Twisted-wing insects


KNOWN TO SPECIALISTS AS STREPSIPTERANS, these tiny insects are virtually unknown to the public. Though quite common — if one only knows where to look — these ingenious winged marvels are so often overlooked by man. I've never seen one, but then again I don't have the taxonomic skills to identify one even if I found it. The biology of very few of the four hundred plus known species has been studied in detail. But those that have illustrate the endless capacity of the Divine Mind for thinking the unthinkable and imagining the unimaginable.

Strepsiptera are parasites of other insects — bugs, flies, cockroaches, bees, grasshoppers, praying mantids and thrips, to be exact. Boy and girl strepsiterans — also known as twisted-wing insects — would never be paired off by a matchmaking agency, that's for sure. Adult males, rarely more than four millimeters long, are free-living and winged. Adult females, by contrast, are grub-like, and generally never leave their host, which is normally rendered sterile by their uninvited guest. The only visible part of the female parasite is the head and thorax; they poke out from between two abdominal segments of the host insect while the bulky abdomen, like the proverbial iceberg, is sunken into the host's flesh where she remains imprisoned for her entire life.

The female strepsipteran

The most fascinating aspect of these deviants is their life-cycle; this is their life. Typically, the winged and free-living male insect starts seeking out a female as soon as he emerges from his pupa and leaves his host. Remember, the females are not free-living, but remain forever imprisoned, cemented into their obliging host. Just how the male is able to find a stylopized insect (that is, one that is playing host to a female strepsipteran) is one of those "little questions" which no one so far has been able to answer. And it is not just a matter of finding any stylopized host; it must be one carrying a female of the same species. Presumably, the complex and bizarre-looking antenna of the male plays some part in this mission impossible.

When the mated female is ready to “give birth" to her brood, she produces about one thousand tiny infants! This she does while still attached to her host. Uniquely in the insect world, the larvae make their way to the outside world by means of a special brood canal that opens at the front end of the mother. In all other insects, the egg-laying or birth canal is located at the posterior end of the animal. How many disasters occurred while this special arrangement was trying to evolve?

Can you spot the parasites?

These larvae are a story in their own right. Shaped vaguely like a woodlouse, of pill bug, these triungulinids, as they are called, are very active little fellows. They are a form of larva almost unique to the strepsiptera. But not quite unique. A number of beetles produce a similar-looking larval stage, and they are also termed triungulinids.

Dorsal and ventral views of two triungulinids

Well, what happens next? The problem that confronts the newly-hatched triungulinids is how to find a new potential host. As soon as they can, these little 'lice' alight from the insect host on which they began their existence. This presumably takes place when the host makes a pit-stop on a flower. Having disembarked, they now loiter on the flower until it is visited by another insect. God has programmed their microscopic computer brain to literally jump aboard any flying insect that alights on the flower. Although not much bigger than a speck of dust, these larvae can spring up to three centimeters in one leap — a logic defying ability. Most of these wee larvae perish soon after hitching a ride simply because they have caught the wrong plane. But by pure mathematical chance, sufficient of them hop onto the correct host to keep the species flourishing.

This is not the end of the story, but almost just the beginning. The triungulinid is not really interested in the animal it has boarded, even if it is the correct kind. It is using this insect as nothing more than an airborne taxi. It is hoping that its driver will take it to its nest. (If it has one.) If it does — and the mathematical chances of this happening are very small, which is why so many are produced — it disembarks in the nest. Here it immediately, instinctively, seeks out any grub larvae that its taxi driver has been nurturing.

In order to keep the life cycle going, it has to get inside the grub. How does it do this? One might think it would simply crawl into its mouth, or one of its breathing holes. But no. It enters by literally dissolving a hole in the grub's skin. It exudes a fluid from its mouth in considerable quantity so that it soon becomes completely immersed in its own exudate. This substance hardens on the outside in three to four minutes, while at the very same time dissolving a hole in the skin of the host grub. Talk about a dual-purpose fluid! The triungulinid is quite active while immersed in this fluid, contracting and expanding its body in an effort to break through the softened skin of its future host. Eventually it succeeds.

Once inside, another remarkable thing occurs. Here the tringulinid molts, which simply means that it casts off its outer skin. (Though there is nothing simple about the process of molting. It's a book long subject in itself.) What emerges from the cast-off skin is not another active little triungulinid, but a legless, maggot-like creature. This 'maggot' lives in the body cavity of the host grub (of a bee, beetle, etc), absorbing food from its benefactor's blood. Usually it passes the entire winter in this secure house.

When spring comes, the miracle of life continues. Generally, both the host insect and its free-loading tenant metamorphose simultaneously, the one inside the other. (Obviously, the strepsipteran doesn't kill its host.) Thus, when the host insect emerges from its winter retreat, its little companion can already be observed jutting out from between the abdominal plates of its landlord. And so the cycle of life for this ingenious example of God's handiwork continues.

All illustrations from The Insects of Australia, Division of Entomology, CSIRO

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