October 2014

Wasp damage on Cosmos

A neighbour of mine recently showed me some Cosmos stems that had linear scars along them.  When she told me that it was wasps that had done the damage, I wasn’t particularly surprised being familiar with their habit of chewing up any suitably soft or weathered wood.  I’ve often seen them on fence posts where they chew little grooves along the grain of the wood.  As far as I know, wasps are not into wood carving or marquetry, so what is going on here?  The simple explanation is, of course, that they are making papier maché.  This is not the papier maché that we made out of torn up newspaper and flour paste in school handicraft classes, but a much finer version made out of wood pulp and wasp spit.  Nor are they making model animals or lumpy boxes, but rather the beautiful, intricate and delicate structure of their nest, which includes both the many layered (and therefore insulated) outer shell and the honeycombs inside where the wasp grubs will incubate.

I don’t know what attracts them to Cosmos in particular, but I suspect that it not only provides the wood pulp for their nest, but also because the sap is not far beneath the surface, it also satisfies their sweet tooth.  It is their sweet tooth that also makes them such a nuisance at picnics, and it’s their nuisance value that makes many people want to destroy their nests at every opportunity, whether they are interfering with your gardening or not.  For good reasons why you shouldn’t do that, I can recommend an article written by Steve Backshall in the Daily Mail on that very subject.  And if you have an aversion to reading the Daily Mail, then it is on-line at http://www.dailymail.co.uk/sciencetech/article-2748653/Why-learn-love-WASPS-They-ve-vanished-summer-But-s-shouldn-t-celebrate-writes-STEVE-BACKSHALL.html (or better still just Google Steve Backshall Daily Mail Wasps.)

German Wasp chewing a garden chair

I mentioned the honeycomb structure above, but it has nothing to do with honey, as there is only one British native (introduced) species that stores honey – the honey bee (Apis mellifera).  I was recently asked by a friend how honey bees manage to build such a regular hexagonal structure.  I didn’t know, of course, but I did vaguely remember some research published in the last year or so that provided an explanation.  I found the article which was published as a news item in Nature magazine, as well as the full text of the research paper.  (See http://www.nature.com/news/how-honeycombs-can-build-themselves-1.13398 )  Basically, what happens is that each bee tries to build a circular cell of a particular diameter (6mm) as close as possible to the next bee doing the same thing.  As the cell grows they turn into a series of adjacent cylinders which are heated by the bodies of the wasps which partially melts the wax.  This allows surface tension to pull the wax into a basically hexagonal shape.  (You can show this by creating identically sized bubbles in soapy water, which then form perfectly regular hexagon-shaped bubbles on the surface.  The research team showed it by melting bunches of plastic drinking straws.)  The bees also knead and reinforce the wax to make sure that the walls don’t get too thin.  The result is the perfect hexagon pattern of the honeycomb which has rounded internal corners, no doubt for the comfort of the grubs.

This does leave some questions unanswered though – like how do the bees measure the circle in the first place?  I suspect that this has to do with the fact that all worker honey bees are roughly the same size and they will measure things like we did before we invented rulers and inches.  We had to use measures like cubits (the length of your forearm) and spans (the width of your outstretched hand).  It’s probably got something to do with the size of the bees head.

The next question is – how do wasps make their hexagons with papier maché which doesn’t melt like wax does?  Maybe it’s something to do with surface tension as the paper dries, but the truth is – I haven’t a clue.  It’s still marvellous though.

September 2014

Keeled Skimmer - Orthetrum coerulescens

Earlier this month we went on a walk with Sussex Wildlife Trust looking for Devil’s darning needles and Horse-biters in Ashdown Forest.  And just in case you don’t recognize those old country names for those creatures, we were looking for Damselflies and Dragonflies. (Note that Damselflies are included in the general term Dragonfly, so I shall just use the latter to save words.)  Ashdown Forest, despite its name is mostly open heathland where there are plenty of muddy pools and slow streams that are ideal habitat for those insects.  There are plenty of dragonflies there and for details of the rarities and photos of what we found see www.pettnats.blogspot.co.uk.

You don’t have to go to Ashdown Forest to see dragonflies, of course.  Anybody with a pond, or who lives near anybody with a pond will see plenty of them in their garden at this time of year.  Despite their somewhat sinister country names they are harmless and beautiful creatures that have fascinating life cycles.  (Even the largest Emperor Dragonfly would be unable to break human skin, let alone a horse’s, though you may get a deservedly painful nip should you provoke one enough.)

Blue-tailed Damselflies - in tandem

Their life-cycle starts with a fertilized egg being deposited in or on a reed stem or even just placed under the water by the female.  If you see one dipping her abdomen into the water or crawling backwards down a reed stem into the water, she will be laying eggs.  She will be doing that with or without the ‘help’ of the male.  The male is not helping, he will just be protecting his investment either by clasping her round the neck to prevent another male mating with her, or by flying close by to chase off any males with similar ideas.  

Each male has a pair of claspers on the end of his abdomen that fits with a specially shaped shield just behind the female’s head called a pronotum.  These are connected during mating and any subsequent egg protection behaviour.  Each species has its own special shape of clasper and pronotum which (mostly) prevents inter-species mating.  Specially shaped genitalia or associated equipment is common in the insect world and is often used by entomologists to distinguish between species when other differences are too subtle or too variable.

Emperor Dragonfly - Egg Laying

When the fertilized eggs hatch out, the larvae will skulk about on the bottom of the pond and prey on any smaller invertebrates that they find there and when they get big enough they will even take tadpoles or small fish.  Because they have a hard external skeleton, they have to moult several times in order to grow to their adult size, and they may take two or more years before they are ready to emerge as adults.  The larva will then wait for suitable weather and summer temperatures, before crawling up a reed stem above the water where it splits its external skeleton for one last time and emerges as the adult insect.  It will stretch out its abdomen and thorax and inflate its wings before the skeleton hardens.

Azure Damselflies in the 'Wheel Position'

The adult Dragonfly will then take to the air, feeding on other flying insects and looking for a mate.  Adult dragonflies live only for a few days, or a few weeks at most.  When the male finds a receptive female, he transfers a packet of sperm from the end of his abdomen to the second segment where his ‘equipment’ resides.  As the second segment is near the front, this involves making a complete loop of his abdomen.  He then straightens out and clasps the female behind the head.  She then loops her abdomen round so that the tip connects with the second segment of his abdomen making what is called the ‘wheel’ shape.  They will then either fly in that position, or will find a convenient perch until the fertilization is complete, whereupon the female will detach her abdomen.  They will then either separate completely, or will fly together with the male still clasping the female until the fertilized eggs have been laid.  And then it all starts over again.  It sounds really complicated written down like that, but fortunately dragonflies can’t read so they manage very successfully with what evolution has provided for them.