Читать книгу Extreme Insects - Richard Jones - Страница 22

NAME	stalk-eyed flies in the family Diopsidae
LOCATION	throughout the tropics, particularly Southeast Asia and Southern Africa
ATTRIBUTE	eyes on thin stalks longer than their bodies

It is a sad fact of life that males often fight each other for the attentions of females. The prize for the victor may be a harem and numerous offspring, but the cost in energy expenditure and bodily damage may be high, and life expectancy short. It is better to be able to size up an opponent before falling to blows, and stalk-eyed flies do this eye-ball to eye-ball.

Many groups of small tropical flies have broad heads, and this is taken to extremes in the family Diopsidae. More than 150 species in this family have heads so wide that the eyes are held out on unfeasibly long, thin horizontal stalks. Very often the head width (12-14 mm) is twice the length of the fly’s body (6-7 mm). Head width, or rather eye-stalk length, is directly proportional to body size, and a good indicator of body strength, which itself is directly linked to the fly’s nutrition when it was a larva. Male diopsids face of fin a head-to-head stalk-measuring contest. The winner gets the females, but the loser walks away unharmed.

This ritual behaviour is thought to have evolved because these tropical flies are relatively long-lived (12 months has been recorded), and because they have something important to guard. Other groups of small flies with shorter lifespans and narrower (but still relatively stout) heads actually come to head-butting bouts: they have little to lose so they just go for it. Male diopsids, on the other hand, have been observed repeatedly contesting for 200 consecutive days.

The valuable resources that male diopsids are defending are string-thin rootlets hanging down from the banks of small streams that run through the woodland in which they live. These apparently mundane bits of straggling vegetation are the prime night-roosting sites for large numbers of females. They gather here and all face upwards, the direction from which any potential predator will come. By fighting, or at least flaunting his broad head, a male diopsid rules the roost and secures his harem.