Immature Stages of Trigonalidae
Immature stages of Trigonalidae were discussed in detail by Clausen (1940), as follows:
The microtype egg of P. thwaitesiii (Fig. 26A) measures 0.12 by 0.07 mm., is ovoid in form, flat ventrally, and arched dorsally, and bears a series of 5-7 longitudinal ridges on the dorsum. The chorion is exceedingly hard and translucent. Other species observed have eggs of similar form, with slight variations in size and in the number and prominence of the longitudinal ridges. Those of N. jezoensis are said to be convex on both sides. The microtype first‑instar larvae have been described only for P. maga (Fig. 26B) and Orthogonalis debilis (Clausen, 1931), and these are very similar in form. They are 0.12 mm. in length, with 12 body segments, broadest in the thoracic region and tapering gradually caudad. The head is broad, largely retracted into the thorax and the mandibles are slender and extruded. The first thoracic segment has a transverse ventral row of five exceedingly heavy hooks, directed caudad, and is heavily sclerotized palmate plate dorsally on the median line. The 2nd & 3rd segments have the row of heavy spines both dorsally and ventrally. Each abdominal segment has a transverse row of minute setae both ventrally and dorsa11y, those on the last two or three segments forming a complete ring.
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The intermediate‑larval instars have been described for P. thwaitesii only, and these were secured by dissection of parasitized Henicospilus larvae taken from their cocoons. The 2nd instar (Fig. 26C) is markedly different from the 1st, having a large hemispherical head, with large but lightly indurate mandibles, and 12 body segments which bear no hooks or setae. The 3rd instar (Fig. 26D) is of the mandibulate type similar to the 1st instar of many other Ichneumonoidea. It has a single pair of spiracles at the anterior margin of the 2nd thoracic segment. The 4th instar larva (Fig. 26E) is more slender, with the head of normal form, and the caudal segments are somewhat attenuated. There are eight pairs of spiracles, situated on the 2nd & 3rd thoracic and the 1st 6 abdominal segments, of which the first pair is much the largest. The 5th instar (Fig. 26F) is very robust in form and bears no integumentary spines or setae. The spiracles are as in the preceding instar. The mandibles are tridentating as compared with the simple form of those of the preceding instars.