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STREPSIPTERA  --  <Images> & <Juveniles>  [Latest Classification]



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This order of insects has a few hundred species that display a prominent sexual dimorphism in adults.  The males have branched antennae, large eyes, paddle-shaped mesothoracic balancers, and large metathoracic wings.  Females are apterous and larviform.  In all families except the primitive Mengenillidae, adult females that have been described lack legs, are not able to move in an orderly way, and never leave the body of the host.  First instar larvae, called triungulinids, are of the planidium type and bear a close resemblance to those of Ripiphoridae and some Meloidae.  The order is considered by some taxonomists to be closely allied to Ripiphoridae, and the free-living females of Mengenilla and Eoxenos have a resemblance to the aberrant genus Ripidius.  Larval development is completed within the body of the host in a way similar to that of Ripidius in cockroaches, while the Ripiphoridae attacking hymenopterous larvae are external except for a time during the first instar (Westwood 1836, 1939; Clausen 1940).  Because of their small size, great activity and short life, adult males are difficult to collect.  Many species ore thus described on the basis of one sex only.  The definite association of adults of the two sexes of a species is possible only by rearing, preferably from the same host individual or from those of the same colony.  The members of the order have come to be known under the common name of stylops from the original generic name of the entire group.  Clausen (1940) noted that a parasitized host is said to be stylopized and parasitization by a strepsipteran is referred to as stylopization.


Hosts of Strepsiptera


Strepsiptera parasitize four orders of insects: Orthoptera, Hemiptera, Homoptera and Hymenoptera, with the latter including the largest number of families.  Records of attack on Orthoptera and Hemiptera are few, while both the Cicadellidae and Fulgoridae frequently are hosts.  However, the Hymenoptera, particularly families Vespidae, Eumenidae and Andrenidae, are most frequent hosts.  Extended early host lists were presented by Pierce (1909, 1918), Robertson (1910), Salt (1927b), Salt & Bequaert (1929) and Ulrich (1933).  Generally, the hosts of the different families, based on the classification and records given by Pierce (1909, 1911, 1918) are as follows:

















Orthoptera: Locustidae

Hemiptera: Pentatomidae

Hymenoptera: Formicidae

Hymenoptera: various families

Hymenoptera: Hylaeidae

Hymenoptera: various families

Homoptera: Cicadellidae

Homoptera:  Cicadellidae, Fulgoridae

Homoptera: Cicadellidae, Fulgoridae


Pierce (1909, 1918) believed that each strepsipteran was confined to a single host species, and each tribe to a family, although recent studies show this to not always be the case.  Clausen (1940) believed that host specificity was not usual in this order.  For example, Bohart (1936) found that many species of Stylops are parasitic in two or more similar species of Andrena.


Generally the order is regarded as beneficial primarily because of attack on Cicadellidae and Fulgoridae.  Muir (1906) found more than 70% of Perkinsiella vitiensis Kirk. on sugarcane in Fiji were parasitized by Elenchoides perkinsi Pierce (= Elenchus tenuicornis Muir), and 30% of Idiocerus atkinsoni Leth. in India contained Pyrilloxenos compactus Pierce (Subramaniam 1922).  Schrader (1924) in studies of Xenos (Acroschismus) wheeleri Pierce as a parasitoid of Polistes, found that the number at first attacked in a nest is low, but that this finally builds up until it causes almost an extinction of the colony.  Piel (1933b) reported parasitization of 25% of Sphex nigellus Smith by Ophthalmochlus sp. and Theobald recorded 50-70% parasitization of Andrena by Stylops sp.  Honeybees are rarely attacked (Beljavsky 1926).  The highest and most consistent parasitization was recorded by Kirkpatrick (1937 a,b) for Corioxenos antestiae Blair on the pentatomid Antestia lineaticollis Stal, a pest of coffee in East Africa.  The portion attacked ranged from 12-84% during the year, with an average of >40%.


In Hawaii several species from various parts of the world were imported for the biological control of sugarcane leafhopper, Perkinsiella saccharicida Kirk, but none were established (Clausen 1940/1962).


Biology & Behavior


Early detailed accounts of the biology and behavior of Strepsiptera were by Nassanov (1892, 1893), Brues (1903, 1905), Perkins (1905c), Hofeneder (1910), Pierce (1909, 1911, 1918), Wheeler (1910), Smith & Hamm (1914), Schrader (1924), Schultze (1925), Ulrich (1927, 1930, 1933), Parker  & Smith (1933b, 1934) and Kirkpatrick (1937b).  Cooper (1938) gave an extended account of the internal anatomy of the immature stages and adults of C. antestiae Blair.


Reproduction.-- Strepsiptera are larviparous.  In view of the manner of development and the position of the adult females, it is improbable that any species lays eggs.  There is a uniformly high reproductive capacity, which is essential because of the hazards encountered by the triungulinids before they reach the host.  Newport (1845-1853) recorded rearing more than 7,000 triungulinids from a single female of Stylops aterrima Newp..  Pierce (1909, 1911, 1918) counted 2,252 in one female of S. swenki Pierce.  Xenos wheeleri, X. auriferi Pierce, and Eoxenos laboulbenei Pey, and Pyrilloxenos compactus have over 1,000 (Clausen 1940/1962).


