Immature Stages of Ichneumonidae
The immature stages of Ichneumonidae were discussed in detail by Clausen (1940) as follows:
The Egg.-- The eggs of the great majority of species of the family are of simple form, without a stalk or pedicel and usually with no sculpturing of the chorion. The shape is variable, ranging from the broadly oval to cylindrical and, in this simple form, to the extremely slender forms represented by those of Echthropsis porteri and Perithous mediator Grav., which are only one‑twentieth as wide as long, curved, and with both ends tapering to points. The eggs of the Cryptinae, Joppinae, Ichneumoninae, and Ophioninae are, with few exceptions, of the above general‑form. In the latter two subfamilies, the stalked type of egg is also found, the extreme development of this modification being in the genera Rhyssa and Megarhyssa, in which the anterior end is drawn out into a slender tube. The stalk of the egg of R. persuasoria is approximately four times the length of the egg body, and the total length of the egg is 12.0 to 13.5 mm.
Surface sculpturing on the chorion occurs in only a few species of the above subfamilies and is not elaborate. In Cryptus sexannulatus Grav. the egg bears light longitudinal markings, whereas that of Ephialtes extensor (Fig. 32E) is covered with closely set "bosses" arranged in rows. The color is usually translucent white, with the eggs of a few species assuming a brownish tinge as incubation progresses.
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The eggs of the Ophioninae are usually of the normal kidney‑shaped or elongate form, but several genera reveal an adaptation for attaching them to the integument of the host larva. This modification is represented by a "pad" or "button" at the mid‑ventral side of the egg by means of which it "adheres" to the inner side of the integument of the host at 3 point in the body opposite that at which the ovipositor is inserted. This form is represented by Therion morio (Fig. 32C), and one that is apparently similarly modified is described by Plotnikov (1914) in Heteropelma calcator.
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The most striking modifications in egg form occur in the ectoparasitic species of the Tryphoninae and Lysiognathinae; in these groups, the eggs either are partly embedded in a puncture in the integument of the host larva or have an adaptive modification of the chorion at the posterior end into some form of anchor, which is embedded therein. In the Paniscini, this adaptation (Fig. 32A) uniformly appears as a short, blunt pedicel, situated somewhat ventrally, from which extends a spiral, looped or " braided " process that is stated to be very elastic at the time of deposition. Only this latter portion is embedded in the wound. Chewyreuv points out that the pedicel is not an extension of the egg chorion, for it dissolves completely in potassium hydroxide. Associated with this form of egg is a distinctive coloration, the chorion being black or brown and shining, thus making it conspicuous upon the body of the host. The darkening of the chorion is most pronounced in the Paniscini and is of varying extent, and at times entirely lacking, in the Tryphonini.
The extreme modifications in egg form are found among the Tryphonini and Cteniscini. Several of these have been described and figured by Clausen (1932a). The egg of Tryphon semirufus (Fig. 33H) has a long thread‑like pedicel, twice the length of the egg body, which bears at its distal end a long, heavily pigmented bar, attached at the middle and serving as an anchor deep within the tissues of the host. Bischoff (1923) figures an identical egg for an undetermined tryphonine species in Europe; that of T. rutilator Holmg., the ovarian form of which is illustrated Pampel, is evidently very much like it. The egg of T. incestus (Fig. 33A, B) is of the same general form; but the pedicel is shorter, the anchor much smaller, and the latter is inserted immediately beneath the integument. That of Tricamptus apiarius Grav. figured by Bischoff is similar to it. The egg of Exenterus tricolor Roman (Morris et al., 1937) is of the same general form and bears a scale‑like sculpturing. In these species, the chorion is exceedingly heavy and tough and is difficult to puncture, even with a needle. In Anisoctenion alacer Grav. (Fig. 33D, F), the anchor assumes a curious and quite different form, in which it is represented by a blackened shield, with serrate margins, on the ventral side of the egg body. This shield, which is slightly larger than the egg, opens out, umbrella‑like, at the time of deposition. The entire egg except the dorsal surface lies beneath the host integument, and the exposed portion of the chorion bears delicate reticulate markings.
The egg of Lysiognatha sp. (Cushman, 1937) is apparently quite similar to that of Tryphon incestus. In all the species that deposit eggs of the pedicellate type, the adaptations that will appear in the laid egg can be detected by an examination of the ovarian egg (Fig. 33D, H).
