Tuesday, 17 April 2012

Having mentioned in my last post that St. Marks Flies (Bibio sp.) are out, I got this shot of a male this morning:

St. Mark's Fly - Bibio marci (male)
We can tell it's a male by the large eyes, which meet in the middle. In Bibios, like many other flies, the male has much larger eyes than the female, and often emerges earlier in order to establish his territory. Bibo larvae are root-feeders on many plants, especially grasses. St Mark's day is 25th April, and the flies get their name from the fact that they usually emerge around that date. I've been seeing them for about a week, so the year is still rather early. Bibios are large flies that fly around in large groups - almost swarms - often with their rear legs hanging downwards.

When I was checking the images, I noticed that I had a decent shot of a couple of interesting features:

Bibio marci head (close-up)
Firstly, we can see how hairy the eyes are. Interestingly enough, the hairs don't interfere with the fly's vision: an insect's compound eyes are made up of tubes with sensors at the inner end and a lens at the outer end. These lenses are all arranged on a curved surface, which makes them all point in slightly different directions, which is good for all-round vision. The hairs grow parallel to the tubes, and as a result have minimal impact on the vision, since the line of sight is along the hairs, not across them. These large eyes are used for navigation and for identification of mates, prey and food.

Secondly, we can see the triangle of three ocelli (auxiliary eyes, arrowed) which react very quickly to changes in light intensity, and are used to detect quick motion in order to avoid predators.

A few years ago I took a series of 'bug's-eye' views of plants. Here's a view of a Dandelion seed clock from an unusual perspective:

Dandelion seedhead
 I rather like that shot.

A few of our hoverflies have grey abdominal bands rather than the usual yellow. This is a female Platycheirus albimanus feeding on Dandelion:

Platycheirus albimanus (female)
This is by far the most common species of hoverfly in my garden at the moment.

1 comment:

Toffeeapple said...

Very good photographs yet again, especially the Dandelion clock.
I recall being bothered by a flock of those flies whilst walking through a field, they just got in the way, nothing more sinister.