Studies on oogenesis and embryology were made by Brues (1903, 1905), Hoffman (1913), Schrader (1924) and Noskiewicz & Poluszynski (1924, 1928).  The latter gave a brief account of supposed polyembryonic development in a species of Halictoxenos parasitic in Halictus simplex Per.


The brood of a single female emerges from the body in a short period of time, usually all in a single day, and several thousand triungulinids of E. laboulbenei emerged from one female in less than one minute (Parker & Smith 1933b, 1934).  Subramaniam (1922, 1932) reported that the triungulinids of P. compactus and Trydactylophagus mysorensis Subr. "shot out into space" through the opening on the ventral side of the cephalothorax.  Exiting from the body of the parent Eoxenos is through the genital opening on the venter of the 7th abdominal segment, while in forms having apodous females the triungulinids at first pass through the genital ducts into the brood chamber and from there to the outside through the genital opening between the head and thorax.  Many of the triungulinids of Corioxenos emerge through the oral aperture of the head (Kirkpatrick 1937b).  The membrane covering the genital opening is perforated at the time of mating, and this provides the normal point of exit in most species (Clausen 1940/1962).


Clausen (1940) pointed out that the use of the term "larviposition" in respect to reproduction in Strepsiptera is inexact, for it implies an act on the part of the female.  The triungulinid leaves the body of the parent female entirely of its own accord unaided by any stimulus of the female.


The simultaneous development of all eggs within the female's body requires the emergence of the entire brood of triungulinids.  Although this is true for the majority of species, a markedly different behavior occurs in Corioxenos antestiae.  Here females have been found to produce young over a period of more than 3 months.  One individual produced 3,720 in 95 days, with 134 remaining within the body at death, and another 2,220 in 49 days.  In the laboratory, a female produced an average of ca. 50 triungulinids per day.  Generally, females of this species continue to produce progeny until the host dies.  Despite the protracted reproductive period, only a single mating is necessary during one lifetime.  No instance was found of spermatozoa being present in the body more than 18 days after mating, and thus embryonic development may cover a minimum period from fertilization to emergence of 6 weeks and a known maximum of ca. 19 weeks (Kirkpatrick 1937b, Clausen 1940).


Triungulinid  Larvae.-- These larvae are thought to leave the maternal host in a relatively short time, particularly if the host is a homopteran.  The jumping habit of the triungulinids, which is common to the planidia of several orders, was first observed by Saunders 91853).  In E. laboulbenei jumping is in an unusual way, the abdomen is raised from the substrate and the caudal cerci are brought forward beneath the body, after which the body is suddenly straightened to project the insect into space.


Triungulinids of several species are believed to be positively phototactic, this in order to aid in attaining a position in which contact with a carrier or host may be made.  It has been shown in Corioxenos that little movement of the triungulinid occurs, and apparently no searching.  Thus, the triungulinid is dependent on direct contact with the host.  The alert or waiting position, which may be held for hours, is characteristic, the terminal cerci being bent forward under the abdomen and the anterior end of the body considerably raised and supported only by the hind legs.  This is in contrast to the habit of the planidia of other orders, which stand erect on the caudal sucker, with the body braced only by the caudal cerci (Clausen 1940).  In Corioxenos, jumping occurs as a result of the stimulus provided by a nearby moving object, and especially in response to certain colors.  Colors most attractive are black, red and orange (Kirkpatrick 1937a), black and orange being the predominant colors of the normal host, Antestia.  If the object to which the triungulinids become attached is not an immature Antestia, they leave at the first opportunity, while if it is an Antestia they make their way to the dorsum of the thorax and abdomen, between the coxae or between the head and thorax, where they cling immobile, with the mouth parts and legs, until the host molts.


Penetration of the host by this species usually occurs at the time of the molt, yet it has been shown that triungulinids will quickly enter adult hosts bearing mature Corioxenos of either sex.  Entry is effected in such cases at the point of extrusion of the older parasitoid, and it is probable that a second generation of parasitoids can develop to maturity in Antestia if that number of individuals is not so high as to cause premature death (Clausen 1940/1962).


Strepsiptera developing in Homoptera reach the host directly, without a carrier, for the triungulinids are released on the foliage visited by nymphs and adults of the host.  These insects are usually so abundant that the chance of a triungulinid reaching a host directly is great.  With hymenopterous hosts the aid of a carrier is advantageous, though possibly not essential.  The larva within the cell, rather than an active stage of the host, must be reached.  Bees and wasps that are subject to attack have two general feeding and brood-caring methods, which have a bearing on the chances of the triungulinids to reach the larvae.  First, the food is stored in the cell, the egg laid on it and the cell is then sealed, or secondly, the egg is first laid in the cell and the larva is fed periodically until maturity, after which the cell is closed.  In the first case, the period in which entry into the cell is possible is very short, probably not more then one day, and thus such species are most likely only infrequently attacked.  However, this omits the possibility that the triungulinids may be able to penetrate the cell after it is closed.  If entry is on the day the egg is laid, the triungulinids have to wait for some time until the egg hatches and the larva is sufficiently developed to withstand attack.  Because of the small size of the triungulinid with respect to that of the host, its penetration into the 1st instar larva seems to be without serious injury to it.  In the second case, the cell remains open throughout the larval feeding period, and the larva is tended one or more times daily by adults, so that the opportunities for parasitization are greater (Clausen 1940/1962).


