Immature Stages of Mymaridae
Immature stages of Mymaridae were discussed in detail by Clausen (1940), as follows:
There is little variation in egg form within the family. The main body of the egg is ellipsoidal, ovoid, or spindle‑shaped, with a slender tapering peduncle at the anterior end ranging in length from one‑tenth that of the egg body in Anaphoidea nitens and Caraphractus to equal its length in Polynema striaticorne (Fig. 44A). The ovarian egg is practically identical with the oviposited egg. These eggs are exceedingly minute, ranging from O.06 mm. in length in Anagrus atomus to 0.25 mm. in P. striaticorne.
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A great deal of confusion exists regarding the larval forms of the Mymaridae. This is primarily due to the exceedingly minute size of the early stages combined with the lack, in the instars following the first, of heavily sclerotized or indurated structures of fixed form. Often several species have been involved in the descriptions of the instars of what was supposedly a single species. This was the case in the account given by Ayers (1884) of Teleas sp., a scelionid parasite in the eggs of Oecanthus in the United States. His figures 2, 3, 8, 12, and 13, of plate 24, are undoubtedly of Polynema (Fig. 46A), which is stated by E. W. Wheeler (1923) to be P. bifasciatipenna Gir. The Polynema sp. figured by Ganin is stated by Bakkendorf to be Anagrus, probably A. incarnatus Hal., though Henriksen believed it to be A. subfuscus Foerst.
There are two general types of first‑instar larvae. The first of these is oblong or flask‑shaped, of the sacciform type, and occurs in the several species of Anagrus that have been studied and in Prestwichia aquatica. The body is merely a bag, without segmentation, and lacks any distinguishing characters. Bakkendorf (1934) figures a somewhat intermediate form between this and the next, in what are considered to be first‑instar larvae of Allaptus minimus Hal. and Lymnaenon effusi Bakk. (Fig. 46B). These are rather spindle‑shaped, with indications of segmentation, and have the caudal segment attenuated and tapering to a point. There is no distinct tail structure, nor are there spines or setae.
The second and more common form of the first‑instar larva is designated as mymariform and occurs in the described species of Polynema, Araphes, Ooctonus, and Anaphoidea. The head is large, drawn out into a curved median conical process, with a second smaller process beneath it, representing the mouth. The thorax and abdomen consist of six to eight segments, often indistinguishable, with transverse rings of long hairs, which are most numerous upon the dorsum. The dorsal spines of P. euchariformis (Fig. 45A) and Anaphoidea nitens are exceptionally long and heavy. The caudal end of the body bears a long, curved or abruptly bent process, often equal to the body in length, which, in some species, is compressed laterally into blade‑like form and bears a single large tooth or one or more smaller paired teeth on the ventral margin. Clark (1931) described two distinct mymariform instars in A. nitens; the first of these has the tail slender and bent at right angles twice, first ventrally and then dorsally, whereas in the second form it is constricted and toothed at several points (Fig. 46C). Clausen (1940) believed this to be highly improbable, and the two forms doubtless represent either two species or different ages of the first instar of the same species. In size the first instar larvae of the Mymaridae are small and range from 0.1 to 0.3 mm. in total length at the time of hatching.
The number of larval instars following the first is very uncertain. Several authors asserted that there are only the first and the mature forms, though in most species there are said to be three. Balduf described four in Polynema striaticorne.
The second‑instar larva of Anagrus is of distinctive form and has been designated as "histriobdellid" by Ganin, who first observed and described it. This larva (Fig. 45C) is cylindrical in form and is divided by constrictions into six segments, of which the first and last are largest. The head bears a pair of large, conical or cylindrical fleshy processes lateroventrally, which are said to be the antennae, and the extruded mandibles, which are long, slender, and curved, lie parallel to each other. The last segment bears a pair of large ear‑like organs, of unknown function, lateroventrally. This type of larva has thus far been associated only with the sacciform first‑instar larva and is not known in any species having mymariform larvae.
The second‑instar larvae of Prestwichia, Anaphoidea, and Polynema (Fig. 45B, D) have few digtinguishing characters except for the relatively large extruded mandibles, which are somewhat fleshy. The body is bag‑like, without segmentation, and lacks appendages, spines, or setae.
The mature larvae of Anagrus and Paranagrus are similar to the histriobdellid form except that the ear‑like processes of the last segment are lacking and the mandibles and antennal processes are much reduced in size. Those of Polynema (Fig. 44C), Prestwichia, Anaphes, and Anaphoidia are indistinctly segmented, and, aside from the large extruded mandibles, have no recognizable characters. The larva of Erythmelus goochi Enock appears to be intermediate in form between Anagrus and the above genera.
In no species is there any indication of an internal tracheal system or spiracles in any of the larval instars.