Greatest sexual dimorphism for a beetle
Platerodrilus , Trilobite beetles
1:12 ratio
Not Applicable ()

The beetles that exhibit the greatest sexual dimorphism (i.e., morphological differences between the sexes within a single species) are the trilobite beetles of the genus Platerodrilus (=Duliticola). Adult males of species in this genus are winged, can measure as little as 5 millimetres (0.2 inches) in total length, and resemble typical beetles. Conversely, the adult females are wingless and can measure up to 60 millimetres (2.4 inches) in total length – a 12-fold length disparity between male and female. Females are neotenous, resembling worm-like beetle larvae their entire life, even as adults. Also unlike the males, the females bear armoured spine-edged plates dorsally, most prominently upon the head, which lend them a resemblance to the prehistoric arthropods known as trilobites, hence this insect's common name. They inhabit tropical rainforests in India and south-east Asia, including Borneo, and the females are often very brightly coloured, unlike the drab, inconspicuous males.

For almost a century after the first discovery of adult (but larva-resembling) female trilobite beetles in 1831, their true identity remained a mystery to entomologists, as it was mistakenly assumed that they were the larvae of some still-undiscovered insect species. In 1925, however, following more than two years of intensive study and investigation both in the laboratory and in the field, Borneo-based Swedish zoologist Eric Mjöberg discovered not only that these larva-like insects were actually adult females but also that the hitherto-undiscovered male was totally different and much smaller in appearance.