Hymenoptera:
7. Poinar, G. O., Jr. 1976. Presence of mermith-
and description of
424 Journal o] Nematology, Volume 13, No. 3, July 1981
idae (Nematoda) in invertebrate paratenic hosts. J.
8, Poinar, G. O., Jr., and R. S. Lane. 1978.
Pheromermis myopis sp. (Nematoda: Mermithidae),
a parasite ol: Tabanus
Tabanidae). J. Parasitology 64:440-444.