Immature Stages of Pteromalidae
Immature stages of Pteromalidae were discussed in detail by Clausen (1940), as follows:
The eggs of the great majority of the species of the Pteromalidae are ovate, ellipsoidal, or cylindrical in form, often slightly curved, and without an anterior stalk or pedicel of any sort. That of Enargopelle ovivora, however, is an exception in that it has a slender stalk, one‑third as long as the egg body, at the broader end. In many species, the chorion is smooth and shining; in others, it is covered with minute spines or spicules, but with the poles bare (Fig. 55A). These spicules often give the egg a grayish color. The eggs of Dibrachys cavus, Stenomalus muscarum, and Habrocytus trypetae Thoms. are covered with "tubercles," or papillae; those of Pachyneuron coccorum L. have a granulate surface. In Dibrachoides dynastes, the chorion bears longitudinal ridges. The sculpturing of the egg is not uniform even within a genus, as shown by the smooth glistening egg of Pteromalus puparum and the spinose chorion of that of P. variabilis Ratz. The sculptured chorion is found only upon eggs that are deposited externally.
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The larvae of practically all species are hymenopteriform with 13 distinct body segments, the head often large, and the integument bare except for three pairs of setae on each of the thoracic segments and two pairs on those of the abdomen, the four pairs of spiracles are situated on the second thoracic and the first three abdominal segments (Fig. 55B). There appears to be a variation among the species in the number of sensory setae. The body of Stenomalus micans is covered with minute integumentary setae, and the last abdominal segment is modified to form a "furca" which serves to hold the larva in a favored feeding position. Habrocytus sp. reared from braconid cocoons by Voukassovitch (1927) has a sucker‑like organ ventrally on the second thoracic segment, which is stated to serve a locomotory function. Merisoporus chalcidiphagus W. & R. has an additional pair of spiracles, which is on the third thoracic segment. Certain of the endophagous species, such as Pteromalus puparum, lack an open tracheal system.
The larva of Pirene graminea, described and figured by Kutter (1934), departs from the normal of the family and Is distinctly mandibulate in form. The head is large, the body segmentation indistinct, the integument without setae, and the large, extruded, falcate mandibles are very widely spaced and lie transversely. Marchal (1907) described a larva of similar form in Tridymus piricolaa Marchal.
The full complement of five larval instars has been described for Eupteromalus nidulans, E. fulvipes Forbes, Dibrachoides dynastes, Pachycrepoideus dubius Ashm., S. micans, and Merisus destructor Say. Dibrachys cavus and H. cerealellae, both of which have been studied in detail, apparently have only four, and Habrocytus sp. discussed by Dustan has only three. Kutter described only two for Pirene graminea, though his prepupa shows distinctive characters representing presumably a third instar.
The second‑instar larvae of all species are of simple form, with the sensory and integumentary setae reduced in size; the latter are often absent. The nine pairs of spiracles occur on the last two thoracic and the first seven abdominal segments. In H. cerealellae, only the four pairs that persist from the first instar are open immediately after the molt, and the additional five, which are smaller, appear later in the stage. The larva of P. graminea is indistinctly segmented, and the head is much reduced, with the mandibles small, curved, and very widely spaced.
The fourth‑instar larva of S. mican is distinguished from other larvae of the family by a heavily sclerotized boring armature on the head. This consists of a transverse plate with a serrate edge situated immediately above the labrum and one or two heavy conical spines on the median line below the antennae.
The mature larvae of the different species are uniform in their characters and present no general distinguishing features. The mandibles are simple, and the integument is smooth, with the sensory setae reduced in size. In H. cerealellae, there are three pairs of prominent spines at the end of the last abdominal segment. This species and Pseudocactolaccus asphondyliae, show pronounced intersegmental ridges dorsa11y. The tracheal system usually has nine pairs of spiracles, situated as on the second instar. However, Haviland (1922b) recorded 10 pairs for Asaphes vulgaris Wlk.; the first of these is situated on the intersegmental membrane between the first and second thoracic segments, and those following are on the third thoracic and the first eight abdominal segments. There is a vestigial tenth pair on the eighth abdominal segment in H. trypetae Thoms. The spiracles of S. micans first appear on the fourth instar, and rudimentary spiracular stalks are also present on the first thoracic and the eighth abdominal segments. Kutter's figure of the prepupa of Pirene graminea, which bears the external structures of the mature larva, shows the last segment produced into tubelike form and bearing two pairs of strong setae at the distal end. This tube is stated to be retractile.
Kearns describes an unusual development of the internal tracheal system in the endophagous first‑ and second‑instar larvae of S. micans (Fig. 56). Both instars possess the usual longitudinal trunks, with dorsal and ventral commeasures at the anterior and posterior ends, respectively, and blunt spiracular stalks in the last two thoracic and the first eight abdominal segments. In addition, a pair of visceral tracheal trunks arises from the anterior commissure and extends over the dorsum of the intestine to the eighth abdominal segment, where they unite. These visceral trunks were not detected in the third‑ to fifth instar larvae. A comparable modification of the tracheal system was not known in any other hymenopterous larva in 1940 (Clausen 1940).
Another departure from the normal respiratory system is described by Dustan (1923) for the larva of Habrocytus sp. parasitic in Rogas pupae. His description of that of the mature larva is as follows: " Perhaps the most amazing thing about this parasite is its immense tracheal system which, as was said previously, almost fills the body cavity. It has a tracheal trunk running down each side of the body and spiracles that can be made out under the 4 mm. objective, which appear to be closed, however; but more unusual than all, it possesses myriads of tracheids packed into every conceivable part of the body. These tracheids are collected into bundles or areas that are held in place by a definite wall or membrane. Just inside the wall of each bundle we find a ring of tracheids, varying somewhat in size but alike in having extremely thin walls. All the space inside the tracheids is packed with blood corpuscles and plasma, so that each bundle really consists of a tracheal sheath, the inside of which instead of being hollow is filled full of blood.... These tracheids open at the hypodermal wall and in this way secure an abundant supply of oxygen from the blood stream of the host. This oxygen is then carried in the tracheal bundles to all parts of the parasite and distributed by the blood stream to the different organs and tissues."
Also please see Gerling & Legner (1968) work on Spalangia cameroni.