Although undoubtedly some triungulinids transfer directly from the maternal host in the nest, this is not thought to be common, for parasitized females do not engage in nest building, at least not at the time that the parasitoid brood escapes.  Pierce (1909, 1911, 1918) considered that transfer was effected mainly on blossoms and at other places frequented by bees and wasps.  Westwood (1839) first suggested that transfer took place on blossoms.  With hosts of social habit, there is an excellent opportunity for transfer by direct contact in or near the nests, in contrast to the improbability of such occurring among solitary species (Clausen 1940/1962).


Penetration of the host is always in the first larval stage, and advanced triungulinids of several species have been dissected from hymenopterous larvae.  They seem to lie free in the body cavity.  Saunders (1853) placed triungulinids with almost fully grown Polistes larvae and observed that penetration occurred soon thereafter.  Dissections a week later showed the larvae just undergoing the first molt.  Kirkpatrick often observed the entry of Corioxenos into its pentatomid host, this usually taking place on the pronotum at the time of molting and is effected within 20-30 min. of the beginning of the molt.  Although entry may occur at any host molt, it is most successful at the first three.  Surprisingly, the newly hatched nymph is never attacked even though in physical condition it is much the same as at the time of following molts.


Certain deformed Antestia are apparently immunity to parasitization by Corioxenos.  A portion of the older nymphs are found with malformed antennae, and rearing experiments where these were subjected to attack by triungulinids of the parasitoid showed that few were able to enter the body (Kirkpatrick 1937a,b).  Only 18 of 54 triungulinids entered the bodies of these individuals, and only 8 of 49 hosts were parasitized.  This is in contrast to results secured with normal hosts, among which over 90% were successfully parasitized when exposed in the same manner.  A deficiency in the molting fluid, resulting in a difficult molt, may be responsible for the malformed antennae.  If this fluid provides the stimulus for penetration by the parasitoid, as is probable, then such a deficiency would reduce the stimulus to the point where triungulinids would not react to it (Clausen 1940/1962).


Development of Larvae.-- In C. antestiae, if penetration of the host body is in the thoracic region, migration of the parasitoid larva to the abdomen usually occurs late in the 2nd stage.  From then on, there is little movement until that incident to the extrusion of the cephalothorax.  In hymenopterous hosts, the larvae also migrate to the abdomen during the early period of development if entry has occurred in the anterior portion of the body, and probably while the host is still in the larval stage.  Early researchers found only 3 larval instars, but later studies found that there are probably 7.  Schrader found that males have only 6 larval instars.  In molting, a transverse break occurs across the anterior end of the body, and the larva wiggles out of the exuviae.  Feeding is mainly by diffusion through the integument, although in Corioxenos movements of the rudimentary mouth parts were observed, but it is not certain whether this represented direct feeding.  Smith & Hamm (1914) examined the alimentary system of S. melittae Kirby and found it to be degenerate and functionless.


The larvae may be determined to sex at a comparatively early developmental stage, especially in the third instar.  Extrusion of the cephalothorax from the host body occurs in both sexes during the 7th stage.  Species attacking Hymenoptera extrude the cephalothorax during the pupal stage of the host or after it has transformed to the adult stage but before the integument has hardened.  In Pentatomidae hosts this occurs only after the adult stage is attained, but in the Homoptera it may occur while the host is still a nymph.  In most cases where the orientation of the cephalothorax has been observed, e.g., in Stylops, Xenos and Elenchoides, it was found that the side of the cephalothorax in contact with the host body is the dorsum, the venter being the outer, exposed side.  However, Saunders (1853) found the reverse to be true in Stylops sp. and Hylecthrus sp., and Kirkpatrick (1937ab) showed that Corioxenos also normally lies with the dorsum as the exposed side.  In each species the orientation of the two sexes is identical.  This difference in orientation among the different species is most likely correlated with the position of the larva in the body of the host.  In Corioxenos, the mature larva lies with its venter against the body wall of the host, and in effecting emergence the cephalothorax is curved ventrally so that when the process is complete the body is U-shaped, the dorsum being outward with respect to the host body.  In other species for the inverted position to be taken the larvae must assume the same internal position with respect to the host body wall, but they lie with the cephalothorax and abdomen in the same plane and effect extrusion by a series of forward thrusts.  Female Dacyrotocara undata Pierce is found in the latter position, the body is straight and cylindrical, and the 1st abdominal segment extends forward beyond the apex of the cephalothorax (Clausen 1940/1962).


