May 2019 (Stylops, male and female flowers)

I took some photos of solitary bees that were foraging round the garden recently, and I uploaded the photos to a Facebook group to help identify them.  One of them was identified as Andrena scotica commonly known as the chocolate mining bee (from its colour rather than from its diet).  The person identifying it mentioned that it had a Stylops.  I had noticed something attached to the abdomen, but didn't know what it was.  Stylops are a rather gruesome parasitic insect with a bizarre lifestyle.  (Because the lifestyle is so bizarre, Stylops has been used as the logo for the Royal Entomological Society.) They had once been classified as beetles or true flies (Diptera), but are now considered to be in an insect order of their own – the Strepistera, meaning twisted winged.  The larvae hang about on flower heads waiting for a bee to come and pollinate the flower.  They then hitch a ride back to the bee’s nest where they burrow into the bee’s growing larvae.  Once inside the larvae they feed on the insect’s blood and go through several moults until the adult bee emerges, by which time the stylops will be located in the bee’s abdomen.  Male stylops then break out between two of the bees abdominal segments and fly off, but the female stylops stays as a pupa with just her head poking out between two abdominal plates and then emits pheromones to attract a male who fertilizes her using an opening just below her head.  Her larvae will then hatch out and leave through the same opening and drop onto a suitable flower to start the cycle all over again.  The presence of stylops seems not to affect the bee’s ability to fly, or to shorten its lifespan, but it does stop the bee reproducing, which in evolutionary terms means that it loses Darwin’s ‘struggle for life’.

Chocolat Mining Bee - Andrena scotica

Close-up of the stylops - it can just be seen poking out between
2 segments just above the tip of the bee's right wing

I have been reading a lot about Darwin recently.  When he embarked on the Beagle, he considered himself a geologist, though he had been recommended for the voyage by his botany professor who had noticed the talent of the young Darwin.  The botanical work that he pursued after the publication of ‘On the Origin of Species’ is considered at least as important as the work he did on evolution.  It is not surprising that Charles Darwin had abilities in botany as his grandfather, Erasmus Darwin wrote widely on the subject.  (While identifying some of the plant photos that I took in Australia, I came across Darwinia fascicularis – Clustered Scent Myrtle, that was named after Erasmus, rather than Charles).   One of the things that Charles Darwin discovered, was that some plants have separate male and female flowers.  This really surprised me as I thought that people would have known that long before the 1860’s when Darwin found out, especially when people like Linnaeus and Mendel had studied plants so carefully many decades before.

People knew that plants such as primroses had two different types of flower, but nobody had thought to ask why?  Darwin, of course, was doing a lot of asking why, in order to find evidence to support his theory of evolution.  Great experimentalist that he was, Darwin carefully tagged each of one type of flower with ribbons so that he could identify them after they had set seed and the petals had gone.  He found that the tagged ones set seed but none of the others did, thus becoming the first person to identify male and female flowers.  Before this, people had assumed that flowers self-pollinated and that bees visited flowers just for the nectar, they hadn't realized that the flower benefited from the encounter.  For Darwin this supported the essential facet of his theory, the ‘mechanism for change’, without which we wouldn't have all the myriad varieties of flowers that we have now.

Clustered Scent Myrtle - Darwinia fascicularis