Not all the Tryphoninae possess eggs of the pedicellate type discussed above. In the Diplazonini, the egg is ellipsoidal in form, with both ends smoothly rounded. That of Hypamblys albopictus is kidney‑shaped, whereas the egg of Exenterus coreensis Uchida (Fig. 33C) is oval in outline, with no pedicel whatever, and is largely embedded in the wound. The egg of E. abruptorius (Fig. 33I) described and figured by Morris (1937) may be considered as intermediate in form between that of E. coreensis and of Tryphon incestus, and it shows an incipient pedicel formation. This is represented by a slender cylindrical extension at the posterior end. At oviposition, the body of the egg is largely embedded in the wound, only a portion of the dorsum being exposed through the aperture in the skin, and the tip of the pedicel also protrudes, though from a separate and minute hole. The variation in egg form and manner of deposition within a genus is illustrated by the three species of Exenterus that have been mentioned.
First‑instar Larva. --What may be termed the normal hymenopteriform first instar larva of the family is that of the ectophagous species of the Ichneumoninae and other subfamilies; it is characterized by a large and often heavily sclerotized head, with large conical antennae and simple mandibles, and 13 body segments of diminishing width. The integument may be bare or clothed with numerous minute spines. Several species that develop internally are of this same general form. The first‑instar larva of P. nigridens bears six pairs of small setae on each segment; in addition, each abdominal segment bears a broad transverse band of minute integumentary setae. The anal opening is usually situated dorsally, though it is said to be on the venter of the thirteenth segment in C. calcitrator (Fig. 34a). In this species, paired fleshy processes occur dorsolaterally on the abdomen; they are of increasing length on the successive segments.
In the Paniscini, the first‑instar larva has been described only for Paniscus cristatus Thoms. It differs from the normal hymenopteriform larva only in the possession of numerous forward‑directed spines on the venter and sides of the last abdominal segment, an adaptation to hold the caudal end of the body more firmly within the eggshell during development. Polysphincta, which has the same habit, is not known to possess this character.
The first‑instar larva of Anisoctenion alacer (Fig. 35B) is markedly different from those thus far discussed, though still of the hymenopteriform type. Each body segment bears a transverse row of long hairs at each lateral margin; these are of decreasing length and number on the successive segments. Each of the first five abdominal segments bears a pronounced welt on the median dorsal line. This larva normally moves upon its back in a looping manner, the welts and the caudal sucker aiding in accomplishing locomotion, while the lateral tufts of long hairs hold the body in a horizontal position. Exenierus coreensis (Fig. 35A) and several others of that genus and Tryphon semirufus have similar larvae, though the lateral tufts of hairs on the latter are much shorter. The larva of Tryphon incestus (Fig. 35C), however, lacks both the dorsal welts and the lateral tufts of long hairs, is densely clothed with minute spines, and does not assume an inverted position when in movement.
The most common type of first‑instar larva among the endoparasitic species is the caudate, which attains its highest development in Ichneumonidae. The body is somewhat cylindrical, with 11 to 13 recognizable segments, and the integument is usually smooth and shining. The tail may equal or exceed the body length, and it may be slender and taper to a sharp point or be almost cylindrical, with the distal end broadly rounded, as in Thersilochus conotracheli (Fig. 36B, C). In some species, as Anomalon cerinops Grav, the terminal portion of the tail is spined. Timberlake (1912) considered the tail of Eulimneria valida Cress. to be a blood gill, whereas the extensive ramifications of the tracheal branches in the tail, illustrated by Tothill (1922) in the larva of Hyposoter pilosulus, which led him to attribute a respiratory function to that organ, have been shown by Thompson and Parker to represent an erroneous interpretation of the structures observed in mounted specimens. Working with Eulimneria crassifemur Thoms., a species of very similar form, they determined that the supposed bundle of tracheids is simply a lobe of the fat body from which the fat globules have been dissolved by the reagents employed.
Thorpe (1932) has studied the tail appendage of a series of species of this and other families with particular reference to its role in respiration. He found an appreciable variation in the extent to which the tracheal branches extend into this organ. In the majority of species, the lateral tracheal trunks extend into it and terminate in the fat body, but in Cremastus interruptor they branch and extend through the basal two‑thirds of the tail.
The newly hatched caudate larvae of Cremastus flavoorbitalis Cam. (Bradley and Burgess, 1934) (Fig. 36A) and C. interruptor Grav. bear a double row of scallops transversely on each body segment; these disappear before the first molt and are believed to be an adaptation to permit of rapid increase in body size. The larva of Anomalon cerinops has a pair of small slender processes ventrally on the first and third thoracic and the sixth and eighth abdominal segments.
