Order Strepsiptera - Stylopids (or Stylops)

• Elenchus species (male) - see picture below

Minute insects, usually less than 5 mm long, in which the females and young stages are nearly always parasitic, living inside the bodies of other insects. The females resemble small grubs, with no eyes, antennae or legs, and live inside the host insect, with only their fused head and thorax sticking out between two of the abdominal segments of the host. A few tropical species have free-living females, with eyes, antennae and legs, but even these remain grub-like and wingless.

The adult males are free-living and have the forewings reduced to small club-like structures, called halteres, which serve as organs of balance; the hindwings are broad and membranous, usually white in colour, with just a few radial veins. The head of the male insect has strongly protruding eyes, branched antennae and degenerate, biting mouthparts. Adult males take no food and their life only lasts a few hours, during which time they fly in search of an insect carrying a female stylops ready for mating. All species are fairly similar in appearance and a typical example of a male stylops is illustrated opposite.

Photo: V.J. Stanek ©

Male stylopid (Elenchus sp.)

The most frequent insect hosts of stylopids are bugs (Hemiptera) and various bees and wasps (Hymenoptera), especially some of the solitary mining bees and their relatives. Other hosts include bristletails, crickets, grasshoppers, mantids and ants. Attacks by stylopids rarely kill the host, but commonly cause sterility and may affect secondary sexual characters (e.g., male hosts tending to become more female in appearance and vice-versa).

Stylopids have a complex metamorphosis and life cycle, with seven larval stages and a pupal stage. Males are rare in some species and reproduction is then largely asexual (by a process called parthenogenesis), but the following account is fairly typical of those species which parasitise bees. The male parasites emerge from their host, usually whilst the host is flying, and they seek out bees carrying female parasites. On the underside of the fused head and thorax of the female there is an opening into a brood canal, which leads to the genital pore at the rear of the female. The male deposits sperm into the brood canal and the sperm eventually finds its way to the genital pore. After fertilisation, the eggs develop and hatch inside the female's body. The first-stage larvae are minute, active, woodlouse-shaped creatures, called triungulins, which find their way on to the body surface of the host. From there they pass to other host insects, probably by way of flowers, but they only use these adult bees for transport back to the nest. Once inside the bee's nest, the triungulin quickly searches out and parasitises a bee larva by burrowing through its body-wall. The triungulin then moults into a legless grub, which lives in the body cavity of its larval host, absorbing food from the host's blood. The parasite matures soon after the host reaches maturity. In its last stage, the stylopid grub works its way outwards and protrudes from the abdomen of the host, ready to change into the adult parasite.

These tiny parasites are fairly common, but rarely noticed by the non-specialist. There are about 400 known species worldwide, of which some 20-30 occur in Europe and the British Isles.