Agriotypidae

There are very few 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 above named species pass winter as adults within the cocoon in the caddis fly case and emerge in springtime when the water temperature rises enough to induce activity, circa 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 circa 66%.

Mating took place very soon after emergence, and oviposition followed about 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.

Immature Stages of Agriotypidae

The egg of Agriotypus gracilis Waterston measures 0. 9 mm. in length and 0. 18 mm. in greatest width and is slightly larger than that of A. armatus. It is slightly convex dorsally and broadest at the anterior end, and the posterior end is smoothly rounded. The chorion is exceedingly thick and tough. At the anterior end is a heavy pedicel of variable length, ranging up to 0.25 mm., the distal extremity of which is irregularly expanded. This "button" is embedded in the integument of the host and anchors the egg firmly in position. Both pedicel and button become black and shriveled after deposition. It is noteworthy that the pedicel mentioned is not represented by any modification in the ovarian egg, which is elongated and oval in form and has the anterior end smoothly rounded. This, and the fact that the pedicel darkens and shrivels quickly after forma­tion, would indicate that it may be formed from secretions of the accessory glands or from material that appears to envelop the anterior end of the ovarian egg, though this aspect has not been studied. It may be emphasized, also, that the pedicel is situated at the anterior end of the egg, whereas in other pedicellate ichneumonoid eggs the pedicel and its "anchor" are represented by definite structures on the ovarian egg and are situated at the posterior end. As the embryo develops, the paired caudal processes can be seen lying along the mid‑ventral line and extending forward to the posterior margin of the head.

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 completre emergence is finally effected. 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).

The first instar larvae (Fig. 26 A, B) of the two species present no apparent points of distinction. The body comprises 13 segments, exclusive of the head, and measures 1.2 mm. in length to the base of the caudal processes. The head is heavily sclerotized, slightly broader than long, and bears dorsally a pair of horn like structures markedly similar to those of the planidia of the Perilampidae. There are four pairs of minute setae dorsally and three pairs ventrally. The mandibles are simple. Each body segment except the last bears a median transverse row of heavy spines dorsally, and these diminish in length caudad. On the first five seg­ments, the rows are continuous across the dorsum, but on those follow­ing the rows are interrupted medially. The first segment bears two pairs of lateral setae, and the following segments bear one pair. The venter of each of the first eight segments bears a broad band of minute setae, and on each of the following four segments the band is interrupted medially. The caudal segment is bifurcate, and the two tapering, heavily sclerotized prongs are 0. 9 mm. in length, diverge at an angle of about 80 deg., and are directed somewhat ventrad. The lobes at the base of the prongs bear numerous robust setae dorsolaterally. The anal opening is ventral on the last segment. There are no spiracles and no visible internal tracheal system.

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.

Henriksen describes the supposed first instar larva of A. armatus (Fig. 27) which he states was found internally in Silo and Goera. Aside from its occurrence internally, it differs markedly in form from the actual first instar larva described by Fisher. The body is 1.4 mm. in length, cylindrical, with the caudal end bluntly rounded and lacking the bifurcate process. Certain characters, however, seem to link it with the Agriotypidae, these being the "horn like" structures on the head and the transverse rows of spines on the dorsum of the body. It seems improb­able that this larva can be of Agriotypus, but if this proves to be the case it must be the second instar rather than the first.

The second instar larva of A. armatus (Fig. 26 D) described by Fisher differs from the first in lacking the heavy integumentary spines, and the long bifurcate caudal process is replaced by a pair of shorter, heavy, opposed hooks. The mandibles are conspicuously toothed. An internal tracheal system is present though there are no spiracles, and the transverse commissures, also, are apparently lacking.

The third and last larval instar is similar to the second, though the caudal hooks are relatively much smaller. The head is quadrate in form, and the mandibles are coarsely dentate. In A. armatus, there are thought to be no spiracles, whereas nine pairs occur in A. gracilis. In view of the conditions under which the mature larva passes the last portion of the stage, in which it is surrounded by air rather than water, open spiracles would seem to be essential (Clausen 1940)

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).