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Immature Stages of Platygastridae


Immature stages of Platygastridae (= Platygasteridae) were discussed in detail by Clausen (1940), as follows:


The egg of Platygastridae is always of minute size, ranging in length from 0.02 to 0.1 mm.  The main body is usually lemon‑shaped; with a stalk at the anterior end which, in L. rhanis (Fig.  108A), is 3X the length of the egg body.  In other species, this stalk is shorter, and the extreme is shown in P. herrickii (Fig. 108C), which has the posterior end somewhat, attenuated and bears several short flagellum‑like processes at the anterior end.  Several species that have a pronounced stalk also bear a blunt protuberance at the posterior end.


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   Fig. 106

               Fig. 107

                 Fig. 108


The first‑instar larvae of the polyembryonic species is the form that frees itself from the enveloping trophamnion; it does not necessarily differ from those of the monoembryonic species.  There are two types of larva of this instar, the first being hymenopteriform and the second cyclopoid.  The hymenopteriform first‑instar larva is elongate‑oval to almost spherical in form and distinctly segmented, with the head small, the mandibles relatively large, no fleshy processes on the body, and three pairs of spiracles, situated on the second and third thoracic and the second abdominal segments.  P. hiemalis (Fig. lO9B) P. ornatus, and P. dryomiaee are. of this type.  A large discoidal body replaces the spiracle on the first abdominal segment, and a spi­racular branch leads to it.  In P. zosine (Fig. IO9A), the body is more elongated, with a constriction between the head and thorax, but it lacks any further indication of segmentation.  There are no spiracles.


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                        Fig. 109


            Fig. 110

           Fig. 111


The cyclopoid type of first‑instar larva is characterized by a cephalothorax usually larger than the remainder of the body and somewhat flattened dorsoventrally, which bears enormous falcate mandibles and conspicuous antennae.  The abdominal seg­ments are narrowed and reduced in number.  The mandibles are widely spaced, being set near the lateral margins, and they lie transversely.  The body terminates in one or more fleshy processes of diverse form.  This type was first studied and figured by Ganin (1869) (Fig. 110) for several species parasitic in cecidomyiid larvae; it was described as  "cyclops‑like."  Two of them described by him have a pair of large fleshy processes lateroventrally on the cephalothorax near the posterior margin, and the caudal segment bears two or more long spine‑like processes which themselves are armed with numerous spines.  Marshal has described a similar larva for Leptacis rhanis (Fig. 108B).  A second form described by Ganin shows the caudal appendage bifurcate, with the inner margins serrate.  That described by Marchal for Inostemma piricola (Fig. 109C) bears a close resemblance to it.  In P. lineatus, P. herrickii, Misocyclops marchali (Fig. 108D), and Sactogaster pisi Foerst (Kutter 1934), the caudal appendage is broad, terminating in a pair of lobes which, in the last named species bears short, heavy spines.  In T.  remulus (Fig. 111), the abdominal segments are very narrow and the caudal process is short and bilobed, with the tips rounded and curved inward.  An undetermined species of Platygaster figured by Marchal (Fig. 109D) has the last abdominal segment somewhat expanded, with the posterior margin serrated.  In some species, such as M. marchali, the paired fleshy processes that generally occur ventrally on the cephalothorax are entirely lacking.


Spiracles are not known to occur upon any cyclopoid larvae, in contrast to the three pairs occurring upon most hymenopteriform larvae of the family.


There is considerable question as to the body parts that make up the so‑called cephalothorax of the cyclopoid larva.  Some authors consider it to consist only of the head, and illustrations show the mandibular muscles attached near the posterior base, though Marchal considered it to include the thoracic segments, also.  The fleshy paired ventral processes are presumably borne on the first thoracic segment.  The fact that the visible segments following the cephalothorax number only 5-7 lends weight to this conclusion and accords with the reduced number of body segments  found in the mature larva.


The consideration of the instars following the first is complicated by a variation in the number of molts recognized among the different species.  According to Hill, P. hiemalis has only a single larval instar.  It is of considerable size when separation is effected from the trophamnion, and a relatively slight growth brings it to larval maturity.  In P. zosine, according to the same author, there is only one molt, and the mature form immediately succeeds the first instar.


The second instar, as here discussed, includes only those of species in which three larval instars have been noted.  That described and figured by Ganin for Platygaster sp. is oval in form with no indication of segmentation; during its early period, it is enclosed within the distended skin of the preceding instar.  There are no fleshy or cuticular process any sort, and no open spiracles are present.  Other species that have been studied by various authors possess second‑instar larvae that likewise present no distinguishing characters.  The mandibles are small in all species. 


The mature larvae, which may be of the first, second, or third instar, depending on the species under consideration, present no characters by which they may be readily separated.  The body is oval in form and distinctly segmented and comprises the head and 10-11 body segments.  No cuticular spines or fleshy processes are to be found.  The mandibles are small and widely spaced.  The tracheal system possesses three pairs of spiracles, situated on the second and third thoracic and the second abdominal segments.  The first abdominal segment bears the large discoidal body mentioned in the discussion of the first instar larva of P. hiemalis and others.  This organ has been detected only in species of Platygaster but may occur in other genera also.


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