Immature Stages of Strepsiptera
Clausen (1940) discussed the immature stages of Strepsiptera as follows:
The Egg.‑‑The fully developed eggs in the body cavity of the female are minute and almost spherical in form and usually have the chorion thin and membranous. That of Eoxenos laboulbenei is 0.07 mm. in diameter and, in its early stages, is seen to have a band of irregular hexagonal cells extending around it. These are not surface markings, however, but represent the developing embryo. In some species, at least, there is an increase in size during incubation, and just prior to hatching the larva lies in a curled position, with the posterior portion of the abdomen sharply bent ventrally beneath the body.
The Larval Instars.‑‑The first‑instar larva of the Strepsiptera is generally referred to as a triungulinid because of its marked resemblance to the larvae of the Rhipiphoridae, which were originally designated by that term. It is, however, essentially a planidium, and various authors have called attention to the similarity of its adaptive characters and habits to those of the planidia of the Eucharidae, Perilampidae, Cyrtidae, etc.
Pierce and other authors have described the triungulinids of a considerable number of species, and the characters available appear to be sufficient to permit determinations of the instar to the family. Further study may reveal a basis for determination to lower groups.
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In general, the triungulinids are fusiform in outline with the head and body segments heavily sclerotized, ranging in color from amber to shining black, the segmental bands terminating in pleural plates lateroventrally, the abdomen bearing one or more pairs of cerci or stylets on the terminal segments and a caudal sucker. After feeding has taken place, the bands become widely separated because of the stretching of the thin intersegmental membranes and reveal the white color of the body contents. In size, the triungulinids are quite small, seldom exceeding 0.3 mm. in length, and many are only half that length. This is understandable in view of the large numbers produced by each female.
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The head in most species is somewhat semicircular in outline, often slightly longer than wide, and occasionally is almost quadrangular. The eyes are usually large and situated slightly beneath the lateral margin. They consist of a group of three to five lenses, variously arranged, which are underlain by masses of pigment. A few species apparently lack the lenses entirely. The antennae may be entirely absent, as in Eoxenos laboulbenei, or occasionally disk‑like or slender and two‑ or three‑jointed, as in Callipharixenos muiri Pierce and Stichotrema dallatorreanum Hof.
In many species, the mandibles are exceedingly minute and spine‑like or entirely lacking, but in the last two species named they are rather large, curved, and directed backward. E. laboulbenei has the axillae very long and spine‑like.
The thoracic segments are longer than the abdominal segments following and are strongly arched. There is considerable variation in the 4‑jointed legs of the various species, particularly in the tarsus. In Stylops californica Pierce (Fig. 218B), the tarsi are spine‑like and equal in length to the tibiae, whereas in C. muiri they are only half as long as the tibiae and terminate in three filaments or claws. In the majority of species, however, the tarsi of the first two pairs of legs are in the form of large disk‑like pulvilli, and those of the hind legs are slender and tapering. All three pairs of legs of Stichotrema dallatorreanum (Fig. 218C,D) are thought to have the pulvilli. The unguliform tarsus of the hind leg of Xenos bohlsi Hoff. is replaced by a ladle‑like pad. The tarsi of all the legs of Stylops swenki are described as being acuminate in form and nearly half the length of the tibiae.
The abdomen consists of 10 segments of gradually decreasing width, though Callipharixenos is thought to have only 7 segments. The first eight segments are short and of approximately equal length, whereas the terminal two are often considerably modified. In Stichotrema dallatorreanum, the ninth segment equals the preceding five in length, and the tenth is small and situated beneath the ninth. The last segment is frequently emarginate and bears two pronounced terminal tubercles from which the stylets arise. Only a single pair of caudal stylets is found in the majority of species, these being one‑half to two‑thirds the length of the body, though two pairs are mentioned for certain species of Xenos, Elenchoides, Corioxenos, Belonogastechthrus, and Mecynocera, and three pairs for Neocholax jacobsoni Meig. If two pairs are present, the anterior pair is shorter and is usually borne on the ninth segment. The three pairs on Neocholax are all borne on the penultimate segment, and in E. laboulbenei the two pairs are both on the terminal segment.
The integumentary armature is almost entirely lacking in a great many species but some, such as Stylops californica (Fig. 218B), bear numerous spines in a transverse ring on the posterior margins of the abdominal segments. A few minute setae are found on the dorsum of the head and thorax. In C. antestiae (Fig. 218A), the thoracic segments bear heavy plate‑like structures, with serrate posterior margins, ventrally between the legs.
The tracheal system has not been extensively studied in any species, and the triungulinid is thought to be without spiracles.
Nassanov as a simple apodous form, tapering toward both ends, with a thin integument and no fleshy protuberances or processes, describes the second‑instar larva of X. vesparum. Brues's description of that of S. melittae noted a median row of 12 fleshy protuberances, considered to be locomotors in function, on the venter of the body. In Corioxenos antestiae, small paired protuberances representing the legs are present on the thoracic segments.
