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HYMENOPTERA, Agriotypinae: Ichneumonidae
(Ichneumonoidea). -- <Images>
& <Juveniles> Agriotypinae
is a Palaearctic subfamily of the parasitic Ichneumonidae. Agriotypus is the only genus in the
group .The known species are aquatic
ectoparasitoids of Trichoptera pupae.
The placement Agriotypus is not clear as it has been classified
with both the Proctotrupoidea and as
a separate family of Ichneumonoidea. There are not many species in this
family, which in 1940 was represented by only two species, Agriotypus armatus in England (Walker 1832) and A. gracilis Waterst. in
Japan (Clausen 1940/1962). Both of
these are aquatic in habit and develop as external parasitoids on prepupae
and pupae of caddis flies. A. armatus
has been found in various parts of Europe, and general observations on its
habits and biology, with incomplete descriptions of the early stages, have
been made by Klapalck (1889, 1893) and Henriksen (1918, 1922). Clausen (1940) noted that it was not until
1932 than an adequate account of its habits and descriptions of all instars
were presented. The Japanese A. gracilis
was observed by Ota (1917, 1918), who thought it to be distinct from the
European form, and its habits and early stages were studied by Clausen
(1931b). Biology & Behavior
Both of the above species pass
winter as adults within the cocoon in the caddis fly case and emerge in
springtime when the water temperature raises enough to induce activity, ca.
to 13°C in the case of A. armatus. Of 21 parasitized caddis fly cases
containing A. gracilis collected
at Lake Hakone, Japan on Mar. 25th and placed in a jar of water that quickly
reached air temperature, complete emergence occurred within two hours. Females predominated in a ratio of ca. 66%
&&. Mating took place very soon after
emergence, and oviposition followed ca. one week later. In order to reach caddis fly cases
occurring on stones, etc., at a depth of 6-15 in. beneath the water surface, the
female crawls down a plant stem or the side of an exposed stone and searches
about for them. There is apparently
no attempt to swim at any time, and thus it is remarkable that cases
parasitized by A. gracilis were found as distant as 25
ft. from the nearest exposed stone or bank.
When an inhabited case was found, the ovipositor explored its
contents. If the caddis fly were
still in an active stage, this oviposition thrust caused it to extrude the
head and thorax from the case, at which time the parasitoid immediately left
it and searched for another containing a prepupa or pupa. The ovipositor is inserted, often with
considerable difficulty, and the egg deposited externally. When emerging from the water, the female
merely releases her foothold and floats to the surface, there being no
movement of either the wings or the legs at this time. The female may take wing immediately upon
reaching the surface, or she may coast for several inches, with the wings
beating rapidly, the middle and hind legs trailing on the water and the
forelegs sharply raised. A. gracilis females were found to remain
under water up to 14 min under experimental conditions, but this was thought
to be exceeded in nature. Upon entry
into the water, the body is completely enveloped in an air bubble that
conforms to the body outline and encloses the antennae, which are held back
over the dorsum and the wings. The
formation of this bubble is made possible by the dense pubescence that
clothes the entire body. The oxygen
contained within the bubble serves to fill the requirements of the wasp while
immersed, and the supply is considered much augmented from the surrounding
water (Clausen 1940/1962). The
antennae, being held within the air bubble, are seemingly entirely
functionless as far as locating the host and determining its suitability are
concerned. During hatching, A. gracilis
eggs form a small break in the tough chorion immediately beneath the mouth of
the larva, and this aperture is slowly enlarged by a steady forward thrust of
the body. The head is bent back over
the thorax, and the venter of the latter is forced through the aperture
first. A further enlargement of the
opening releases the head, and complete emergence is finally affected. The emergence hole is circular in outline
and 2/3rds the width of the egg. The
edges are curled back, and there is no splitting along a longitudinal line
such as occurs in many other Hymenoptera.
From 5-8 hrs are required for hatching of the larva from the egg
(Clausen 1940/1962). Modifications in form of the 1st
instar larva are adaptations for locomotion and to prevent it from being
washed out of the host case. The
dorsal rows of spines can be raised to a nearly vertical position and serve,
in conjunction with the head and the bifurcate caudal appendage, to
facilitate ready movement between two curved surfaces such as are presented
by the caddis fly body and the wall of the case. Respiration is obviously cutaneous, and the oxygen supply is
derived from the water that flows through the case. The point of feeding of the young A. armatus larva is
usually on the underside of the thorax of the prepupa and beneath a wing pad
on the pupa. The first molt takes
place ca. one week after hatching. There is thought to be an
internally parasitic phase in the development of the larva, as indicated by
the supposed 1st instar larva of A.
armatus found by Henriksen
(1922). Only three instars have been
described, all of which feed externally.
The normal number of instars for the order is 5, and two are
consequently not accounted for. If
the larva found by Henrikesn is actually Agriotypus,
the habits and manner of development are of special interest, because entry
into the body of the host would be by 1st instar larvae, followed by an
immediate molt, after which two stages would be passed internally and these
succeeded by the two external stages that are now known as the 2nd and 3rd. After the host body contents are
completely consumed, the Agriotypus
larva spins its cocoon within the host case.
The last larval exuviae of the host, and the pupal remains, are left
in the form of a pad at the posterior end of the case and are partitioned off
by the parasitoid cocoon. This cocoon
lines the sides of the host case, and its wall is thickest at the anterior
end. The ribbon-like appendage, that
is characteristic of parasitized cases, is then formed, being extruded
dorsally at the anterior end of the case.
This ribbon is 1.0-1.5 mm in width and may be almost 5.0 cm in
length. It consists of a closely woven
outer covering enclosing a mass of tangled silken strands. Ota considers the ribbon to be a
protective device. That it serves in
respiration is certain, as experiments of Muller (1889, 1891) revealed that
the larvae and pupae invariably died when the band was removed, although they
survived if removed from the water.
The respiratory requirements of the early larval stages upon the
living host are met by the absorption of oxygen from the water flowing
through the case; but after the cocoon is spun the parasitoid larva and its
following stages are surrounded by air, and some means are necessary to
replenish the oxygen supply during the many months passed within it. The way in which oxygen from the
surrounding water reaches the parasitoid in the cocoon is not definitely
known, but Clausen (1940) thought that a lower air pressure within the cocoon
may draw the gas from the water and through the interstices of the silken
ribbon into it. Fisher (1932)
concluded that the gas content of the cocoon may at first be CO-2 exhaled by
the larva and that this escapes and is replaced by oxygen as soon as the
ribbon begins to function. Following spinning of the cocoon,
the larva remains quiescent for 7-10 days before pupating. The meconium is cast by the prepupa and is
found in the form of a ring surrounding the tip of the pupal abdomen but
separated from it by the last larval exuviae. There is one generation each year; adults usually emerge during
April, and the adult stage is again attained at the end of September. Then the water temperature is declining
and adults remain quiescent in the cocoon until the following spring (Clausen
1940/1962). For detailed
accounts of the immature stages of Agriotypidae, please see Clausen
(1940/1962). References: Please refer to <biology.ref.htm>, [Additional references may be found at: MELVYL Library ] |