When it comes to vials, these are the smallest of the small. These come in two lengths: 16 and 21mm.
For those who aren't familiar with these types of vials: some similar insect species can only be properly identified by their genitalia. These organs are carefully removed and stored separately from the rest of the body. Genitalia are usually stored in glycerine, requiring a vial that seals very well and is small enough to allow easy viewing.
Main body of the vial is polypropylene, which is clear and easy to see through.
Caps are silicone, which can be pierced by a pin, so that the vial can be kept alongside the insect from which they were removed. (Realistically, if the genitalia are separated from their body, good luck figuring out which bits go with which bugs.)
Come in packages of 10.