I have a particular interest in insect scale fine structure, especially when used as test subjects for microscope development in the 19th century. An early test for lower power achromatic optics were silverfish scales (Lepisma saccharinum, formerly L. saccharina), but these are quite undemanding to resolve and test subjects such as diatom frustules became established for higher powers. When assessing the single lens optics supplied with an 1830s Cary-Gould type pocket microscope, I found the variation in scale fine structure useful for quantifying their low NAs after 'calibrating' a strew with modern optics.
I recall seeing silverfish regularly in my youth where they were often seen scurrying from under the side boarding of the bath in a pre-centrally heated family house. I have never seen them since and to study the scales a kind reader sent me a live example some years ago.
In the past year or so have found an occasional critter scurrying out of the airing cupboard which houses the hot water cylinder. The cupboard remains warm throughout the year so towels and bedding are stored there as well as a useful place to allow homemade bread dough to rise. Initially was pleased to see a local silverfish colony available to study but they were not silver and seemed larger than remembered. The habitat also did not seem correct as they typically prefer cool and humid places. I wondered if they were another member of the so-called bristletail family, the firebrat (Thermobia domestica) which do prefer warmer habitats and also more tolerant than silverfish of low humidity which the airing cupboard has. So a particularly larger example found recently was duly studied live.
The video below shows that unlike the silverfish it did not move with a fish type movement. The body was ca. 12 mm long, the antennae 16 mm and longest tail 12 mm. Some of the pest resource websites note that firebrats do have antenna longer than the body. A reference (1) in the Wikipedia entry for the 'Firebrat' cites a 1992 paper by Pedro Wygodzinsky. This includes many species not found in the UK but the key is useful to see what features are used to differentiate between the silverfish and firebrat. A smaller example had been preserved in the past so had a stab at insect dissection in order to use the key.
The antenna are distinctly longer than the body, one feature that is noted to distinguish a firebrat from a silverfish.
The body also has a mottled look rather than the grey-silver typical of a silverfish.
The key diverges splitting the two species by whether the macrochaetae are 'glabrous' (Lepisma) or 'barbed or feathered' (Thermobia). These features did not survive the softening in lactic acid but they were inspected in the live specimen and were barbed. Further species in the key were not British. At the final key out to Thermobia domestica it states the maxillary palps to have six segments not five. To my eye it seems to have five unless have not counted one segment underlying the body (image above).
The number and grouping of 'bristle combs' on the 'abdominal terga' were also a feature keying out T. domestica from other US /Caribbean species but unclear what they were, my poor dissection not helping. If a further specimen appears will study these features on a freshly preserved undissected specimen.
It's interesting to note that scale shape and/or fine structure did not feature in the key. Scales are readily shed and can be collected from a live specimen by allowing it to shimmy under either a coverslip or upturned slide. The former presenting scales uppermost the latter for the undersides uppermost. The variety of shapes and fine structure did seem to be very similar to that of Lepisma.
Zeiss 10/0.22 and 16/0.4 phase objectives.
The upper left image shows a section of exoskeleton after softening that retained a cluster of scales. The other two from dry mount scale strews transferred from the live specimen using a coverslip so scales are uppermost.
Part of one of the bristletails.
Comments to the author David Walker are welcomed.
Footnote added January 24th 2024. One of our contributors Dr. Andrew Chick who is a Lecturer in Forensic Entomology (University of Cumbria) has offered some valuable information on the species of bristletails currently found in the UK. He had a letter published in the journal Zootaxa in 2018 entitled 'A Revised Checklist of the UK Silverfish (Zygentoma: Lepismatidae).' He notes that two additional species have been recorded in the UK and offers a key for the now four species. For non-affiliated readers a copy for personal use only can be obtained from the Micscape Editor. The letter also cites valuable references including the now public domain key published by the Royal Entomological Society in 1954.
References
1) Wygodzinsky, Pedro (1972). "A Review of the Silverfish (Lepismatidae, Thysanura) of the United States and the Caribbean Area". American Museum Novitates. American Museum of Natural History (2481). Cited by Wikipedia 'Firebrat' entry. Their link is no longer valid but the Internet Archive holds a copy.