The first‑instar larva of Omorgus mutabilis Holmg. bears a pair of prominent tusk‑like sense organs on the head that project downward and backward from the posterior ventral margin of the head capsule. They represent one of the four pairs of sense organs present on the venter of the head of larvae of this family.
Many of the caudate larvae have the head comparatively large, heavily sclerotized, with falcate mandibles, approaching that of the mandibulate type. The larva of Syrphoctonus maculifrons Cress. may properly be considered as of the latter type, for the head is equal to the thoracic region in width and the tail is hardly evident (Kamal, 1939). It bears a strong resemblance to the mandibulate larvae of the Braconidae, particularly of Opius. In Diplazon and Homotropus, of the same subfamily, the head is smaller and the tail more fully developed, though still short.
The vesiculate type of larva is not nearly so common, nor is the vesicle so highly developed as in the Braconidae. Usually it is in an incipient stage, is small in size, and often is not readily recognized because of being retracted at the time of examination. A number of the caudate larvae of the Ichneumoninae and Ophioninae, such as Glypta rufiscutellaris, Nemeritis canescens, and Anomalon cerinops, bear the vesicle dorsally at the base of the tail. A typical ichneumonid vesicle is that of Banchus femoralis Thoms., illustrated in Fig. 37.
The polypodeiform type of larva is found in Hypamblys albopictus (Wardle, 1914) in which the paired thoracic processes are lobe‑like and those of the abdominal segments rather sharply pointed. The tail is approximately one‑fifth the length of the body.
There is apparently no essential distinction between the respiratory systems of ecto‑ and endoparasitic first‑instar larvae. Some are stated to be entirely devoid of tracheae~ whereas others have a complete internal system corresponding to that of the mature larva except for the lack of spiracles. The tracheal system of Phaeogenes nigridens, which has been fully described by Smith, consists of a main lateral trunk on each side of the body connected by main transverse commissures dorsally in the first thoracic and ventrally in the ninth abdominal segment. Accessory lateral commissures connected with the main trunks by three branches, extend from the posterior margin of the first thoracic to the anterior margin of the first abdominal segment. In each of the first nine abdominal segments, the ventral branches are connected to form secondary transverse Gommissures.
With very few exceptions, the first‑instar larvae of this family lack spiracles. Paniscus cristatus is said to have a pair on the prothorax; Meyer (1922) illustrates that pair, and eight additional pairs on the abdomen, in Tryphon signator Grav. Imms (1918b) found nine pairs of spiracles on the first‑instar larva of Pimpla pomorum; Speyer (1926), studying the same species, noted an additional pair, very minute, on the thorax. The general lack of an open tracheal system is in contrast to the Braconidae and other extensively studied families of the order, in which the ectoparasitic first‑instar larvae are quite consistently provided with open spiracles.
Intermediate‑instar Larvae. --The information available as of 1940 was insufficient to make an adequate comparison of the larval instars between the first and last, due primarily to uncertainty as to the total number. A considerable number of species are stated to have only three instars, and others four; many are known to have five instars. Unquestionably, some of those said to have only three will reveal, on closer examination, a greater number. Rosenberg mentions the occurrence of six instars in occasional larvae of Cryptus sexannulatus Grav. and Hemiteles hemipterus, though the normal number is five and four, respectively. In the species of Paniscini and Polysphinctini that retain connection with the eggshell during larval development, the number of instars can be readily determined by a count of the exuviae forming the pad beneath the posterior portion of the body.
In species having hymenopteriform first‑instar larvae, there is little change in general form in the following instars, but those of caudate form in the first instar usually show a progressive reduction in the appendage, with its complete absence in the last instar. In Thersilochus conotracheli, it disappears entirely with the first molt, and in some other species it persists only through the second instar. The bidentate mandibles appear in the second instar in Ephialtes examinator. The second‑instar larva of Collyria calcitrator (Fig. 34c, e) is of a pronounced mandibulate type, with the head wider than the body and the mandibles large and falcate in form. The fleshy dorsolateral processes on the abdomen persist in this instar.
The stage of development at which the spiracles appear is variable. In Ephialtes examinator and Phaeogenes nigridens the nine pairs are evident in the second instar, though in the latter species, which is internal, they are nonfunctional. Angitia fenestralis Holmg. reveals the spiracles in the penultimate instar, but in the majority of species they appear only in the last one.