In the third instar of Corioxenos, the male still retains traces of the legs, but these are lacking in the female. The latter now shows the first indications of median dorsal protuberances on the body segments. The fourth instar is quite similar, with the leg rudiments conspicuous in the male, and the dorsal prominences of the female are distinct. The latter are conspicuous on the fifth instar of both sexes. In apparently all species, the sexes can be differentiated in the third instar by the presence, in the female, of two or three columns of massed cells extending the length of the abdomen on each side of the intestine.
The sixth‑instar female larva shows the differentiation of the cephalothorax, which is as yet unsclerotized, with a pronounced constriction at its juncture with the abdomen, and is deflected ventrally. The spiracles of both sexes and the mouth parts of the female are sclerotized.
The seventh‑instar larva of both sexes of Corioxenos retains the fleshy dorsal processes on the abdominal segments, though they are much larger in the female (Fig. 219). In the male, there are sclerotized bands on the cephalothorax, and after extrusion the entire structure in both sexes becomes heavily sclerotized and brown in color. The side in contact with the host body, which in this species is the venter, is appreciably flattened in the female. The cephalothorax of the male is almost quadrangular in form and larger than that of the female. This greater size of the male is also found in the species attacking Homoptera, as contrasted with the opposite condition in the species attacking Hymenoptera. The fleshy dorsal processes disappear as the body becomes distended with eggs. These processes are considered to serve principally to increase the absorptive area of the body, but they also function in a limited way in locomotion. In all species, the cephalothorax becomes heavily sclerotized, the color being white at the time of extrusion but changing in a few hours to yellow, cinnamon, or brown. In certain species, the segmentation of the cephalothorax is indicated at the lateral margins, and in Corioxenos distinct lines reveal it in the male and definite constrictions in the female. Throughout the Strepsiptera the exerted cephalothorax of the two sexes can be readily distinguished, that of the female being appreciably flattened dorsoventrally, whereas that of the male is almost cylindrical, its depth very nearly equaling the width.
The Male Pupa.‑‑The male pupa is distinctive in form and is found within the abdominal portion of the unbroken larval exuviae. According to Pierce, the seventh instar is really a pupa, which is enclosed by the sixth larval exuviae, and a second pupal instar then follows. After transformation, the adult male is thus enveloped in three skins. In Corioxenos, the pupal skin is pushed downward to the posterior end of the puparium.
The Adult Female.‑‑Because of the retention of the larval form by the adult females of the order, their persistence within the host body, and the peculiar development of the reproductive system, it seems desirable to give here a general description of that stage. It has been mentioned that in the apodous forms the seventh‑instar female larva transforms directly into the adult without the intervention of a pupal stage. Much controversy arose among early entomologists as to the orientation of the body of the female, some claiming that the exserted portion was the head and others that it was the posterior portion of the abdomen. The latter appeared logical in view of the emergence of the triungulinids at that point. There was also considerable doubt as to the sides of the cephalothorax that were to be designated as dorsal and ventral. The true interpretation was arrived at only by an examination of the nervous system.
The adult female, as usually described, consists of the cephalothorax of the last larval instar plus the abdomen of the adult. Whether or not exuviation is complete is not known; if it does occur, the head and thorax of the adult female must be undifferentiated, with the cuticle thin, transparent, and following closely the inner surface of the cephalothorax. Certainly, the spiracles of the last larval instar, as well as other structures of the cephalothorax, serve the adult, als0. The exuviae of the abdomen remains closely enveloping the adult except for a definite area ventrally, which draws somewhat away from the body to form the brood chamber (Fig. 220A). According to Muir, the last larval exuviae of Elenchoides perkinsi is broken over the dorsum of the abdomen as a result of the great distention of the body of the female that accompanies egg production. According to Nassanov, the adult female of S. melittae is enveloped in two larval exuviae rather than only one.
In many species, the portion of the exuviae forming the brood chamber becomes considerably darkened, in contrast to the white of the remainder of the body. The brood chamber varies in width and length among the different species and extends to the posterior margin of the fourth segment in Dacyrtocara undata and to the eighth in Xenos vesparum, while in other species it is of intermediate length. This depends on the number and position of the genital pores. The chamber usually extends one, and occasionally two, segments beyond the last pore. The epithelium lining the ventral surface of the body and forming the dorsal wall of the brood chamber is appreciably modified, with the cells produced into spiny processes (Fig. 220D). This spiny surface is said to facilitate the escape of the triungullinids.
Westwood originally called attention to the existence of a cord or tube connecting the base of the abdomen to the integument of the host, and it was suggested that food is taken into the body by this means. Pierce observed the same feature in Halictoxenos crawfordi Pierce and in several Xenidae, but no definite information is available regarding either its structure or function. Ulrich stated that it is of host origin and that it serves as a means of attachment. It is quite possible that this structure represents an ingrowth of the integument, or rather a scab formation, analogous to the respiratory funnel formed about the posterior end of parasitic tachinid larvae that maintain, for respiratory purposes, a constant communication with the exterior through an aperture in the body wall of the host. The wound resulting from the extrusion of the cephalothorax is large, and, without complete healing, resulting in a close union of the intersegmental membrane with the "neck" of the parasite, the host would soon die from desiccation or disease.