Among strepsipterans developing in Hymenoptera, the cephalothorax of the parasitoid is extruded through the intersegmental membrane of the abdomen at any point on the periphery, although a greater number are found dorsally than ventrally or laterally.  The natural curvature of the abdomen of the bee or wasp, with the dorsum decidedly convex, allows for more ready extrusion in that area.  The cephalothorax of the parasitoid is always directed toward the rear, a position induced by the overlapping of the host's abdominal segments.  Those of the males are usually found overlying the 3rd segment, while those of the females are generally on the 5th segment.  However, they may be found on any of the other segments with the exception of those comprising the genital system.  The point of extrusion of the cephalothorax of the species attacking Hemiptera and Homoptera seems to present a greater consistency within each species than is the case with those infesting Hymenoptera.  The cephalothorax of both sexes of Corioxenos is extruded dorsally through the intersegmental membranae between the 3rd and 4th abdominal tergites.  That of the male if found on the median line, or close to it, while that of the female is most often found near the lateral margin (see Clausen 1940 for diagrams).  Those on Fulgoridae are regularly upon the pleural region only, while the species attacking Cicadellidae are found in either the dorsal or the ventral position (Perkins 1905c).  Perkins also found that those of the males of Halictophagus are found either dorsally or ventrally, while those of the females are in the pleural region only.  Pierce (1909, 1911, 1918) found males of Stenocranophilus quadratus Pierce to be located on the 5th segment of Stenocranus and invariably dorsal in position, while the females are on the 3rd segment and usually lateral.  In Tettigoxenos orientalis E. & H., most are ventral and located on the 4th or 5th segments of adult hosts and on the 3rd segment of nymphs.  Generally, it seems that in hymenopterous hosts the point of extrusion of the females is farther back than that of the males, while in Homoptera the opposite holds.  Pierce (1909, 1911, 1918) considered this condition due to the fact that female parasitoids are larger than males in the first case and smaller in the second.


Pupation.-- In species with apodous females, the individuals of this sex transform to the adult stage directly from the 7th instar larva without an intervening pupal stage.  However, the male undergoes a complete metamorphosis where the pupa develops within the 7th larval exuviae in situ in the host.  It may not completely fill this "puparium," and in some cases the body is withdrawn from the cephalothorax so that it lies entirely within the host body.  The adult male may remain within the puparium for several days after casting the pupal skin and it then effects emergence by pushing off the operculum or head portion of the cephalothorax.  This occurs during early morning hours, especially in bright sunlight.  The host dies prior to this time, but this does not effect the parasitoid, for adult males have been reared from hosts that had died nearly a week previously (Clausen 1940/1962).


Parker & Smith (1933b, 1934) found in Eoxenos and Silvestri (1933) in Mengenilla quaeseta Silv., which have free-living adults of both sexes, that there occurs what was considered to be a pupal stage of the female as well as of the male.  The pupa of Eoxenos is found within the last larval exuviae, and at emergence of the adult the pupal skin is left almost intact.  Sometimes the female remains permanently within the larval exuviae, in which case the pupal skin is torn into fragments and pushed back into the posterior end.  The larval shell remains intact except that the head is detached, and the opening thus made serves as a means of exiting for the triungulinids.  Males also pupate within the last larval skin, and at emergence of the adult the head and first thoracic segment are broken off.  The pupal shell is usually found within the larval exuviae (Clausen 1940/1962).


Mating.-- Because males are so scarce in Strepsiptera, it has been thought that reproduction is unisexual in most species.  The activities of the males during their brief adult life (usually less than 1 day) is of interest, because they are greatly attracted to the host insects, and the finding of a host is followed by a search for the female parasitoid.  Attempts at mating were noted in several species, and Pierce (1909, 1911, 1918) noted several instances where this was seemingly successful.  Smith & Hamm (1914) concluded that fertilization cannot take place, however.  Their evidence supports the claim for unisexual reproduction based on (1) that there is no opening of apparatus in the female adapted for conveying the spermatozoa to the eggs; and (2) the eggs remain throughout their development encased in the follicular epithelium of the ovary, so that access to them by spermatozoa which may have entered the body cavity is difficult; (3) parthenogenesis must occur as a normal rule in parasitoids of Halictus where males are extremely rare; (4) the known stages in the polar body formation of Stylops are inconsistent with the view that fertilization by a spermatozoon has been effected; and (5) actual copulation by the male has never been adequately observed. 


Copulating pairs of S. aterrima were observed by Perkins (1918b).  A close examination after killing pairs revealed that the aedeagus was inserted into the brood chamber.  Hofeneder (1923) found Stylops in copula on a female of Andrena flavipes Perez. for 2 1/2 min.  Such observations are nevertheless not conclusive, and it was not until Schrader studied X. wheeleri that the apparent mating, as observed by others, was proved to result in egg fertilization.  mating was observed in 10 cases and over a period of 20-50 sec.  In each case females were of a certain age, or stage of development, for the cephalothorax had been exserted only 4-5 days.  This partially explains the difficulties experienced by other earlier investigators, who may have been using older females that had already been mated.  The spermatozoa are released through the genital opening into the brood canal and enter the body through the four trumpet-shaped ducts on the abdominal venter.  They then disperse throughout the body, penetrate the egg membrane, and effect fertilization.  A cytological examination of virgin females showed that the eggs develop as far as the metaphase of the first maturation division, which is reached 4-5 days after extrusion of the cephalothorax.  This condition is maintained for 10-14 days, after which degeneration occurs.  Thus, it was demonstrated that not only does fertilization take place but that parthenogenetic reproduction cannot occur in this species.  In species attacking Halictus, mating may take place in the autumn and only the gravid females persist until the following spring.  Collections at this time would suggest that the entire population is exclusively of the female sex.


