Looking at a spider's belly

 I posted this photo a couple of days ago, and now want to look at it again, closely.

Araneus diadematus

These are beautiful spiders, and I mostly look at the patterns on the top of their abdomen; white patches on orange or brown, these patches arranged to form a cross. Here, she hangs quietly in her web, allowing me to see her underside, and even get it all more or less in focus.

She's a female. First, her pedipalps, the two spiky, curved appendages in front of her fangs, are slim. Mature males have a large "boxing glove" tip to theirs.

And then, near the forward end of her abdomen, she has an appendage called the epigyne. (The cream-coloured, curvy hook crossing the border between yellow and dark brown.) This aids in mating, holding the male's pedipalp in place, and later serves as an ovipositor, the egg-laying organ.

The eight legs are all connected to the front half of the body, the cephalothorax, in a circle around the sternum (in humans this word is used for the breastbone; spiders have no internal bones). They are moved by hydraulic pressure regulated inside the cephalothorax.

At the rear end of the abdomen, you can see her spinnerets. Silk from glands inside her abdomen is squeezed out through these and hardens quickly to form her web.

Our Araneus diadematus has three pairs of spinnerets. Each pair produces threads with specific characteristics. For example, the web’s orbital or circular threads are sticky, good for snaring prey. Others, like the radial threads linking the hub to the periphery of the web, are extra strong but are not sticky. (Nature'sDepths/wondrous-webs)

I found this photo of Araneus diadematus spinnerets, taken with an scanning electron microscope:

From JeremyPoolesem.org.uk

This page also has photos of the epigyne, the pedipalps, and other details of this spider. Go look!

~~~~~~~~~~

Subí esta foto el otro dia, pero quiero volver a mirarla con más atención.

Foto: Araneus diadematus

Estas son unas arañas hermosas, y por lo general estoy mirando los diseños en la parte superior del abdomen; manchas blancas sobre un fondo en anaranjado o café, las manchas organizadas en forma de cruz. Aquí en esta foto, se quedaba quieta en su telaraña, mostrándome la parte inferior de su cuerpo.

Es una hembra. Esto se sabe por la forma de sus pedipalpos, esos dos apéndices con espinas, en frente de sus quelíceros; su extremo es delgado; los pedipalpos de arañas machos maduros son inflados; se les ha comparado con los guantes de un boxeador.

Y además, cerca del frente de su abdomen, se ve el apéndice llamado el epigino, que es el ganchito que cruza el borde entre lo amarillo y lo café en la foto. Este apéndice ayuda en la cópula, deteniéndo el órgano del macho, y luego sirve de ovipositor.

Las ocho patas están conectadas a la sección delantera de la araña, el cefalotórax y rodean el esternón. (En humanos, el esternón es el hueso en el centro del pecho. Las arañas no tienen huesos.) Estas patas se mueven a base de presión hidráulica, regulada en el interior del cefalotórax.

En la parte trasera del abdomen, verás las hileras. La telaraña se produce en glándulas dentro del abdomen y el líquido se extruye desde las hileras. Al llegar al exterior, se endurece.

Nuestra Araneus diadematus tiene tres pares de hileras. Cada pareja produce hilos con sus características particulares. Por ejemplo, los hilos orbitales o circulares son pegajosos, sirviendo para atrapar la presa. Otros, como los hilos que unen el centro de la telaraña a su perímetro, son muy fuertes pero no son pegajosos. (de Nature'sDepths/wondrous-webs)

Encontré esta foto de hileras de Araneus diadematus hecha con el microscopio electrónico:

Foto de JeremyPoolesem.org.uk 

Esta página tiene también fotos de los pedipalpos, del epigino y otras partes de la araña. ¡Hay que verlas!

The lure of plastic

A male Philodromus dispar spider moved into my spider house while it sat outside with the lid open, airing out. I closed it up and brought it in, but after a couple of days, the spider was unhappy with the arrangement. He kept running and running and running, never stopping, for hours. Looking for a mate, I guessed, and opened the top for him. He scampered up and out; I let him go.

On my desk, there's a plastic bottle of aerated water that I use to spray critters that need to be kept damp. When next I saw "Philo", he was climbing up the bottle. When he got to the top, he stopped abruptly, put his pedipalps down against the plastic, his rear in the air, not a customary pose for these active spiders. And then he just stayed put there, as though his pedipalps had been glued down. I shooed him along, and he moved an inch or two and repeated the pose, pedipalps to the plastic.

"Interesting stuff! Smells like ..."

He's a mature male, shown by the large "boxing glove" ends on his pedipalps*; he uses these to transport sperm to the female's genital pore. But why does he want to make contact with this particular plastic? Is there some chemical in the plastic that reminds him of a female's pheromones?

Now that I think of it, was it the smell of the plastic, just outside his house, that drove him frantic in the first place?

Questions, questions. I haven't found an answer.

After a bit, since he wouldn't leave the top of the bottle on his own, I shook him off by the outside door. In the morning, I found him on a vinyl chair in the kitchen.

*Alex Webb has an astounding photo of a male black widow's pedipalps; go take a look!