The characters that have been utilized for describing the larviform females are as follows (Pierce, 19O9, 1918):
Head--width at emargination at base of mandibles in relation to thorax.
Cephalothorax--width at base of head, at spiracles, and at base; length from front edge of spiracle to apex of head and from base to apex of cephalothorax.
Mandibles--presence or absence, mld forms.
Spiracles--number, position, and form.
Genital pores--number and position.
Genital openings--number and form.
Distinct mandibles are present in the heads of the adult females of the Stylopidae, Xenidae, and Hylecthridae. In a few species, these are simple and small, but in the majority they bear a distinct tooth at the inner apical angle and may be widely separated. They are vestigial or absent in the Halictophagidae and Elenchidae. These widely spaced thorn‑like mandibles may be utilized by the mature larva in effecting the extrusion of the cephalothorax from the body.
The tracheal system of the mature larva usually comprises a pair of large spiracles on the cephalothorax, with branches extending throughout the body. In X. vesparum (Fig. 22OE) and S. melittae, according to Nassanov, the two main anterior trunks are directed caudad from the spiracles; just before leaving the cephalothorax, each one gives off a branch that extends forward, and the two then unite to form a dorsal commissure. The main trunks divide immediately after entering the abdomen and reunite in the eighth segment, thus giving a dorsal and ventral trunk, with various branches, on each side of the body, and a posterior commissure. Smith and Hamm, however, noted only a single trunk on each side, with numerous ramifications, in S. melittae. According to Pierce, the paired lateral trunks are normal for the order. With few exceptions, the adult females have a single pair of rather large spiracles situated at the lateral margins of the metathorax. Callipharixenos muiri and Chrysocorixenos siamensis Pierce have two pairs, and D. undata is distinguished by three pairs of small, slit‑like spiracles at the margins of the abdominal brood chamber, in addition to the thoracic pair. Presumably, the metathoracic spiracles are closed until the extrusion of the cephalothorax from the body of the host. The exact stage in larval development at which the spiracles appear is unclear, though Nassanov and others have implied that they occur in all instars after the first.
The reproductive system of the female is of remarkable form and not at all comparable with that found in any other insect. The eggs lie free in the body cavity, though Brues detected a rudimentary oviduct and vagina in Xenos wheeleri. On the median ventral line of the abdomen, there first appears a series of invaginations of the cuticle that develop into funnel‑like structures, the inner ends of which become open about the time the larvae are mature. These are the genital duct or pores, through which insemination takes place and the larvae later escape. These pores are usually found on the second and following abdominal segments, though in Corioxenos antestiae the first one is on the third segment. The number of pores is two in the case of Dacyrtocara undata; three in Elenchinus japonicus E. & H., Elenchoides perkinsi and Carwfordia labiata Ogl.; four in X. bohlsi, X. vesparum, (Fig. 221), X. wheeleri, Tridactylophagus mysorensis, and Corioxenos antestiae; and five in Halictoxenos viridulae Pierce, Halictophagus curtisii Dale, Chrysocorixenos siamensis, Callipharixenos muiri, and several species of Stylops. A very unusunl arrangement of these pores is found in Stichotrema dallatorreanum, in which species they occur in three transverse rows of 12-14 each on the first or second abdominal segment. Hofeneder has erected the superfamily Stichotrematoidea on the basis of this character.
The genital opening usually occurs as a transverse crescentic slit ventrally at the juncture of the head and thorax; it is through this opening that insemination takes place and the triungulinids later emerge. There is considerable variation in the form of this opening among the different species; it is almost semicircular in Elenchoides and Elenchinus. The opening is usually closed by a membrane, which is perforated at the time of mating. A striking departure in respect to the genital opening is found in Corioxenos antestiae in which, instead of being single and medially placed, it is paired, with one opening occurring at each side of the cephalothorax. This arrangement is said to be essential in order that mating may be accomplished, for the cephalothorax, except for one lateral margin, is covered by the wing of the host.
The free‑living adult females of Eoxenos (Fig. 217B) and Mengenilla differ in many morphological characters from those which are permanently internal in habit. They are wingless and have primitive compound eyes, and the single genital opening occurs ventrally at the posterior margin of the seventh abdominal segment. In Eoxenos, the antennae are five‑jointed and the legs have four‑jointed tarsi terminating in a pair of claws, whereas in Mengenilla they are four‑ and three‑jointed, respectively.
The reproductive system of E. laboulbenei, which has been studied by Parker and Smith, is quite different from that already described and approaches the type common to that of most other insects. The vagina is distinct and terminates anteriorly in the fifth abdominal segment, at which end it is apparently open. The eggs lie free in the body cavity, and some may even be contained in the head. There is no evidence of segmental genital pores on the median ventral line of the abdomen. Spiracles are present on the first seven abdominal segments.