Mature Larvae. --The normal last‑instar larva of the Ichneumonidae has 13 distinct body segments, the integument usually smooth and glistening, and it bears no fleshy processes or appendages. In Phaeogenes nigridens, there is a very characteristic dorsal hump on the third thoracic and first abdominal segments, a modification in form said by Smith to be necessary because of the manner of feeding of the larva. In the majority of species, the mandibles are simple, often with minute spines on the margin, though a few are bidentate and those of Echthropsis porteri are 5-dentate. In Xylonomus brachylabris Kr., the mandible has a concavity on the inner side flanked by ridges crowned with distinct teeth. The mandibles of Polysphincta are stated to be curved outward at the tips, and the puncture in the host integument is made, not by a pinching action, but by the tips being brought together, inserted, and then spread apart. Each body segment usually bears a row of small, delicate spines transversely that may encircle the segment. In Philopsyche abdominalis Morley (Skaife 1921b), there are two distinct bands of short spines on each segment, those of the first band being directed cephalad and those of the posterior band caudad. This is presumably an adaptation for movement within the case of the bagworm host. The larva of Pimpla pomorum bears numerous minute papillae upon the integument.
The tracheal system consists of the two main longitudinal trunks connected by dorsal anterior and ventral posterior commissures, with a supplementary lateral trunk on each side extending from the posterior margin of the first thoracic segment to the anterior margin of the first abdominal segment and connected with the main trunk by three branches. There are usually nine pairs of spiracles, the first of which, though mesothoracic in origin, is situated at the posterior margin of the prothorax, the remainder being near the anterior margin of the first eight abdominal segments (Fig. 38). Angitia fenestralis (Meyer 1915) is stated to have 11 pairs of spiracles, situated on all thoracic and the first eight abdominal segments. Imms (1918b) called attention to the occurrence of 10 pairs in Pimpla pomorum, the additional pair on the 2nd thoracic segment being vestigial and nonfunctional. Thorpe (1930) mentioned this in a discussion of P. ruficollis Grav. and stated that the occurrence of the vestigial pair on the 2nd thoracic segment is probably general in the family but has been largely overlooked. There are 10 pairs in Polysphincta tuberosa, also, but those of the thorax are on the 1st and 3rd segments, while in Collyria calcitrator (Fig. 38) they occur on the 2nd and 3rd. The tracheal system of the latter species differs also from the normal for the family in the lack of the lateral accessory and the posterior ventral commissures. Salt pointed out the general similarity of the larval characters of the species to those of the Braconidae. In Scambus detrita and other species, the ventral branches in each abdominal segment unite to form accessory ventral commissures.
The greatest modification in mature larval form and in functional adaptation occurs in the tribe Polysphinctini and in certain other Ichneumoninae. These species are parasitic upon spiders or are predaceous in their egg capsules. The morphological modifications are of two forms and serve distinct purposes. The first of these is the occurrence dorsally of retractile "welts" (Fig. 39C), surmounted by a number of hooked spines or of patches of straight spines, which serve to hold the larva in the web during the spinning of the cocoon or to facilitate movement in the egg capsule. The second modification is the development of paired fleshy processes ventrally on certain abdominal segments to attach the body firmly to the exuviae and thus to the body of the host spider.
The mature larvae of a considerable number of species have been described by Nielsen (1923), and the dorsal welts, bearing the hooked spines, occur in most if not all species of Polysphincta, Schizopyga, and Zaglyptus. The number of welts is usually seven or eight, and they occur in a single row on the median line of the third thoracic and the following seven segments in P. tuberosa Grav. (Fig. 39A), P. eximia Schm., and P. nielseni Roman. Four welts only are recorded on the larva of P. gracilis Holmg., whereas in P. clypeata Holmg. (Fig. 39B), P. pallipes, and S. podagrica Grav. they are paired, rather than single, on each segment. In the last species, they occur on the first six abdominal segments (Nielsen, 1935). Laboulbene (1858) records them on the first seven body segments in P. fairmairii Lab., and Maneval (1936) stated that they are on the first seven abdominal segments in Z. variipes Grav. In these two species, also, the welts are single rather than paired. The hooked spines that surmount each welt are directed outward from the center of the welt; and when one of these, or more, is drawn over a strand of the host web and the welt then retracted into the body, the larva is very securely held in position. In Tromatobia oculatoria F., the spines are simple and straight and arranged in transverse bands at the anterior and posterior sides of the welt. Those at the front are directed cephalad, and those at the rear caudad.
Many if not all of the species of Polysphincta have a pair of fleshy conical processes (Fig. 39D) ventrally on the fifth and sixth abdominal segments, and these are embedded in the exuviae beneath the body. They are present upon the intermediate instars, also. In S. podagrica, there are four pairs of these processes rather than two, and they occur on the fifth to the eighth abdominal segments.