Description & Statistics
Adult beetles are rather striking with a markedly streamlined body, pectinate male antennae, but the color pattern of many species is variable. Females of Macrosiagon pusillum Gerst. may be completely red or black, or the thorax may be of one color and the elytra and abdomen of the other. Silvestri (1905) described the genus Rhizostylops as having certain characters and habits that seem to place it as an intermediate form between Rhipiphoridae and Strepsiptera, and the adult females bear a striking resemblance to those of the genera Mengenilla and Eoxenos of Strepsiptera. Adult females of Rhizostylops as well as those of several species of Ripidius are apterous, degenerate and larviform (Clausen 1940/62).
All species seem to be parasitic, passing at least a portion of the larval period internally in the host body. This adaptation is virtually unknown elsewhere in the Coleoptera. Development is accompanied by a hypermetamorphosis that is comparable with that in Meloidae and certain parasitic Staphylinidae.
Most Rhipiphoridae seem to attack larvae of Hymenoptera in families Andrenidae, Scoliidae, Vespidae and Tiphiidae. Those most often encountered belong to the genera Metoecus, Ripiphorus, and Macrosiagon. Rather extensive parasitization of scoliid and tiphiid larvae in cocoons has been observed on several occasions. In India, Tiphia pullivora A. & J., 28.4% of field collected cocoons yielded Macrosiagon pusillum adults. Generally all representatives of the family developing on Hymenoptera are harmful (Clausen 1940/62).
This is a small, cosmopolitan family with over 200 species known. Characters include a serrated female antenna, male antenna pectinate or flabellate, 11-segmented in both sexes; humpbacked, wedge-shaped beetles; pronotum large, distinct, narrowed anteriorly; tarsal formula 5-5-4; elytra entire; abdomen with 5 visible sternites, blunt at apex. The maxillary palps are 4-segmented; labial palps 3-segmented; legs slender; trochantin absent. In some species females are apterous and larviform.
All known species are solitary parasitoids during their immature stages. Most attack larvae of Hymenoptera in the family Andrenidae, Vespidae, Tiphiidae and Scoliidae. Some parasitize adult and nymphal cockroaches. Both primary and hyperparasitic species are known. First instar larvae are phoretic. Larvae under hypermetamorphosis. Cockroach parasitoids are internal, while those species parasitizing Hymenoptera are internal only during the 1st instar. Adults are free-living.
Biology & Behavior
Ripidius spp. departs from the normal behavior for the family, both in host preferences and in relationships. Ripidius pectinicornis Thbg. was originally described as early as 1808, as a parasitoid of Blatella germanica L. under the name of Symbius blattarum Sund. by Sundervall in 1831 (Clausen 1940). Mature larvae were found in the bodies of cockroaches on a ship, and adult females were observed to lay their eggs abundantly. Stamm (1935, 1936) extended studies on the behavior and larval forms of this species. Schultze, cited by Clausen (1940) recorded rearing R. scutellaris Hell. from Blattidae in the Philippines, and R. boissyi Abeille is parasitic in nymphs of Ectobia in Europe. The whole genus seems restricted to Blattidae. It is also distinguished in habit from those developing on larvae of Hymenoptera, by passing its entire larval period within the host. R. pectinicornis is gregarious, with 1-5 developing in each host, while those on Hymenoptera are consistently solitary.
Extensive observations have been made on Metoecus paradoxus L., which is common in Europe as a parasitoid of Vespa spp. larvae. The parasitic relationship was recognized early in 1864 by Westwood (cited by Clausen, 1940). Chapman (1870, 1891, 1897) first thought this species was a commensal in the nest. Murray (1870a,b) agreed with the conclusions of Westwood. Rouget (1873) obtained oviposition in the laboratory and thought that under field conditions the eggs are laid on blossoms, foliage, etc., and that the young larvae are then carried to the nest by Vespa adults. Chapman later found the much distended 1st instar larva, 10X their original length, within the bodies of the host larvae, just beneath the integument of the 4th of 5th segment. Only a part of the 1st stage is passed internally, and the 2nd instar larva is found as a collar encircling the cervix of the host.