Extended observations were made by Kirkpatrick (1937ab) on the mating behavior of Corioxenos.  The male is strongly attracted to the host insect, even if it is unstylopized, and is not at all attracted to females of its own species after they are removed form the host.  It apparently is attracted first to the host by sight and then to the female by touch.  To mate it is first necessary for the male to penetrate the membrane covering the exposed lateral genital opening.  In Eoxenos, Parker & Smith noted that the aedeagus of the male apparently penetrated the body all along the median ventral line rather than the genital opening on the 7th segment.  Females remain capable of being fertilized over variable periods.  In Corioxenos mating may occur successfully 4-7 days after extrusion of the cephalothorax, but in one case a female kept in confinement was successfully mated 119 days after extrusion.


Life Cycle


The life cycle is exceeding variable, especially in species which attack the host in its immature stages but do not themselves reach maturity until the host attains adulthood.  This applies in particular to those attacking Polistes and other wasps of similar habit which are subject to attack throughout the larval period and to species having pentatomid hosts which are attacked in any nymphal instar but mature only in adults.


The pupal stage of the male of X. vesparum Rossi takes 28-32 days (Nassanov 1892, 1893), while Saunders (1853) found that only 8 days elapse from the extrusion of the cephalothorax to emergence of the male of Hylecthrus rubi Saund.  In E. laboulbenei, the pupal stage is finished in 12 days.  Males of C. antestiae complete the cycle from entry of the host to adult emergence in a minimum of 50 days, of which the pupal stage represents ca. 12 days, while the female attains the adult stage in 34-36 days, of which 28-30 days represents the time from entry of the triungulinid to the extrusion of the cephalothorax (Clausen 1940/1962).


Mated females of those species attacking bees and wasps, usually hibernate.  However, Hylecthrus is believed to overwinter in the early larval stages within the Prosopis larvae, this difference being made necessary by the host's hibernation, which occurs in the larval rather than in the adult stage.  In Eoxenos there seems to be generally only one generation per year.  Adult gravid females are found during autumn and conceal themselves under stones and other objects during winter.  Embryos do not develop in their bodies until the following spring, and the triungulinids leave the parent body early in July.


Triungulinids are adapted to a relatively long period of free life, which is required to allow them to reach their hosts. Corioxenos survives for a max. of 19 days in the absence of hosts and 34 days if external attachment to the host occurs.  Most of this longer period is passed in an entirely inactive condition on the host body while awaiting the molt, and this seemingly accounts for the greater longevity.  The first molt occurs 3-6 days after entering the host, and a corresponding internal period of one week or less has been found in other species (Clausen 1940/1962).  The mature larval stage is attained in hymenopterous hosts only when the host is nearing the time for transfer to adulthood.  Therefore, it is probable that the histolytic processes associated with pupation provide the stimulus for completion of growth (Clausen 1940).  Developmental delays until this host condition is attained is known to be obligatory in other parasitic insects.


The cephalothorax of X. vesparum is extruded from the host abdomen 5-8 days after the Polistes adults emerge from their cells.  In Ophthalmochlus sp. and Stylops spp., adult males usually emerge the day hosts first leave their nests.  This does not, however, aid much in determining the time of extrusion, for the hosts may have been in the adult stage in their nests for a considerable period.  The cephalothorax of X. pallidus Brues may not be extruded until several days after the wasp has left its pupal cell (Pierce 1909, 1911, 1918.  Extrusion in Corioxenos occurs not more than 36 hrs. after the final transformation of Antestia.  Saunders (1853) recorded the activities of male larvae of Hylecthrus rubi underneath the abdominal integument of fully developed Prosopis pupae, and extrusion took place almost immediately after the host had assumed its adult form.  Clausen (1940) believed it logical for extrusion to take place very soon after the pupal skin was cast, for the integument is more easily penetrated at this time than when it has hardened.


Sex Ratios of Host & Parasitoid


Pierce (1909) provided an extended discussion relative to sex ratios of several Hymenoptera that were stylopized, the % of each sex stylopized and the sex ratio of the parasitoids.  From 1,553 Polistes annularis L. collected during October, only 15.6% were female.  Of this number, 17.2% were parasitized, but the % was 19.7 among males and only 2.8 among females.  Sex ratio of Xenos pallidus favored males by about 1:2.  In Andrena crawfordi Vier, 43.3% of 266 bees collected were female, and of these 53.1% were parasitized compared to only 20.6% of the males.  The sex ratio of the parasitoid favored females slightly.  September rearings of Xenos wheeleri from Polistes gave a majority of more than 2:1 favoring males, while May and June collections yielded female parasitoids only (Schrader 1924).  Earlier records of the same species taken during late August gave males in excess by 4:1 (Wheeler 1910).  Apparently not only does the sex ratio of the host vary widely among seasons, but the ratios of the parasitoids differ markedly during the year and in different hosts.  Male Polistes do not survive winter well, and few of those of either sex from which parasitoids have emerged would survive.  Stenocranophilus quadratus males predominated in a ratio of 1.7:1, and parasitization of female Stenocranus was a bit higher than of males (Clausen 1940).  Females of Elenchinus japonicus, parasitic in Delphacodes, predominated 2.5:1, while in Tettigoxenos orientalis, parasitizing Parabolocratus, the sexes were about equally abundant (Esaki & Hashimoto 1931).


