October 2017

Warning: This article makes reference to creatures with eight legs.  Well now that we’ve lost the arachnophobes we can talk about creatures that at this time of year suddenly seem to be everywhere.  They have, of course, been everywhere since the spring, it’s just that now they are much, much bigger, and consequently they build bigger and stronger webs to catch bigger meals for themselves.  And talking of size – now that the arachnophobes have gone – the average person is at least 150 times taller than the biggest UK spider, and at least 30,000 times heavier, so what’s the problem?  Having said that, however, I have seen some sizeable spiders recently.  On a recent survey at Rye Harbour I found a four-spot orb-weaver spider and two very impressive wasp spiders.

The four-spot orb-weaver was a large female with a brick-red abdomen that accentuated the four white spots on her back – a real beauty.  She had just caught a turnip sawfly and was injecting venom into it as I was taking photographs.  She had a very large abdomen that suggests that she was carrying a heavy load of eggs.  It has been found that a female with her egg load can weigh as much as 2.5 grams, and that after laying her eggs into the protective cocoon that will keep them safe until the spring, she only weighs 0.8 grams – less than half the weight of her eggs.  After the sawfly had been subdued, it would be eaten.  Unlike most other spider species that inject digestive fluids into their prey, and then suck out the resulting ‘soup’, the four-spot orb-weaver chews its prey and doesn’t leave the empty husk.  One bizarre fact I discovered while fact checking for this column was a quote from a research scientist who had done a spider taste test and found that… of the spiders tasted that day, ‘the four-spot orb-weaver – with a slightly nutty flavour – was the best’!

The turnip sawfly is part of the Hymenoptera family that includes bees, wasps and ants.  Like bees, the turnip sawfly adults feed on nectar and pollen, but unlike bees, wasps and ants, they don’t have a sting.  They get the name sawfly because the females have an ovipositor with a serrated edge that is used to cut into the plant stems in which they lay their eggs.  The turnip sawfly is bright orange with a few contrasting black bits – one of nature’s most reliable warning signs.  So if they don’t sting, why the bright colours?   It’s called a turnip sawfly because the larvae feed on brassicas – like turnips, cabbage, etc.  Brassica leaves contain substances called glucosinolates that give the leaves a bitter taste to discourage insects from eating them (and possibly to give children a reason to dislike sprouts).  But the sawfly larvae ingest the glucosinolates and concentrate it in their blood.  This can then be released in foul-tasting droplets to try and prevent predatory insects from eating them.  Even though the adult turnip sawfly doesn’t eat turnip leaves, its blood still contains the glucosinolates, hence the ‘don’t eat me I taste horrible’ colours.  Sadly for the turnip sawfly in the four-spot orb-weaver’s web, the spider ignored the warning.