Reproductive capacity of Rhipiphoridae is relatively high, which is expected because of a high mortality in the 1st larval stage. Chobaut (1891) noted that the female of Macrosiagon flabellatum F. lays ca. 500 eggs, and Silvestri recorded ca. 3,000 for R. inquirendus Silv. Eggs are usually laid in clusters, with the site of oviposition being variable. M. flabellatum lays its eggs in clusters in the soil, covering them lightly with earth. jarvis (1922) found that M. cucullatum Macl. laid the eggs close together among the hairs on the undersides of the leaves of Urenia and Ficus. Over 100 were found on a single leaf, covering an area of ca. 9-10 sq-cm. Metoecus paradoxus lays the eggs in crevices in decaying wood. Ripiphorus subdipterus Bosc. was found to oviposit in the blossoms of Eryngium (Chobaut 1906), and R. solidaginis Pierce does so in the green buds of goldenrod, Solidago rigida (Pierce 1904). There are numerous adaptations correlated with the location of the host stages and with the habits of the host adults in case the latter serve as carriers of the triungulinids. In no case were eggs found to be placed on or in close proximity to the host stages on which development of the larva occurs (Clausen 1940/62).
Of particular interest is the manner by which the triungulinids gain access to the host, because it involves transportation by some agency from the vicinity of hatching to the host larvae in their cells. It is believed that the triungulinids themselves do not take an active search for either the host stages or the carrier but rather that they take up a position favorable to contact with a carrier and then wait for it. Triungulinids of M. flabellatum attach themselves to Odynerus adults and are thus carried to the nest (Chobaut 1906). Pierce (1904) thought that the triungulinids of R. solidaginis are carried by the Ripiphorus adults themselves, which are thought to hibernate in the holes of Epinomia. This explanation is in view of the occurrence of the triungulinids on opening buds of Solidago, a plant that is not frequented by Epinomia adults. However, many of them were found on the bodies of bees of various genera living in the Epinomia community. Triungulinids of R. subdipterus are found on Eryngium blossoms and are thought to attach themselves to Halictus adults frequenting this plant (Clausen 1940/62).
Macrosaigon cucullatum is parasitic on larvae of Campsomeris spp. in Australia. The wasps are external parasitoids of scarab grubs in soil. Triungulinids of Macrosaigon are found on the foliage of certain trees and the problem of reaching host larvae in the soil, which are themselves parasitic and thus receive no attention from the parent females, is more complex than that facing the species mentioned previously. Laboratory studies indicated that the triungulinids probably attach themselves to the Campsomeris females and are thus carried into the soil at the time the latter oviposit and that at this time they transfer to the scarab grub and await the hatching of the Campsomeris egg and its subsequent development as a larva. One triungulinid was found to remain motionless on an egg on a paralyzed grub for 3 days, during which it made no effort to pierce the chorion. Although development is completed only on the mature larva in the cocoon, it is probable that the triungulinid attaches itself to the partially grown larva or enters its body prior to cocoon formation (Clausen 1940/62). Triungulinids do not effect parasitization of scoliid or tiphiid larvae after the cocoon has been spun.
Among scoliid and tiphiid hosts of various Rhipiphoridae, it is evident that if the triungulinids of the parasitoid are carried into the soil by the females at the time of oviposition, the extent of parasitization of the different species will vary greatly in the same locality, due to diverse feeding habits of the adults. Scoliid females feed mainly at blossoms, while the spring species of Tiphiidae feed almost exclusively on insect honeydew and the summer and autumn species mostly on the secretions from various nectar glands of plants. The relatively high mortality of Tiphia pullivora previously mentioned, is possibly linked to a more general tendency to feed at blossoms than is shown by other species in the field during the same season (Clausen 1940/62).
A simple parasitic relationship in this family seems to exist in respect to the Ripidius species which attack nymphs and adults of cockroaches. In this genus the eggs are thought to be laid indiscriminately in crevices, etc., and the triungulinids attach themselves directly to passing hosts and enter the body to develop, thus eliminating the requirement of a carrier.
