Diplura

The dipluros (Diplura, from the Greek διπλός diplos, double and ουρα oura, tail) are a class and order of hexapod arthropods very close to true insects (class Insecta). Dipluras are small animals, abundant members of the edaphon (soil biota). They receive their name due to the presence of two lateral rings at the end of their body. About 800 species are known.

Features

They are small in size (a few millimeters) although some can reach 50 mm. As is common in the interstitial fauna of the soil, they do not present noticeable pigmentation. The body surface appears more or less densely covered with setae (like hairs). The body is flattened, slender and flexible.

The head bears two very characteristic antennae, moniliform (beaded) with very recognizable tufts and each one muscled independently, an evolutionarily primitive trait that is also observed in springtails, but has been lost in insects. There are no eyes in dipluros. They are entognathous, with the mouthparts fitted in an inferior position within a groove on the head, a trait in which they coincide with other non-insect hexapods (prothurans and springtails).

The thorax is made up of three segments, as in other hexapods, which in this case are articulated in a particularly flexible way, which contrasts with the relative rigidity of the thorax of insects.

The abdomen is made up of ten segments, including the telson (last segment) that carry in a ventrolateral position small non-articulated appendages called styles, which may be evolutionary residues of ancient locomotive legs. Paired, evaginable vesicles are also present in them, which have to do with water regulation, facilitating the absorption of dew. At the end of the abdomen there are two appendages, cerci, like those observed in certain orders of true insects, and which give their name to the group. In the Campodeidae family the cerci are long and moniliform, but in the Japygidae and Anajapygidae they have evolved into short, hard pieces that in the former are true pincers.

Ecology

Diplures are part of the soil fauna, where they occur in considerable densities. Some are cave dwellers. The Campodeidae are skilled runners that also dig galleries in the ground; The others are specialists in forcing their way through pre-existing cracks and are incompetent walkers. Campodeids autotomy their cerci, in the style of lizards, detaching them when caught by a predator. The Japygidae use their hind claws as a defensive weapon and little credence is given to the interpretation that they are also useful for capturing prey. Dilures are essentially omnivores that devour organic waste, soil fungi and small animal prey.

Reproduction and development

There is no mating. Males release small spermatophores (solid packets of sperm) that females locate and manipulate, inserting them into their genital pore. The laying consists of sets of pedunculated eggs. There is no metamorphosis, the juvenile stages are similar to the adults. In some cases, parental care of juveniles has been observed. The first two stages (phases between molts) do not have the capacity to feed, which is reached, like the typical adult morphology, with the third stage. Adults continue to molt throughout their lives, which facilitates organ regeneration, and they can be very long-lived (up to two years) given their small size.

Classification and relationships

There are about 800 species that are classified into three orders and eight families. They are distributed throughout the world, except in regions with an arctic climate. Fossils have been known since the Carboniferous.

The diplures have often been classified in a taxon called Entognatha with the elipures (Protura + Collembola), based on the hidden position of the mouthparts, which contrasts with the patent one they have in true insects (Insecta, called by them Ectognatha). However, this character is surely a plesiomorphy. Molecular analyzes have shown that they are closely related to insects.

The relationships would be the following: