The use of the word ‘primitive’ often carries with it a sense of disparagement, an inferred inferiority and a sense that the object being described is not as advanced as perhaps it should be. When the term is used in a biological context it has a different meaning, albeit one that is still tarnished by the wider negative interpretation. Primitive organisms, at least those still in existence today, have a long pedigree but may (to our eyes) be of simple construction. Such simplicity, however, underlines that the organism has a body plan that works, a plan that has enabled it to survive through countless generations with little or no change.
The term ‘primitive’ is often used to describe the bristletails, a group of small, wingless and carrot-shaped insects with a long and ancient lineage. Included with these are two creatures that are somewhat less familiar to us now than they were a few generations ago: the silverfish and the firebrat. Of these, the silverfish is likely to be the one you have encountered yourself, a silvery fish-shaped invertebrate, just a few millimetres long that has simple long bristles on its head, with finer ones emerging from its narrow rear end. I occasionally come across them in the bathroom, where their wriggling fish-like movement further emphasises that these are something different and interesting. I remember finding them quite frequently as a child in the warmth of the airing cupboard, where the temperature and humidity was to their liking.
Silverfish are easily overlooked, their small size and nocturnal habits reason enough for them to go about their business unnoticed. As such, you will find little written about them in general books on insects, which is a shame because they are fascinating creatures with a highly ritualised mating display. In this, the male and female approach and touch one another with their antennae, periodically backing away as if unsure, before returning for further investigation. This behaviour, which may last for many minutes, is then followed by a chase before the two individuals come to a halt side-by-side, the male vibrating his tail against the female and then releasing a capsule full of sperm. The female will then collect this and use it to fertilise her eggs, of which there are many hundreds.
Related to the silverfish, but less commonly encountered today, is the firebrat. Brown in colour, and certainly less fish-like, the firebrat does best in bakeries, around furnaces and near the warmth of open fires. The widespread use of domestic central heating is probably the reason for its disappearance from most homes. Both species are regarded as pests, their liking for carbohydrates (including paper and textiles) means that they can damage a range of products.