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Origami Triangular Spider




Triangular spider is a family of spider whose abdoment has triangular (or heart) shape. Its most striking anatomical features are its predatory arms. The first two pairs of legs has spikes to capture prey. More details in http://www.findaspider.org.au/find/spiders/182.htm.

Source: http://www.findaspider.org.au/

It is so cool, and I haven't seen any origami models based on this creature yet. So I decided to give it a try.

Design

First thing came to my mind was the legs placement. Intuitively, we would put them along the paper borders. So this placement came up:


To create the spikes, I will use the "comb" structure in the excess paper formed in the corners. We can easily tune the number of spikes, the length, and the spacing.


However, there is an issue. We can't put this comb structure for the second pair of legs. To overcome this, I switched the legs position so the paper's four corners can be fully utilized. As a bonus, there's a neat left-right and top-bottom symmetry.


Now for the head and body. The head much narrower than the body. This means I need to do "level shifter"-like fold as in box pleating, to narrow down the head. That process will cost its length. Therefore we can just make two flaps of the same length, and the head will have the correct length once level shifter was placed. We can simply inflate circles in the middle.

The next step is filling each polygon with the molecule. It hits me that we have hexagons, whose molecule is not easy to work with. Therefore I introduced stubs to decompose it to 2 quadrilaterals and 2 triangles. Maybe I can use this stub for other body parts, like pedipalps.

Now all polygons are either quadrilaterals or triangles. The molecules should be easy to fill with. Unfortunately the quadrilaterals will have to use gusset molecules. I typed "unfortunately" because somehow I felt that gusset molecule is not really desired in origami design. All this time checking famous designers' crease pattern, it is very rare to see gusset (except if the designer specializes in circle packing method). Perhaps it makes collapsing harder?

Anyway I decided to go with gussets. If that is the cost of maximizing paper efficiency, so be it!
Next I did some math to figure out the proportion, and fill in the molecules. Here's the final crease pattern.


I changed the comb structure a bit, because I didn't want to deal with extremely narrow triangles. It is very hard to get clean fold on those structures.

Fold

Locating the references were easy, since it is just 1/3 and 1/4 of the paper. The final result have the proportion I wanted. Although there was problem in utilizing the stub to make pedipalps. The paper got too thick in the middle flaps area, and I could no longer fold anything there. In the end I simply tuck in the stubs under the abdomen. Perhaps, on "real paper" it will be easier to work with.



This leg arragement reminds me of "Mega Metagross" from Pokemon.

At that time I didn't have good knowledge about paper. My mind simply think: thinner is better. So I used a 30 cm x 30 cm craft paper (not kraft paper) with 35-40 gsm that I bought cheaply from Daisho. I used white glue to bind the layers in the legs. In the end, I couldn't utilize the stubs, it is still tucked under the abdomen.

Remark

This was my first successful attempt at doing circle packing in systematic fashion. I am satisfied with this result. Months after finishing this model, I found out that Derek McGann made one in 2014. Checking the crease pattern, apparently the paper arrangement I did was similar to his. Perhaps an analogy of convergent evolution, where efficient designs may independently develop.


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