The greatest number of individuals that are able to develop in a single host is about 31 larvae of Xenos reported by Brues (1903, 1905) in one larva of Polistes.  The highest number to have reached the exserted stage was 15, these being male pupae found in a male of P. annularis.  However, regularly the number in each host ranges between 1 and 5.  Esaki & Hashimoto (1931) found 1 to 3 in each individual homopterous host, and Subramaniam (1922, 1932) studying Pyrilloxenos compactus, found 1-3 in Idiocerus sp.  A maximum of 7 was recorded by Misra (1917) for this species in nymphs of Pyrilla aberrans Kirby and 12 in the adults.


Brues (1903, 1905) concluded, as a result of studies on several species of Xenos, that there is a tendency for all parasitoids developing in a single host to be of the same sex.  In X. pallidus parasitizing Polistes annularis, of 100 male hosts containing more than one parasitoid and averaging 2.99, 26 lots were males only, 11 were females only  and 63 were of both sexes (Pierce 1909).  This disparity between the number of pure colonies of the two sexes may be explained by the predominance of males in the sex ratio of the parasitoid.  Wheeler's studies on X. wheeleri showed an average of 2.4 parasitoids developing in Polistes sp., the maximum being 11.  Wheeler believed that in all instances where the number was in excess of 4, all of them were of the same sex.  This is suggestive of a differential death rate among larvae of the both sexes, the males dominating because of their smaller size and lower food requirements (Clausen 1940).  Studies on the same species by Schrader (1924) supports Wheeler's contention regarding the differential death rate among larvae, and dissections of larvae, pupae, and adults of Polistes showed that the larvae of the two sexes of Xenos were present in about equal numbers.  Figures for X. vesparum in P. gallicus L. presented by Vandel (1932) show that 18 out of 31 wasps containing more than 1 parasitoid bore only one sex, of which 16 were male broods.  In 13 mixed broods, 9 had an excess of males, the numbers were equal in 3, and only one contained an excess of females.  However, 21 out of the 25 solitary individuals were males.  Limited observations on Ophthalmochlus sp. in Sphex by Piel (1933b) showed that males were the minority in mixed broods.


Detailed information on sex ratio in studies of Corioxenos, of which 1-5 develop per host, show field collected material numbering 1,000 hosts had 44.7% with only a single parasitoid, among which females predominated in a ratio of 1.5:1.  Clausen (1940) considered this to be the normal sex ratio of the parasitoid.  Where hosts contained two parasitoids, the female preponderance was higher, the ratio being 2.4:1.  With a greater number in each host the sexes were present in about equal numbers.  Such records show no tendency for the brood in a host to be of only one sex.  Of 249 Antestia containing 3 or more extruded Corioxenos, 16 of the broods were 100% male, 9 were 100% female and 224 were mixed.


Stylopization Effects on the Host


Many noticeable changes are brought about by strepsipteran parasitization, among which are included various internal changes due to the feeding of larvae and often a pronounced influence on the primary and secondary sexual characteristics of the adult host.  Most seem to result from nutritional balance upsets of the host through feeding by the parasitoid.  Early studies on the effect of stylopization were by Perez (1886), Pierce (1909), (Wheeler (1910), Smith & Hamm (1914), Perkins (1918a), and Salt (1927a, 1931a).


Stylopization effects various bee and wasp hosts by either accelerating or retarding development to the adult stage.  Westwood (1840) noted that parasitized Andrena adults appear ca. 1 month earlier in springtime than do healthy adults, and Saunders (1853) observed earlier emergence in the case of Prosopis.  Pierce (1909) recorded that the first adults of A. crawfordi to be found in the field were parasitized by 59-79% while those taken a few days later showed a much lower percentage.  Only adult Xenos were found in Polistes wasps of the autumn broods, which were considered to belong to earlier broods that had been retarded in their larval and pupal development (Wheeler 1910).  Such results indicate that there is only a single parasitoid generation annually, though Schrader (1924) demonstrated that there are really two.  The latter found that the host larvae and pupae which remain for a longer period in the nest are the most heavily parasitized and that their random distribution in the nest indicated an actual retardation.  It thus appears that there is an actual acceleration in development among the bees in contrast to a retardation among the wasps (Clausen 1940).  Studies by Salt (1927) show that because of the presence of the parasitoid in the body of the host larva and its continuous abstraction of food materials from the blood, the host is kept in a continuous state of hunger.  Thus, stylopized bee larvae feed more rapidly and consume the food supply in the cell earlier than unparasitized individuals.  They then pupate and emerge at an earlier date than do the unparasitized individuals.  However, in Polistes the food supply is not of limited quantity, and the larvae are thought to be provided with food as long as they need it.  Parasitized individuals have the same persistent hunger as is found among the bees, but are able to continue to feed enough to compensate for the materials taken by the parasitoid.  This leads to the larval stage being prolonged, and adult emergence is correspondingly delayed.  In Fulgoridae, which are able to secure whatever added food is required, the parasitized individuals likewise show a retardation in development.


