It's like a shirt made out of air.
This is the sleeveless version of Castelli's Core Mesh Base Layer. And as with the short sleeve version, it incorporates an update to the classic fishnet style design. The older design used the holes of what felt like real fishnet in order to trap air between the skin and the second layer of clothing.
However, with the new fully-open mesh design, it still serves to hold the jersey off of the skin. Essentially, this creates a layer of air between your skin and the jersey in order to help you maintain a constant body temperature. But, besides these thermo-regulating properties, the improvements that Castelli brings to the table are several. The mesh is made from a polyester hollow core fiber that's soft-to-the-touch, efficiently wicks away moisture, and has ample stretch. In terms of shape, Castelli calls this 'body wrap,' because it's designed to wrap the body so that the gaps are only in the mesh, not between the mesh and the body. Accordingly, the entire base layer is mesh, save the hems and the 'V' that resides just below the collar.
The Core Mesh is cut long in order to stay comfortably tucked inside your shorts and bibs. Additionally, it was constructed with minimal seams for a maximum level of comfort.
The Castelli Core Mesh Sleeveless Base Layer is available in the color White and in the sizes Small/Medium, Large/X-Large, and XX-Large.