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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 13C 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 ]