Triungulinids of all species are equipped with a caudal sucker and 1-2 pairs of cerci of varying length which they use to assume an erect position, with the legs entirely free, while waiting to attach to passing insects, etc. They are thought to have the jumping habit which is common to larvae of this kind.
The fee-living phase of larval life may extend over a considerable length of time, during which food does not seem to be required. However, Pierce (1904) believed that the triungulinids of Ripiphorus solidaginis fed on the plant tissues or sap of Solidago soon after hatching. He based this conclusion on (1) that they are of considerably greater size than the egg, and (2) that they are found only on Solidago, which is not frequented by host bees. It was assumed that this plant was utilized in preference to others, in order to fulfill these food requirements. A transitory plant feeding habit such as this is not in accord with the habits of larvae of this type, and the evidence presented does not definitely establish its occurrence. The increase in size may possibly have been the result of imbibing moisture from the leaf surface (Clausen 1940).
With exception of Ripidius pectinicornis and Ripidius spp. which pass the entire larval feeding period within the cockroach host, all known species develop externally, having an internal phase only in the 1st stage. Sometimes this internal period is short, but in M. flabellatum, entry into the Odynerus larva occurs during late summer, and the parasitoid larva does not emerge for external feeding until the following June. The developmental cycle and larval habits are comparable to those of certain Perilampidae, in particular species with hyperparasitic habits. Usually the host larva is not killed until it has completed feeding and it prepared to pupate. The cells containing parasitized Vespa larvae and those of other host groups of similar habit as well are thus closed in the normal way. In the case of Scoliidae and Tiphiidae, the cocoons are spun before death (Clausen 1940/62).
Transition from internal to external feeding has been observed in Macrosiagon flabellatum and Metoecus paradoxus (Grandi 1937). In the former species, the greatly distended triungulinid, which is several hundred times as large by volume as when newly hatched (see Clausen, 1940 for diagrams), emerges through a puncture in the 3rd thoracic segment of the host, immediately casts it exuviae, which remains in the puncture, and then assumes the feeding position in which it is found as a collar around the 1st or 2nd thoracic segment (see Clausen, 1940 for diagram). The triungulinid increases in length from 0.5 mm. at hatching to 2.5 mm. just prior to the first molt. The host larva is eventually consumed.
Most species of Rhipiphoridae seem to have only one generation per year, which is closely correlated with the cycle of the host. Ripiphorus solidaginis overwinters in the adult stage and lays eggs early in springtime, with the adult stage attained again in August (Pierce 1904). However, Metoecus paradoxus lays its eggs in late autumn, and the fully developed embryo persists in the egg until springtime. Macrosaigon flabellatum lays its eggs in late summer, and overwinters as 1st instar larvae within the body of Odynerus larvae. M. pusillum is thought to have the same hibernation habit, for adults emerge from Tiphia cocoons during July. Barber (1939) discussing observations of J. C. Bridwell on Ripiphorus sp., parasitic on Augochlora pura Say, mentioned that the triungulinids are found attached to the hairs of hibernating inseminated female hosts. They overwinter in this way, on the hibernating female bee, and transfer to her brood cells when these are formed in spring. R. solidaginis is believed to have 2 generations annually; Ripidius pectinicornis, developing in cockroaches in the tropics, probably has a short cycle, with several generations each year (Clausen 1940/62).
In M. flabellatum and M. cucullatum, the incubation period i 17 and 7.5 days, respectively. Larval feeding of Metoecus paradoxus covers only 12-14 days.
For detailed descriptions of immature stages of Rhipiphoridae, please see Clausen (1940/62).
Linsley, E. G. & J. W. MacSwain. 1951. Bull. Calif. Ins. Surv. 1: 79-88.
Linsley, E. G., J. W. MacSwain & R. F. Smith. 1952. Univ. Calif. Publ. Ent. 9: 291-314.
Selander, R. B. 1957. Ann. Ent. Soc. Amer. 50: 88-103.