Immature Stages of Rhipiphoridae
Some detailed information on immature stages of Rhipiphoridae = Ripiphoridae) was given by Clausen (1940)
The eggs of the various species are relatively minute, ranging from 0.3 mm. in length in Macrosaigon flabellatum to 0.7 by 0.15 mm. in Rhizostylops inquirendus. They are usually broader at one end, white, and in Meteocus paradoxus, at least, covered with a mucilaginous material.
The triungulinid first‑instar larvae are of the planidium type, with the body smoothly tapered and the segments each bearing a heavily sclerotized band, with pleural plates, and separated by thin and very elastic intersegmental membranes, which are visible only after the commencement of feeding. The body color is blackish, owing to the heavily indurate segmental plates. The thoracic segments are large and broad, and the abdominal segments, which apparently range in number from 8 to lO, are of diminishing length. The head (Fig. 235) is somewhat triangular in form and heavily sclerotized and bears several ocelli, placed close together, at each side. The antennae are usually two‑jointed, the second terminating in a long hair‑like process that is believed to be a segment. The mandibles are heavy, simple, and sharply curved. The legs are long and slender, and the tibiae usually terminate in large leaf‑like pulvilli. The body segments bear a variable number of integumentary setae and spines. In the majority of species, the two caudal segments each bear a pair of cerci of varying length, and thc abdomen terminates in a suctorial disk.
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The characters of the triungulinid, which show marked variation between species, are: the antennae, the claws or pulvilli of the legs, the setae and conical spines on the body segments, and the caudal cerci. In most species, the antennae are two‑jointed, the first often bearing a spine or branch near the distal end, though that of Macrosiagon tricuspidata is believed to be three‑jointed. The large leaf‑like pulvillus that is borne at the end of the tibia, in lieu of the tarsus, is replaced in R inquirendus by a pair of minute claws, whereas in Ripidius denisi (Chobaut 1919) the claws are present and also a small pulvillus. In M. flabellatum, pleural plates are prcsent on the sccond and third thoracic and the first seven abdominal segments (Grandi, 1937). Perhaps the most conspicuous specific character is the number and arrangement of the spines and setae upon the different body segments. A number of species, including M tricuspidata, M. flabellatum (Fig. 234B,C), and Ripiphorus solidaginis, apparently have only a single seta at each lateral margin of the segments, and Metoecus paradoxus has a row of small setae dorsally and ventrally on each segment. Ripidius denisi and Rhizostylops inquirendus are distinguished by long setae at the posterior margins of the segments and heavy conical spines on the head and thorax, a pair of these occurring also on each of the first three abdominal segments in the first‑named species. With respect to the caudal cerci, Ripiphorus solidaginis has only two very short pairs, whereas Ripidius denisi has one pair, these being only slightly longer than one or two segments. M. paradoxus and Macrosiagon cucullatum have a single long pair, also, but these are equal to the body in length. Other species have two pairs of unequal length; those on the last segment are longest and equal or exceed the body length in M. tricuspidata and M. flabellatum. In the latter species, the first pair occurs on the ventral plate of the penultimate segment, and the second pair is on the dorsal plate of the last segment.
In Rhizostylops inquirendus and M. flabgellatum, the spiracles occur only on the last two abdominal segments, and transverse commissures occur at the caudal end of the body and in the second and third thoracic segment, whereas there are nine pairs in the case of M. flabellatum, these being situated on the pleural plates of the mesothoracic and the first seven abdominal segments and at the lateral margins of the dorsal plate of the eighth segment.
Little is known of the second‑instar larva of any species. Stamm mentioned it in the case of Ripidius pectinicornis stating that it is quite similar to the mature form. It is to be expected that all the distinguishing characters of the triungulinid would be lost at the first molt; and the second instar, being strictly a feeding form, is grub‑like. What is presumably the second instar of Metoecus paradoxus was figured by Chapman; it is white in color and distinctly segmented, the legs are entirely lacking, and the tracheal system has spiracles on the first and third thoracic and the first six abdominal segments and accessory longitudinal trunks in the thorax.
The mature, and presumably third‑instar, larva is known in Macrosiagon flabellatum, Metoecus paradoxus, Ripiphorus stylopides, and Ripidius pectinicornis. These present considerable differences, the last‑named being markedly different from the others. In Macrosiagon flabellatum (Fig. 233A), the body is rather elongated and greatly curved, with large fleshy processes on all segments, and the legs are short, fleshy, and nonfunctional. Open spiracles occur on the mesothorax and on the first seven abdominal segments, and atrophicd spiracles are found on the metathorax and on the eighth abdominal segment. The larva of Metoecus paradoxus is also rather elongated, with the head and thorax sharply bent ventrally and the caudal segments tapering and likewise curved ventrally. A number of fleshy tubercles occur upon the thoracic segments, and the legs are rudimentary, being conical, somewhat pointed, and divided by mere constrictions into three parts. There are eight pairs of spiraclcs, situated as in the second instar. The mature larva of Ripiphorus stylopides was figured by Böving and Criaghead (1930) and presents several features that distinguish it from M. paradoxus. Two pairs of large fleshy conical processes occur dorsally and dorsolatcrally on each body segment, and the eight pairs of spiracles are situated as in Macrosiagon flabellatum.
In contrast to the larvae already described, that of Ripidius pectinicornis (Fig. 237) may be considered as intermediate between them and the normal coleopterous larva. The fleshy segmental processes are lacking, and the legs (Fig. 237c) are larger and distinctly four‑segmented and terminate in a heavily‑sclerotized claw like tarsus. The head and mouth structures are much reduced (Fig. 237a, b), the antennae being short and two-jointed, and the most conspicuous feature is the labium which ends in three- jointed palpi. No mandibles are discernible. Dorsally on the head there is found a pair of curved, converging, heavily chitinized ridges, which are present also, though widely separated, on the thoracic segments. They are believed to serve in perforating the integument of the host at the time of emergence from the body. Such an adaptation is not required in the species that develop externally upon hymenopterous larvae.