Parasitism by male Strepsiptera seems to result in greater injury and causes more profound changes in the host than does that by females.  Many of these changes occur while the host is in the mature larval and pupal stages; and thus the abstraction of large quantities of food materials from the body for the production of eggs by the parasitoid female is not involved (Clausen 1940).  When parasitized adults emerge, the male parasitoid is a more highly organized body than the female and has been more of a drain on the host's vitality.  However, this explanation is not in accord with that given for the preponderance of males in heavily parasitized hosts, which is attributed to a differential death rate.


A general marked loss of vitality occurs among parasitized host adults, ranging from merely reduced activity to an almost complete cessation of normal functions.  Parasitized male bees and wasps have been found to mate normally, which also is occasionally the case with females.  The females among these hosts seldom attempt to carry pollen or to build and furnish nests.  Some species of Andrena when parasitized never carry pollen, while others and Chloralictus also, may do this frequently or occasionally.


The host often sustains severe injury to its internal organs, which is caused by the removal of food from the blood rather than by direct feeding and tissue laceration.  Atrophy of the ovaries is so far advanced so that mature eggs cannot be produced (Kirby 1828, Clausen 1940).  The secretory and poison glands of Andrena are reduced, the tracheal system is reduced and with the vesicles few and imperfect, the nerve ganglia of the abdomen are atrophied, and the intestine is empty and dislocated (Newport 1845-1853).  The male reproductive system seems unimpaired, though Perez (1886) mentioned that these organs are atrophied only on the side bearing the parasitoid and Salt (1927) noticed a slight size reduction.  However, in Antestia spp. parasitized by Corioxenos antestiae, most males proved to be sterile as determined by breeding experiments with unparasitized females, even though apparently normal spermatozoa were present and mating apparently occurred normally.  Functional sterility may be more general than is obvious, however.  Among females that are parasitized at the time of the final molt and that contain only a single parasitoid, a small number (ca. 1%) contain 5-15 apparently mature eggs each.  However, despite this no female ever oviposits.


Hosts do not always die as a result of Strepsiptera parasitism.  Death of parasitized leafhoppers usually follows soon after emergence of the male parasitoid, this being due to the empty puparium providing a large opening to the body cavity through which desiccation of the viscera occurs and which also allows disease organisms to enter (Perkins 1918a,b; Misra 1917).  Female leafhoppers with male parasitoids merely become sluggish at the time of triungulinid exiting and may live for a considerable time.  Perkins believed that the presence of a fungus disease was essential to maximum effectiveness of these leafhopper parasitoids.  Because of the injury to the reproductive system of the female hosts, this seems to be inconsequential, for reproduction has ceased and these individuals cannot contribute further to population increase.  In Antestia spp., adults containing either male or female Corioxenos live almost as long as healthy individuals.  Adult Polistes bearing empty male puparia may live for a considerable time and may even be able to hibernate successfully (Clausen 1940).


The first extended study on the external changes brought about from Strepsiptera parasitism was by Perez (1886) on a large number of Andrena spp., with other researchers following (see Clausen 1940).  Apart from mechanical distortion and occasional direct injury induced by the extrusion of the parasitoid's cephalothorax, the general external changes are evident in the relative size of the body parts: integument, puncturation, pilosity and wing venation.  The principal secondary sexual characters that are affected, with the direction of the changes that take place include, antennae (color and proportionate length of segments); pubescence (color: pollen-collecting apparatus (size of tarsi and number of bristles); clypeus (color and shape); mandibles (color); and genitalia (size, shape and proportions of parts) (Clausen 1940/62).


In Polistes no great modification in secondary sexual characters results from parasitism, but the most marked changes are found in Andrena.  Perez concluded that the modifications in the latter constitute actual inversions of development, "The stylopized Andrena, male or female, is not merely a diminished male or female; it is a female which takes on male attributes; a male that takes on the characters of the female."  Other researchers later corroborated this.  Such changes are so pronounced that in many instances parasitized individuals have been described as new species (Clausen 1940).


There is no uniformity in the effect of stylopization on the external characters of hymenopterous hosts.  Even among individuals of a single species, the differences may be very large, probably due to a variation in the stage of development at which parasitization occurs.  In some cases particular effects are revealed in only one sex, and these differences are correspondingly greater between species and genera.  Such various changes occur in a definite order (Salt 1927).  Esaki & Hashimoto (1931) found among Fulgoridae a marked "neutralization" of the genitalia of both sexes of Delphacodes fucifera Horv. resulting from parasitism by Elenchinus.  However, no such effect was found in the cicadellid, Parabalocratus prasinus Mats., parasitized by Tettigoxenos.  No changes in secondary sexual characters were observed in the pentatomid Antestia sp. when attacked by Corioxenos, however (Kirkpatrick 1937ab).


For detailed descriptions of immature stages of Strepsiptera, please see Clausen (1940).


Further Description


          The Strepsiptera (also known as twisted-winged parasites) have ten families making up about 607 species. The early stage larvae and the short-lived adult males are free-living but most of their life is spent as endoparasitoids in other insects such as wasps, bees, leafhoppers, cockroaches and silverfish.


