Tuesday, 16 January 2018

It's not that simple

Do you like figs? Wasps? Well you can't have one without the other. When I was in Grad School in Boston, I was along the corridor from the lab of Tom Kunz, who taught us Mammalogy 504. He and his people worked on bats, not only the insect-eating chaps we're familiar with in Ireland but also the great flying fruit bats of the tropics. These lads are often part of a complex interacting network of mutually dependent species. The bats eat and disperse the fruit [bat gnawing fig L from Tim Laman] all over the jungle by eating the fruit off the tree and flying off somewhere quiet to digest it. In due course the seeds pass through the bat-gut and finish up as a smear on a tree-trunk, whence they sprout as a network of vines to grow up encircling and eventually strangling the tree. The flowers of the fig "tree", in contrast to garden flowers, develop like an ingrowing toe-nail with all the delicate bits on the inside and a tough exterior; it's called a syconium. Think daisy Bellis perennis rather than daffodil Narcissus poeticus because each syconium consists of multiple florets. There is a little pore at one end, through which the pollinating wasp enters. The female wasp is there to lay a clutch of eggs - for her the pollination is incidental - which hatch as larvae, eat some of the fruit and then exit as new flyers. Without the wasp no fruit is set. Without the bat, no dispersal. Don't think for a minute that figs are 'suitable for vegetarians' though: part of the treat is dead wasp bits. Getting your reproductive parts eaten away inside and out is a small price to pay for thus getting your offspring launched in the world. YMMV! And we have barely even mentioned the tree which supports the fig-tree and eventually gives its life for the system.

From our fuzzy-hearted perspective, some of the players enumerated above are nicer than others. These sort of woowah value judgments have given trouble to some god-botherers, including St Chuck: "I own that I cannot see as plainly as others do, and as I should wish to do, evidence of design and beneficence on all sides of us. There seems to me too much misery in the world. I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of Caterpillars, or that a cat should play with mice " (Charles Darwin). Did someone mention Ichneumons? There are at least 25,000 species of these hymenoptera (the bees, ants and wasps), most of which make a living by parasitising the larvae of other insects. But only because those other species will leave fat, juicy, nutritious morsels lying about unclaimed.

The females who lay the eggs which grow into the larvae sought by ichneumons go to some efforts to hide their progeny from predators; and the wasps exercise themselves to see or smell through these attempts at concealment. One of the better studied species is Ichneumon eumerus which is super-fussy about its host: only a larva of a particular blue butterfly Phengaris rebeli [synonym Maculinea rebeli] will do. Over the evolutionary eons, the butterflies got rather pissed off about this decimation of their progeny until one member of the species was born with a peculiar biochemical mutation. Its larvae produced a pheromone which made it smell remarkably like the larvae of Myrmica schencki a species of small ant; the ants thought so aNNyway and were therefore programmed to carry the butterfly larva back home to their nest and care for it - like cuckoos. For a while, the butterflies were safe from the attentions of the Ichneumon, until one of the latter learned how to find the nests of Myrmica schencki and determine if there were any butterfly larvae in there. If so, the gravid female darted into the ant nest and tried to deposit a clutch of eggs on the butterfly larva. Which led to a bit a barney as the ants tried to defend their trophy "offspring".  The wasps which succeeded left more of their own offspring into the next generation. One avenue to success was to develop and use another 'alarm' pheromone which made the ants run around in circles biting each other . . . thus allowing the wasp a window of egg-laying opportunity. These coils of mutual dependency are now impossible to dis-entangle.  Picture [R] of the dramatis personnae the ant is beige-on-grey to the L from the BBC.

Apart from the baroque inter-dependencies, the peculiar thing is the specificity of it all. No other butterfly larva smells quite right to either of the hymenopteran species. It looks probable that the larval pheromones of the butterfly and the ant are subtly different. Nat Geog suggests that teasing out the differences could help develop chemical agents to control the behaviour of insect pests. The butterfly pheromone is good enough to fool the ant, but different enough to encourage the wasp to assail the nest only if a butterfly larva's reek was wafting out the nest hole. You couldn't make it up! Then again, it suggests that our science is just skimming the surface of what we know about the natural world.

The wasp-ant-butterfly system is covered in a long list of natural peculiarities on http://bizarrecreature.blogspot.com/

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