Stylopidia

Classification after Kinzelbach (1971)

Introduction

The Stylopidia are obligate parasites of other insects. This group is distinguished from the other suborder of Strepsiptera, the Mengenillidia, by the females, which are totally endoparasitic in their hosts. The Stylopidia is the larger of the two suborders, comprising 7 families and 584 species described so far.

The female Stylopidia possess no external characters typical for normal adult insects, such as eyes, antennae, mouthparts, legs, wings and external genitalia. The only visible feature is the cephalothorax, that extrudes through the host at the neotenic adult stage (fig. 1). The free-living male (fig. 2) emerges from the puparium in the host (fig. 3) and seeks an endoparasitic female and fertilizes it (fig. 4). The extreme sexual dimorphism is most pronounced in this suborder. In the suborder Mengenillidia, both the males and females emerge from the host to pupate externally. For more information on the insects parasitized by stylopidians see the page on Stylopidia Host Relationships.

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Fig. 1. Delphacid female with three female Elenchus varleyi Kathirithamby cephalothoraxes (arrows). Image from Kathirithamby (1989b), copyright © 1989 CSIRO Publishing. Fig. 2. Frontal view of male Elenchus tenuicornis (Kirby). Image copyright © 2002 J. Kathirithamby. Fig. 3. Male Elenchus tenuicornis (Kirby) emerging from Javesella dubia (Kirschbaum) (Delphacidae). Image from Kathirithamby (1989a). Copyright © 1989 Blackwell Science. Fig. 4. Male Stylops pacificus Bohart mating with female endoparasitic in Andrena complexa Viereck (Hymenoptera). Photograph by Edward S. Ross, from Kathirithamby (1998a), copyright © 1998 Edward S. Ross.

Most stylopidian families are cosmopolitan in distribution. The only exceptions are:

• Callipharixenidae: now only known from South East Asia (Kinzelbach 1971, Kathirithamby unpublished)
• Bohartillidae: known from the Neotropics (Honduras and Ecuador, Kinzelbach 1969b and Kathirithamby unpublished) and in Dominican amber (Kathirithamby & Grimaldi 1993, Kinzelbach & Pohl 1994)

Characteristics

The autapomorphic characters of the suborder Stylopidia are:

1. Neotenic females are totally endoparasitic in the hosts (fig. 1), unlike the Mengenillidia, which emerge during the last larval instar to pupate externally.
2. Female Stylopidia have a number of genital openings from the body cavity into the apron.
3. MA1 vein in the wing of male with only residues.

The suborder Stylopidia is distinct from the Mengenillidia, in that the females are totally endoparasitic in their hosts, and remain within their larval cuticles, except for the extruded cephalothorax. They do not undergo a pupal stage but become neotenic adults after the IVth larval stage, when the cephalothorax is extruded. The ventral surface (fig. 5) has a peritrophic matrix (fig. 6) that is analagous to the gut in insects and is referred to as the "brood canal/apron" (Kathirithamby 2000). Several genital tubes open from the body cavity of the female into the apron (fig. 7). The brood canal/apron is initially used for the entry of sperm and later as a passage to the outside for the emergence of the viviparous host-seeking 1st instar larvae from the body cavity of the mother to the outside (fig. 8).

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Left: Fig. 5. 5 μm section of integument in the region of the apron in female Stichotrema dallatorreanum Hofeneder showing microvillate cells (arrow head) and peritrophic matrix (arrow). Right: Fig. 6. Neotenic female Stichotrema dallatorreanum Hofeneder. Diagrammatic representation of a longitudinal section revealing the 4th instar unshed cuticle which is sclerotised anteriorly as the cephalothorax (cp) and collar (co), posteriorly as a thin unscleroticed cuticle (ep). The neotenic female (neo) lies within this persistent cuticle. The region of the apron is on the ventral side (v) of the cephalothorax (cp), consisting of a cuticle analogous to the peritrophic matrix (pm). The ectoperitrophic space (ecp) is lined with microvilli (mv), and the anterior abdominal segments have invaginations (in) that lead into the female body cavity. The endoperitrophic space (enp) is the host hemocoel. The dotted line indicates the margin of the host cuticle with dorsal (d) and ventral (v) sides of the neotenic female. Images from Kathirithamby (2000). Copyright © 2000 Blackwell Science.

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Left: Fig. 7. Neotenic female Stichotrema dallatorreanum Hofeneder with cephalothorax (arrow) and apron (arrow head). Scale bar= 5mm. Image from Kathirithamby et al. (2001). Copyright © 2001 The Netherlands Entomological Society. Right: Fig. 8. SEM of 1st instar larva of Stichotrema dallatorreanum Hofeneder emerging from an invagination/genital tube into the brood canal.

Discussion of Phylogenetic Relationships

There are 7 families in this suborder, but many of the families are described only from females or males such as the females of Bohartillidae and males of Callipharixenidae are unknown, and only six females out of the 106 described Myrmecolacidae are known. Most of the species in the family Myrmecolacidae have been described from free-living males that have come into traps.

While the relationships among stylopidian families are largely unresolved, preliminary evidence indicates a sistergroup relationship between the Myrmecolacidae and Elenchidae. Autapomorphies that distinguish these two groups from the rest of the families are (Kinzelbach 1971):

1. Narrow antennal joints
2. Short female cephalothorax
3. 2-4 genital openings in the female
4. Wide brood canal opening
5. Mesothoracic tarsus of 1st instar larva pointed

The sister group relationship is also seen in the analysis by Halbert et al. (2001) where seven Myrmecolacidae were compared to Elenchidae and Stylopidae. However, Pohl (2002), who studied the 1st instar larvae, does not support this sister group relationship. A phylogenetic analysis of the molecular data, which is at present being gathered would clarify this question.

References

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