Immature Stages of Trichogrammatidae
Immature stages of Trichogrammatidae were discussed in detail by Clausen (1940), as follows:
The egg forms of very few species of the family were known by 1940 (Clausen 1940). Those of several Trichogramma are somewhat elongate, with the middle portion distinctly expanded, and both ends are smoothly rounded (Fig. 48A). The egg of Oligosita utilis. is of similar form, though with a short, heavy peduncle at one end. In Poropoea stollwercki (Fig. 49A), the main body is 0.5 mm. in length, elongate and irregularly curved, and it bears a slender peduncle at the anterior end. The narrower posterior portion represents a somewhat ringed appearance. The egg of P. defilippii Rond. is similar, although the peduncle is shorter and more delicate. In Chaetostricha pulchra, figured hy Bakkendorf (1934), the main egg body is cylindrical, and the anterior peduncle, which is half as long, is set at an angle with the main axis of the egg.
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There are two distinct types of first‑instar larva, the first of which is sacciform almost globular or cylindrical, and lacks sensory and integumentary setae and other external characters. The mandibles are minute but distinct. Those of Trichogramma (Fig. 48B), Chaetostricha, and Oligosita are of this type. The second is much more highly specialized, being mymariform, with the head and thorax appreciably larger than the abdomen, the segmentation distinct, the caudal segment drawn out into a tail, and the thorax and abdominal segments bear long setae. This type occurs in Poropoea and Ophioneurus. The first‑instar larva of P. stollwercki (Fig. 49B), as described by Silvestri, is 0.28 mm. in length, with the head and thorax exceptionally large, and the abdomen consists of six ring‑like segments and a seventh that is curved ventrally and extended into a point. There are 16-18 long, slender setae in a transverse row at the posterior margin of the thorax, situated dorsally and extending to the lateral margins. All abdominal segments except the last bear a smaller number of these setae dorsally near the anterior margins. The larva of Ophioneurus signatus Ratz. (Fig. 50), described by Bakkendorf (1934), has a distinct head; but the thorax and abdomen are unsegmented, almost spherical, and the last segment is produced into a slender, curved tail which bears a tooth at the mid‑ventral margin. In addition, there is a long, heavy process or spine arising dorsally slightly in front of the base of the tail.
There has been considerable disagreement as to the number of larval instars in the family. Taylor stated that there is only one in Oligosita utilis, and Bakkendorf was unable to find evidence of intermediate molts in Chaetostricha pulchra. Flanders described three instars in Trichogramma, and Silvestri recorded five for Poropoea stollwercki, though here, also, the evidence of a corresponding number of molts is incomplete.
The second instar of Trichogramma (Fig. 48C) is somewhat elongate and tapering anteriorly, and the segmentation is indicated only on the anterior half of the body. The mandibles are extruded and only slightly curved. The presumed second‑instar larva of P. stollwercki (Fig. 49C) is still mymariform, with the abdomen further reduced, and the tail exceeding the body in length. It appears more probable that this larva is still of the first instar rather than a distinct second. Even the third instar as figured (Fig. 49D) is identical with the first except for the elimination of the segmental lines, possibly due to an increase in volume through feeding.
The mature larvae of all genera are of similar form, being robust, more or less distinctly segmented, and without spines or setae. The mandibles are elongate and extruded and lie parallel to each other. They are immovable and consequently not used in feeding, though they may serve to lacerate the remaining embryonic tissues in the host egg. Several authors emphasized the complete lack of a tracheal system in the larvae of Trichogramma, and it has not been mentioned in other genera.