          The males have wings, legs, eyes, and antennae, and appear as flies, although they usually have no useful mouthparts. Many of their mouthparts are modified into sensory structures. Adult males are very short-lived (usually less than 4.5 hours) and do not feed. Females, in all families except the Mengenillidae, do not leave their hosts and are neotenic in form, lacking wings and legs. Virgin females release a pheromone which the males search for. In the Stylopidia the female has its anterior region extruding out of the host body and the male mates by rupturing the female's brood canal opening which lies between the head and prothorax. Sperm passes through the opening in a process termed hypodermic insemination. Each female produces many thousands of triungulin larvae that emerge from the brood opening on the head, which protrudes outside the host body. These larvae have legs without a trochanter, the leg segment that forms the articulation between the basal coxa and the femur) and actively search out new hosts. Their hosts include members belonging to the orders Zygentoma, Orthoptera, Blattodea, Mantodea, Heteroptera, Hymenoptera, and Diptera. In the Strepsipteran family Myrmecolacidae, the males parasitize ants while the females parasitize Orthoptera.


          The eggs hatch inside the female and the planidium larvae can move around freely within the female's haemocoel, which is unique to these animals. The female has a brood canal that communicates with the outside world and it is through this that the larvae escape. The larvae are very active, as they only have a limited amount of time to find a host before they exhaust their food reserves. These first-instar larvae have stemmata (simple, single-lens eyes) and once they latch onto a host they enter it by secreting enzymes that soften the cuticle, usually in the abdominal region of the host. Some species have been reported to enter the eggs of hosts. Larvae of Stichotrema dallatorreanurn Hofeneder from Papua New Guinea were found to enter their orthopteran host's tarsus (foot). Once inside the host, they undergo hypermetamorphosis and become a less mobile legless larval form. They induce the host to produce a bag-like structure inside which they feed and grow. This structure, made from host tissue, protects them from the immune defences of the host. Larvae go through four more instars and in each moult there is separation of the older cuticle but no discarding ("apolysis without ecdysis") leading to multiple layers being formed around the larvae. Male larvae produce pupae after the last moult, but females directly become neotenous adults. The colour and shape of the host's abdomen may be changed and the host usually becomes sterile. The parasites then undergo holometabolous metamorphosis to become adults. Adult males emerge out of the host body while females stay inside. Females may occupy up to 90% of the abdominal volume of their hosts.


          Adult male Strepsiptera have eyes that are different from those of any other insect.  They resemble the schizochroal eyes found in the trilobite group known as Phacopida. Instead of a compound eye consisting of hundreds to thousands of ommatidia, each with a single lens and capable of producing a picture element (pixel), the strepsipteran eyes consist of only a few dozen ommatidia separated by cuticle and/or setae, giving the eye a blackberry-like appearance.


          Many females may be seen within a stylopized host. Males are rare. They may sometimes be captured at light traps or may be attracted using cages containing virgin females.


Strepsiptera may alter the behaviour of their hosts. Myrmecolacids may cause their ant hosts to climb up the tips of grass leaves, possibly to increase the spread of female pheromones to increase the chances of being located by males.


          The order, named by William Kirby in 1813, is named for the hind wings (strepsi=twisted + ptera=wing), which are borne at a twisted angle when at rest. The forewings are halteres (and initially were thought to be dried and twisted wings).


          Strepsiptera are an difficult for taxonomists. Originally it was believed they were the sister group to the beetle families Meloidae and Ripiphoridae, which have similar parasitic development and forewing reduction; early molecular research suggested their inclusion as a sister group to the flies, in a clade called the halteria, which have one pair of the wings modified into halteres, and failed to support their relationship to the beetles. More recent molecular studies, however, suggest that they are outside the clade Mecopterida (containing the Diptera and Lepidoptera), yet there is no definite evidence for affinity with any other extant group. Study of their evolutionary position has been a problem due to difficulties in phylogenetic analysis arising from long branch attraction. The most basal strepsipteran is the fossil Protoxenos janzeni discovered in Baltic amber, while the most basal living strepsipteran is Bahiaxenos relictus, the sole member of the family Bahiaxenidae. The earliest known strepsipteran fossil is that of Cretostylops engeli discovered in middle Cretaceous amber from Myanmar.




          There are two major groups Stylopidia and Mengenillidia. The Mengenillidia include three extinct families (Cretostylopidae, Protoxenidae, and Mengeidae) plus two extant families (Bahiaxenidae and Mengenillidae; the latter is not monophyletic, however, they are considered more primitive and the known females (Mengenillidae only) are free living, with rudimentary legs and antennae. The females have a single genital opening. The males have strong mandibles, a distinct labrum, and more than 5 antennal segments.


          The other group, Stylopidia, includes seven families Corioxenidae, Halictophagidae, Callipharixenidae, Bohartillidae, Elenchidae, Myrmecolacidae, and Stylopidae. All Stylopidia have endoparasitic females having multiple genital openings


          Stylopidae have 4 segmented tarsi and antennae with 4-6 segments and the third segment has a lateral process. The family Stylopidae may be paraphyletic. The Elenchidae have 2-segmented tarsi and 4 segmented antennae with the third segment having a lateral process. The Halictophagidae have 3-segmented tarsi and 7-segmented antennae with lateral processes from the third and fourth segments. The Stylopidae mostly parasitize wasps and bees, the Elenchidae are known to parasitize Fulgoroidea while the Halictophagidae are found on leafhoppers, treehoppers as well as mole cricket hosts.



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