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| Lepinotus patruelis, a common bark louse. Photo by David Jones |
By Judy Wearing
“I think I
have insects in my house,” the Bavarian woman living in England told the secretary at the Department of Zoology where I worked. “I hear these knocking
sounds all the time. I think it is an insect. Do you have anyone who could come and check it out for me
and tell me what it is?”
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The deathwatch beetle, which hits its head against wood to call for a mate.
Photo by Josef Dvořák.
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One candidate for the cause of the sounds she was hearing was the deathwatch beetle, a small insect with a
hard head that burrows into wood beams. The beetle knocks on wood to attract
mates, making a noise that sounds a little bit like a miniature woodpecker.
People don’t want deathwatch beetles in their houses, because when the larvae burrow/eat their way into wood beams they leave tunnels behind. The tunnels weaken the beam and can cause structural damage.
I went to
this woman’s house looking for another kind of insect, however. I was looking
for Psocoptera, a group of tiny insects otherwise known as bark and book lice,
or barkflies. Measuring from 1 to 2 millimetres in length, these obscure
creatures are not lice at all, but rather distant cousins. They have long
antennae, and many species lack wings. They feed on tiny bits of this and that:
algae, crumbs of other insect carcasses, and fungal spores. They do not bite anything,
and frankly, they can seem rather boring. You might find them on logs and tree
trunks, the undersides of mushrooms, or other damp places – if you look closely enough. But, they are easy to ignore, and largely go unnoticed.
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| Trogium pulsatorium, whose mating call sounds like the ticking of a clock. |
Some book
and bark lice have a remarkable habit: they ‘sing’. One species, Trogium pulsatorium, creamy white from
head to toe and small enough to fit on the end of a pin, produces a noise that
sounds exactly like the ticking of a clock. I'm inclined to think this creature is the original deathwatch, rather than the woodpecker-like
beetle. The death watch was a sound in people's houses that terrorized Medieval Britain. The ghostly ticking of a clock was thought to mark the final hours of someone in the household. The female bark louse makes this ticking noise by vibrating its abdomen. Like
the noise of the deathwatch beetle, it is a mating call.
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Lepinotus patruelis female, who 'sings' to attract mates by vibrating her abdomen.
Photo by David Jones |
Another singing barklouse, Lepinotus patruelis, is slightly more
colourful. Females are dark brown, and nearly 2 mm in length, so their rear ends
might dangle off the head of a pin. The males are smaller and golden
brown. In this species, when males vibrate their abdomens, they make a sound
like a quacking duck. Quack, quack, quack – four to six times. They do this several times a minute when calling for a mate. When females
vibrate their abdomens, they make a series of clicks like dragging a fingernail
across the teeth of a comb. The ‘songs’ are used by both males and
females to attract mates. Both sexes, especially females, keep singing when a
mate draws near. And females seem to compete with each other, just like male
crickets do, by singing at each other in the presence of a male. To hear these noises in the laboratory, I used a
listening device, a sound magnifier, sold at a local electronics shop. The
insects are pretty tiny, so it is not surprising that their sounds are not
audible to the naked ear. If, or how, the Bavarian woman could hear these insects added to the mystery.
I searched out places in her main rooms where these bark lice might hide. In the kitchen,
around the counter there were lots of bread crumbs and some mold spores in the
corners of the window sill. In the living room, there were plenty of tropical house
plants, and the air was warm and moist. Perfect conditions for these insects:
damp with lots of food. As I sat on the sofa with a cup of tea in hand, I heard
the sound of a fingernail running across a comb. I listened and heard it again.
It was a female Lepinotus patruelis, calling to attract a mate.
And I could hear it loud and clear! I followed the persistent noises to a large
tropical plant in the corner, and began searching among the leaves. I found her
nestled in the crux of a large, curved leaf – she had found a natural amplifier
and was using it to broadcast her song to the whole room.
This feat, of finding an amplifier to broadcast their sound makes these bark lice the smallest known creature to make an
audible noise (to my knowledge). But this broadcasting skill is not the only
unusual feature of the singing behaviour. It is also highly unusual for females of any species to be the ones calling for mates, let alone competing in 'singing competitions.' But that is another story, for another blog.
References:
Wearing, J. (1996) Reproductive biology of Lepinotus patruelis (Psocoptera):
Implications for courtship theory. (Unpublished doctoral dissertation)
University of Oxford, UK.
Death Watch Beetle (Xestobium
rufovillosum)
http://www.arkive.org/death-watch-beetle/xestobium-rufovillosum/video-09a.html
.jpg "Lachnum subvirgineum")
