Immature Stages of Encyrtidae
Immature stages of Encyrtidae were discussed in detail by Clausen (1940), as follows:
The Encyrtidae reveal an exceptional diversity in form of the immature stages, and many of the modifications are strictly adaptive. These are made necessary not only by the wide range of hosts attacked, but by the varied conditions under which development takes place.
The Egg. --Two general types of egg are produced by the Encyrtidae, there being the stalked and the encyrtiform, the latter representing an adaptive modification of the first. In both forms, the ovarian egg is two‑bodied, and the contents of the anterior body, or bulb, are forced into the egg proper at the time of oviposition, leaving the stalk as a slender tube at the anterior end. In the stalked form, this stalk is functionless after deposition but may, in some instances, serve to attach the egg to the integument or to other of the internal organs of the host. Representative genera having this type of egg are Aphidencyrtus, Cerapterocerus, Eusemion, and Anarhopus. In Tetracnemus pretiosus, according to Clancy, the stalk is reduced to a broad, blunt petiole one‑third to one‑fourth the length of the egg body.
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The encyrtiform egg is distinguished from the stalked form by a heavy surface rib, termed the aeroscopic plate by Silvestri (1919), which extends the length of the stalk and of the greater portion of the egg itself. Well‑known genera having encyrtiform eggs are Encyrtus, Microterys (Fig. 73A, B), Aphycus, Metaphycus, Blastothrix, and Ooencyrtus. The plate of O. johnsoni is described by Maple as granulate in appearance and is composed of a mosaic of cells upon the outer surface of the stalk and egg body except for a thickened area near the base of what remains of the bulb of the ovarian egg. In deposited eggs, the plate is much darker than the remainder of the chorion.
In two species of Isodromus parasitic in Chrysopa larvae, it has been found by Clancy that the egg of one, I. niger Ashm., is typically encyrtiform, whereas the other, I. iceryae How. (Fig. 74), lacks the aeroscopic plate and bears merely a melanized ring and a delicate membranous collar on the stalk.
First‑instar Larvae. --Among the monoembryonic species of the family, four forms of first‑instar larvae may be distinguished, based upon morphological modifications having a functional nature. The hymenopteriform larva has a body of 12-13 visible segments, is widest in the thoracic or anterior abdominal region, and has no sculpturing or segmental processes. These larvae lie free in the body cavity of the host and lack the open tracheal system. A typical representative of this group is Comperiella bifasciata How. (Compere and Smith, 1927).
The second is the encyrtiform, so called because it hatches from the encyrtiform egg previously described. The number of body segments is reduced, there being only 10-11 visible, and the last segment, which bears the single pair of spiracles, apparently represents several that have fused. The last four or five segments are usually closely enveloped by the eggshell, and this connection persists through the greater portion of the larval stage. The larva of Isodromus iceryae, which hatches from the modified encyrtiform egg already described, is hymenopteriform, for it lacks the posterior spiracles and consequently does not derive its air supply through the stalk. Both the egg and first‑instar larva of this species appear to represent transitional stages between the hymenopteriform and the encyrtiform type. The genus Microterys contains many well‑known species having encyrtiform larvae; yet it has been shown by De Bach that in M. titiani it is hymenopteriform, with a full complement of spiracles.
The caudate larva (Fig. 77A) is frequently found among the species attacking Coccidae and Aphididae and is characterized by the development of the last abdominal segment into a tail‑like organ that may exceed the body proper in length and may bear setae on the distal portion. It is associated with the stalked type of egg. These larvae do not possess open spiracles. Many genera have larvae of this type, the best known being Aphidencyrtus, Cerapterocerus, Cheiloneurus, Eusemion, and others.
The vesiculate form is similar to the hymenopteriform, except that the proctodaeum is evaginated to form a caudal vesicle. This modification is rare among the Encyrtidae and is at present known only in the genera Anarhopus and Clausenia (Fig. 76C), both of which parasitize mealybugs. Tetracnemus pretiosus (Fig. 76A, B) may be of the same type, though it is uncertain whether or not the small expanded organ on the caudal segment corresponds to the vesicle in the above‑named species. Both A. sydneyensis and T. pretiosus are distinguished from other known Encyrtidae by the presence of a ring of fleshy processes or protuberances on the first 12 body segments; and the former has also a single, curved medium process dorsally on the last segment, immediately above the vesicle.
Intermediate‑instar and Mature Larvae. --The greatest diversification in form occurs in the first instar, and the succeeding forms tend to become more uniform as the final instar is reached. The hymenopteriform larva, which lies free in the body cavity of the host, progresses through the series of molts without appreciable change in its essential characters. In the caudate forms, the tail becomes considerably reduced in size in the second instar and practically disappears in the third. The vesiculate forms, on the other hand, show an enlargement of the vesicle in the second and third instars. In Anarhopus and Tetracnemus, the ring of fleshy protuberances on each body segment of the first‑instar larva is lacking after the first molt.
The number and position of the spiracles of the larvae are an exceedingly variable character in Encyrtidae. In the hymenopteriform larva, the spiracles are lacking in the first and second instars, but they appear on the second to the tenth body segments in the third or a later instar. Among the species having caudate larvae, Cerapterocerus mirabilis Westw. is stated to lack spiracles until the fourth instar, at which time the nine pairs appear in the position already mentioned. In Carabunia myersi (Fig. 73), they are first found on what is stated to be the third and final instar, and only three in number, the anterior pair being on one of the thoracic segments and the remaining two pairs on the abdominal segments immediately preceding the caudal appendage
Information regarding the spiracle arrangement of vesiculate larvae is available only for Anarhopus sydneyensis. In this species, they are lacking on the first instar and occur on the second to tenth body segments of the second and last instar. Tetracncmus pretiosus has no open spiracles until the final instar, when the full complement appears.
The first‑instar encyrtiform larvae possess a single pair of spiracles on the last apparent abdominal segment. This arrangement persists in the following two instars, and the nine pairs of spiracles then appear on the fourth instar. In Microterys speciosus (Ishii, 1923), they are stated to appear on the third instar. Clancy mentioned that the second instar of Isodromus is readily distinguished from the first by the presence of the spiracular spurs in the second to ninth body segments, and this character may be common to many second‑instar larvae of the encyrtiform type. A marked departure from the normal for the family occurs in Metaphycus lounsburyi (Smith and Compere, 1920), in which the single caudal pair of spiracles of the first instar is followed by three additional pairs, situated on the second to fourth body segments, on the second instar, and by the usual nine on the third instar.
A further modification in spiracular arrangement is found in certain species of Encyrtus having encyrtiform larvae, which acquire in their later stages an intimate connection with the host respiratory system. In E. infelix (Thorpe, 1936), the fourth instar has 3 pairs of spiracles, one of which is on the prothorax and the remaining two at the posterior end of the abdomen. The caudal spiracles are borne at the end of a pair of slender tube‑like processes, merely enclosing the tracheal tubes, half to‑two‑thirds the length of the body proper. The fifth‑instar larva bears only two pairs of spiracles, one at each end of the body. This spiracular modification occurs also in E. infidus, though one instar apparently was overlooked, and the described third (Fig. 70B) is identical with the fourth of E. infelix. Ishii (1932a) described the supposed first‑instar larva of E. barbatus Timb. which has the three pairs of spiracles arranged in identically the same manner as is given above for the fourth instar. The large size of this larva indicates that it may be a later instar than that stated (Clausen 1940).
In considering the various adaptations, it is seen that the characters mentioned are common to a number of genera and that, in some instances, all species of a given genus do not reveal the same modifications.