Relationships after Kinzelbach 1978.

Introduction

Strepsiptera are obligate parasites of insects, with hosts ranging across 7 orders and 34 families. The name of the group is derived from the Greek words for twisted (streptos) and wing (pteron) and refers to the peculiar twisted wing of the male's hind-wings while in flight. Representatives of the suborder Mengenillidia generally show more primitive characteristics (fig. 1). The Mengenillidae parasitize Thysanura (Lepismatidae), the only known order in the sub-class Apterygota to be attacked by strepsipterans, while Mengeidae are known only from fossil males from Baltic amber. We have very little information about their life history, therefore.

Strepsiptera exhibit extreme sexual dimorphism, which is most pronounced in the suborder Stylopidia. Strepsipteran males emerge from the host after endoparasitic pupation in the host. Adult males are free-living, and their sole mission is to find and fertilize a female. They have reduced forewings and fan-shaped hind wings, branched antennae, and raspberry-like eyes (title image and fig. 2); the latter are very unusual among living insects and form a modern counterpart to the structural plan proposed for eyes of trilobites (Kinzelbach 1971, 1990, Kathirithamby 1989, Buschbeck et al. 1999).

Females of the family Stylopidae are neotenic (i.e., they retain juvenile features even in adulthood) and totally endoparasitic in their hosts. They are highly modified morphologically, lacking adult external characteristics such as eyes, antennae, legs, wings and external genitalia (fig. 3). Apart from the adult males, the only free-living stages in this suborder are the viviparous 1st instar host-seeking larvae (fig 4). In contrast, males and females in the family Mengenillidae leave the host at the end of the last larval instar to pupate externally (fig. 5, 6). After eclosion, the females are free-living, with the presence of all other adult characteristics such as eyes, mouthparts, antennae, legs and a ventral genital opening, but with the absence of wings (fig. 7).

The combination of morphological reduction and modification, and the bizarre and unusual life history of Strepsiptera, have puzzled biologists for over two centuries (Rossi 1793; Latreille 1809; Kirby 1802, 1813. 1815; Lamark 1816 Pierce 1909; Crowson 1960, 1981; Arnett 1963; Kinzelbach 1971, 1990; Kathirithamby 1989), and the Strepsiptera's phylogenetic position has been the most enigmatic question in ordinal level insect systematics (the "Strepsiptera problem", Kristensen 1981, see below).

Strepsiptera are cosmopolitan in distribution (Table 1), but are extremely difficult to locate, and one often has to find the host in order to find the female. To date, 596 species of Strepsiptera have been described and many more await description. Many of the described species are of free-living males that have come into traps. At present, since sites have been located where Strepsiptera can be collected, some of the species are being reared in the laboratory. Work on the extraordinary reproductive and developmental biology and behavioural ecology of this group is under way.