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ight cted

m/

Do Polar BearsGet Lonely?

THE LAST WORD

Big and scary

While in Costa Rica we were visited

by this beast (see photo). It was

about the size of a cigarette packet,

could fly (but not very well), and

dogs seemed nervous of it. We

haven’t managed to identify it

and neither could locals or tour

guides. We’re not even sure what

sort of insect it is. Can any of your

readers help?

■ The insect in the picture is a male dobsonfly , which is in the order Megaloptera and the genus Corydalus. However, a species identification is not possible without the full insect being visible.

Dobsonfly larvae inhabit fresh running watercourses before crawling out and hatching into the adult shown. The genus Corydalus attracts attention throughout both north and south America as a result of its size – its wingspan can reach 16 centimetres. The males possess

very large mandibles, as shown in the photograph, which can be up to 3 centimetres long. They are too long to be a threat to humans, though, as the leverage is too poor to puncture the skin.

Dobsonflies are nocturnal but, like other insects, are attracted to lights, which is probably what attracted this specimen to you. The dogs were most likely bothered by the sight of a large insect fluttering around them.

If you had asked a fisherman to help identify the insect you may have had more luck. They tend to have an in-depth knowledge of local insects as they make similar-looking fly hooks to help catch fish. Peter ScottSchool of Life SciencesUniversity of SussexBrighton, East Sussex, UK

■ The insect is a dobsonfly and part of the Megaloptera order, which also includes the alderflies and fish flies. The individual shown is almost

certainly a species of Corydalus and its large jaws indicate that it is a male.

Males use their jaws in mating displays and in threat postures directed at rival males, as well as to grasp females during mating. They have little leverage, though, so are not as fearsome as they may seem to the human observer. In contrast, the female’s short, pincer-like jaws can inflict a painful nip.

Dobsonflies spend most of their lives as aquatic larvae, hiding under stones and pouncing out at prey. Some fishermen, who use North American species as bait, call the larvae hellgrammites. The larger species can take several years to mature. When mature, the insect can eject a foul-smelling, and no doubt foul-tasting, secretion from the anus as a deterrent to predators. The adults are short-lived and do not feed.

The larger species of dobsonfly can have a body length of 12.5 centimetres, with males having jaws up to 2.5 centimetres long. There are 30 species of Corydalus, found mainly in Central and South America. Three occur in North America, the most common being Corydalus cornuta.Chris O’TooleHope Entomological Collections,Oxford University Museum of Natural History, UK

This week’s questions

MEAN GREENS

Of all the vegetables I buy, whole iceberg lettuces have the greatest longevity. They are edible up to three weeks after their “best before” date. Other vegetables succumb sooner. Why does it last so long compared with, say, tomatoes, broccoli or radishes?Finn de BoerRotterdam, the Netherlands

TIMELY QUESTION

I look after a pendulum clock at the University of Cambridge which I hope will achieve an accuracy of less than 1 second of error per year ( www.trin.cam.ac.uk/clock). It has a temperature-compensated pendulum, but is sensitive to air pressure variation. If the mean global temperature was to rise by, say, 4 °C, would there be any change to mean air pressure at sea level? Put simply, would global warming cause the clock to speed up or slow down? Hugh Hunt Keeper of the Clock, Trinity College, University of Cambridge, UK

HOT TOPIC

One suggestion to combat climate change is that we should become vegetarians as livestock is more environmentally damaging than growing crops. However, if we stopped eating meat, livestock would still live, so is the suggestion correct? Or are we expected to cull any remaining pigs and cows?Ella GribbenLondon, UK

Last words past and present, plus questions, at www.last-word.com

“While male dobsonflies have mandibles that are 3 centimetres long, they pose no threat to humans”