Silverfish samples were collected from several museums and libraries. Some of them were collected by hand picking or insect aspirator and others were trapped by blunder traps. The silverfish were captured in areas where temperature and humidity were controlled around 20-30°C and 50-60% RH, respectively. Trapped specimens were removed from traps with hexane and stored in 99% ethanol. They were used for molecular examination. Alive individuals were bred in humidity-controlled cages. Some of them and their descendants were treated and stored in 80% ethanol and used for both morphological and molecular examinations.

DNA was extracted from some legs, antennae or caudal appendages separated from the body using a Qiagen DNeasy Blood and Tissue Kit (Qiagen, Inc., Valencia, California, USA). The specimens used for DNA extraction were deposited in the collection of Tokyo National Research Institute for Cultural Properties (TBK), Tokyo, Japan. Polymerase chain reaction (PCR) amplification of COI was carried out using a TaKaRa PCR Thermal Cycler Dice (Takara Bio Inc., Kusatsu, Shiga, Japan) with an amplification programmed: 3 minutes at 95°C, 35 cycles of 20 seconds at 98°C, 15 seconds at 55°C, 1 minute at 72°C and a final extension step for 5 minutes at 72°C. A single fragment of 658 bp was amplified from the PCR amplification using the primer pair LCO1490 (5’-GGTCAACAAATCATAAAGATATTGG-3’) and HCO2198 (5’-TAAACTTCAGGGTGACCAAAAAATCA-3’) (Folmer et al. 1994). KAPA HiFi HotStart ReadyMix PCR Kit (Nippon Genetics Co., Ltd., Tokyo, Japan) was used for the enzyme. PCR products were visualised via 2% agarose gel electrophoresis with Midori Green Direct (Nippon Genetics Co., Ltd., Tokyo, Japan) to confirm their amplification. PCR products were purified with NucleoSpin Gel and PCR Clean Up Kit (Takara Bio Inc., Kusatsu, Shiga, Japan) according to the manufacturer’s instructions. Purified PCR products were sequenced by Macrogen Japan Sequencing Service (Macrogen Japan Corp.; https://dna.macrogen.com/eng/index.jsp). The DNA sequence was edited with Chromas Version 2.6.6 (Technelysium Pty. Ltd.; https://technelysium.com.au) and MEGA (Molecular Evolutionary Genetics Analysis) Version 11 (Tamura et al. 2021).

Morphological data were mainly taken from specimens stored in 80% ethanol and slide-mounted specimens, using the Leica MZ125 high-performance stereomicroscope (Leica Microsystems GmbH, Wetzlar, Germany) and Olympus BX53 (Olympus Corp., Tokyo, Japan). Specimens stored in 80% ethanol were treated in 20% potassium hydroxide (KOH) for 5-6 hours at room temperature, substances in the body were removed with glacial acetic acid/Clear Lite 3/absolute ethanol (2:2:1) and the specimens were dehydrated with a graded alcohol series before being slide-mounted in Euparal. Some specimens were stained with acid fuchsin. Photographs were taken with a Leica DFC290 HD (Leica Microsystems GmbH, Wetzlar, Germany) and Nikon D5300 (Nikon Corp., Tokyo, Japan) camera. Image J version 1.53c (U.S. National Institutes of Health, Bethesda, Maryland, U.S.A.) was used to take measurements. The final images were prepared with Photoshop CC2021 and Illustrator CC2021 (Adobe Inc., San Jose, California, U.S.A.).

Nine specimens (8 adults and 1 juvenile) of C. calvum were used for scanning electron microscope (SEM) examination. Specimens examined were stored in 80% ethanol and dehydrated with a graded alcohol series, then the specimens were air dried prior to mounting on 10 mm aluminium stubs. Specimens were held in place with carbon double-sided tape (Nisshin EM Co., Ltd., Tokyo, Japan) and coated with gold in a Quick Auto Coater SC-701AT (Sanyu Denshi Co., Ltd., Tokyo, Japan) for 60 seconds. We used an S-3700N scanning electron microscope (Hitachi Ltd., Tokyo, Japan) and photographed screen images.

The COI gene sequences of Ctenolepisma calvum 22TBK001-22TBK004 have been deposited in the EMBL/GenBank/DDBJ databases under the accession number LC719153-LC719156.