Immature Stages of Aphelinidae
Immature stages of Aphelinidae were discussed in detail by Clausen (1940), as follows:
The ovarian eggs of many species of the Aphelinidae that have been observed are of the two‑bodied type such as is found in several other families. At deposition the contents are forced into the main body, and the "bulb" remains as a somewhat collapsed stalk or hook. This form of egg is common to Aphytis, Centrodera, and Aspidiotiphagus. In Marietta zebra, and other species, the laid egg (Fig. 66D) is much like the ovarian form except that the anterior stalk is appreciably reduced in size. This species is distinguished from others of the family by having the chorion of the main body covered with small papillae.
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The eggs of Ablerus, Casca, Encarsia, Eretmocerus, and Physcus, and those of Coccophagus of primary habit, are of simple form, ranging from lemon‑shaped to kidney‑shaped and cylindrical. That of Eretmocerus serius (Fig. 65B) is distinctly flattened, owing presumably to its being compressed between the host body and the leaf after deposition.
Smith and Compere recorded an unusual modification in the egg of Euxanthellus philippiae Silv. The ovarian egg is simple; yet after its deposition it is found to be attached to the host larva by a pedicel arising at the center of the ventral side (Fig. 66E). This pedicel apparently is formed in the same manner as that of the male eggs of Coccophagus Iycimni and C. heteropneusticus Comp., which are discussed in the following section, and it may, in fact, be a male egg, also.
The first‑instar larvae of the species that show no sexual dimorphism are of two types, the hymenopteriform and the caudate. The hymenopteriform larvae range from elongated to almost spherical in form and bear no appendages or integumentary setae. To this group belong the larvae of Aphelinus, Eretmocerus, Prospaltella, Aphytis, and Aspidiotiphagus. The larva of E. serius (Fig. 65C) is pear‑shaped and shows no evidence of segmentation. Those of endophagous habit usually have only a simple internal tracheal system and no spiracles. Among the species that develop externally the number of spiracles is variable, ranging from five in M. zebra to eight in Aphytis mytilaspidis and A. longiclavae Mercet, though none was detected in Eretmocerus.
The caudate larva is not encountered frequently in this Aphelinidae, though it has been described for Aspidiotiphagus citrinus.
It is often difficult to determine the number of instars because of the minute size of the larvae and the lack of heavily sclerotized parts which might distinguish the successive exuviae. There are apparently only three, in contrast to the five found in several related families. The second and third instars possess no distinctive features and need not be described. The mature larvae usually have nine pairs of spiracles, though Coccophagus hawaiiensis and Aphelinus semiflavus are thought to have only five.
The pupae of a number of genera that attack diaspine Coccidae are conspicuous because of the extreme compression in the dorsoventral plane. In some species of Aphytis, the thickness is only one‑fourth the width of the body. The pupa of Physcus intermedia has an exceptionally heavily sclerotized integument, black in color and similar to that found in various Eulophidae.
Sexual Dimorphism in the Immature Stages of Coccophagus.-- In the species of this genus that develop, in both sexes, as primary parasites, there is no departure from the normal of the family in the egg or larval forms, those of the two sexes being identical. Sex differentiation in host relationships in a group of species has involved a specialization in habits of the male that is reflected in several modifications in form to meet more successfully the conditions encountered. Flanders has described and figured the female and male eggs and early larval instars of several species.
The ovarian and laid female eggs of C. Iycimnia and C. heteropneusticus (Fig. 66A) are somewhat cylindrical, though slightly broader at one end, slightly curved, and smoothly rounded at both ends. The male egg (Fig. 66B, C), on the other hand, is drawn out at the anterior end into an irregular stalk‑like process which, at deposition, is twisted into a button‑like pedicel when it is inserted into a puncture in the host integument.
The first‑instar female larva of C. capensis (Fig. 68B) is of the caudate type, whereas the male (Fig. 68A) is of a distinctive form, having the segments clothed in long setae and the last abdominal segment extended into a long, spine‑like process. Both sexes have only an internal tracheal system. The male larva of C. scutellaris is of similar form.
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The first‑instar female larva of C. heteropneusticus (Fig. 68C) is rather elongated and lacks an open tracheal system, whereas the male (Fig. 68D) is more robust and has spiracles on the last thoracic and the second, fourth, and sixth abdominal segments.
Coccophagus gurneyi has a first‑instar larva female (Fig. 68F) that is slender, tapering posteriorly, and has only an internal tracheal system. The male (Fig. 68E) is of a modified planidium type, with heavily sclerotized segmental plates and two pairs of spiracles, situated on the intersegmental membrane just in front of the third and fifth bands, representing the first and third abdominal segments.
The ectophagous male larva of C. lycimnia is robust in form, with spiracles on the metathorax and on the second, fourth, and sixth abdominal segments.