Chordeumatida - an overview | ScienceDirect Topics (2023)

Life-Forms

At least five morphological body plans and a few life-forms can be distinguished among millipedes (Golovatch and Kime, 2009). The pin-cushion millipedes (ie, the representatives of the worldwide distributed order Polyxenida) are soft bodied, very small (usually <5mm long), extremely bristly, swift, and capable of withstanding much drier conditions than most other Diplopoda. Many polyxenidans live under loose tree bark or are characteristic inhabitants of microcaverns and small crevices under stones, in the uppermost soil, in litter, and in similar substrates.

The pill millipedes (the largely Holarctic order Glomerida and the Afrotropical and Indo-Australian order Sphaerotheriida) are collectively termed “rollers” because of their ability to roll themselves into mostly glossy balls. Pill millipedes are generally litter dwellers, but some smaller Glomerida are either troglobionts or geobionts.

The colobognathans (ie, the “sucking millipedes” with reduced mouthparts) and some Chordeumatida, which mostly possess flexible, worm-like, strongly tapered bodies, and shorter legs, are termed “borers.”

Many Polydesmida and numerous Chordeumatida, often conspicuously ornamented and relatively shorter, long-legged, and displaying more or less strong paranota (wing-like dorsolateral projections of the diplosegments), are referred to as “wedge types” and are characteristic of forest litter.

The lifestyle, or ecomorphological type, most common and widespread among Diplopoda is that of “bulldozers” or “rammers.” Their long, cylindrical, hard body with numerous diplosegments (hence, numerous pushing legs) penetrates the substrate like a bulldozer, using the broad head as a ram. Most juliform millipedes (Julida, Spirobolida, Spirostreptida, Callipodida, and some others) belong to this ecological type. This burrowing habit must have been critical in ensuring diplopods much or even most of their present-day high ecological and geographical performance. In particular, unlike the remaining ecomorphotypes, only juliforms, among Diplopoda, appear to have colonized virtually any suitable habitat. In fact, in Juliformia belong the few littoral dwellers and deserticoles, but this body plan also provides most of millipede diversity in troglobionts and geobionts, in addition to anthropochores. Indeed, the northernmost record of a millipede belongs to the subcorticolous European species Proteroiulus fuscus (Julida) in the forest–tundra belt of Yamal Peninsula, Russia׳s north, whereas perhaps the most characteristic deserticole among millipedes is Orthoporus ornatus (Spirostreptida) in the southern United States and adjacent parts of Mexico. Both these species are rare except in their native environments (Golovatch and Kime, 2009).

There are sound reasons to believe that, due to their capability to escape adverse conditions by burrowing in the soil, rotten logs, and similar shelters, juliforms (namely, species of the order Julida) dominate in Europe. This youngest, fully migratory nucleus of the millipede fauna shows a clear-cut inclination to dwell on open terrain. In contrast, all the remaining millipede orders display very evident geographical trends in diversification. In Europe, even the relatively uniformly distributed Polydesmida have a remarkable center of secondary diversification in Slovenia, whereas the Chordeumatida are particularly species-rich within the Atlantic climatic zone of western Europe (Kime and Golovatch, 2000; Golovatch and Kime, 2009). Furthermore, only a few species of Julida, apparently in response to the strongly adverse conditions that existed in Europe during the Ice Age, appear to have developed a reproductive strategy unique among terrestrial animals – the periodomorphosis, ie, as previously mentioned, the extension of male life by means of intercalary stadia.

Most millipede lineages are currently in a phase of rapid evolution and speciation; however, there are a few apparently relict groups (Hoffman, 1980). Polydesmida seem remarkably diverse largely due to their wedge type of burrowing allowing ecological niche partitioning mainly in the litter and at the soil–litter interface. This is particularly obvious in tropical and subtropical faunas. However, examples of troglobionts, geobionts, and myrmecophilous as well as arboricolous species are about as numerous among Polydesmida as among juliforms, whose type of burrowing and lifestyle seem the most characteristic, widespread, and ecologically progressive among all recent Diplopoda. It is forest litter that seems to have always been and still remains the main and primary environment for millipedes (Kime and Golovatch, 2000; Golovatch and Kime, 2009).

Physiological Problems Faced by Millipedes in Freshwater

As a first look and rough answer, we could consider that there are no “truly aquatic” millipedes anywhere in the world because all millipedes lack a gill or other structure to extract oxygen from water, unlike almost all other aquatic organisms. Their respiratory system is specially adapted for extracting oxygen from air. The exchange of oxygen and carbon dioxide between the cells of these arthropods and the atmosphere takes place via tracheae. The tracheal respiratory system opens via cuticular spiracles located on sternites (Blower, 1985; Hopkin and Read, 1992). Being air-breathing, most millipedes living in flooded or waterlogged soils usually come to the surface, move to upper litter layers, or even climb trees to avoid drowning.

The excretory system of millipedes is also not adapted for life in water. Terrestrial millipedes primarily excrete nitrogenous wastes as ammonia (which has to be excreted rapidly as gas) and uric acid (which is stored before excretion), with the latter requiring more energy but less water to produce and excrete. They have not evolved to release nitrogenous excretory products directly into water as liquid ammonia.

Several cases of such aquatic or semiaquatic diplopods have been documented in different localities, ecosystems, and biogeographical regions in the world. Examples are reported for species in Polyzoniida, Julida, Chordeumatida, and above all, Polydesmida.

Millipedes of Tropical Islands and Amazonian Floodplains

Although almost nothing is known about the small chelodesmid Pandirodesmus disparipes Silvestri, 1932 from Guyana, it is suspected of being semi-aquatic. It is good climber, swimmer, and glider, and several morphological features of the legs, tubiform spiracles, sternites, and hydrophobe setae strongly suggest a special ecology closely associated with freshwater, despite the absence of cerotegument and microtrichia in the spiracles (Golovatch and Kime, 2009).

Millipedes of Swamps and Rivers

Several reports of “aquatic” Polydesmida (mainly Paradoxosomatidae) were recently noted for streams in North America (Shelley, personal communication). Numerous specimens, thousands of them, were observed in the middle of a stream, covering water algae; all were alive and well as far as it could be determined. These polydesmid millipedes are sightless, and their mass migrations seem not to be deterred by crossing a stream. They cannot be considered as a truly aquatic species of millipedes, but they certainly seem to show capacity to survive in water. Paradoxosomatid species observed in the Hudson Valley were under wet soggy leaf litter all along a very small headwater stream. They are not “aquatic” but living and well adapted to damp areas close to the streams. Similar situations have been observed in Australia, and they are thought to have occurred in swamps, floodplains, and river banks in Central Europe during major floods last century (Tajovsky, 1999). European millipedes can survive in floodplains using a risk-strategy, due to a combination of high reproductive rates, dispersal, and re-immigration following catastrophic events (Adis and Junk, 2002; Golovatch and Kime, 2009).

Aquatic Australian Millipedes

Populations of two millipede species were discovered under stones submerged in a creek on the Macquarie University campus in the northern suburb of Sydney. One was a Polyzoniida belonging to the family Siphonotidae, and the second was a Polydesmida belonging to the family Pyrgodesmidae. The latter is closely related to pyrgodesmid species described in oceanic islands and Amazonian inundation forests. This was the first published report of “aquatic millipedes” in Australia. They became targets in the center of a hot debate about protection and conservation of invertebrate populations threatened by construction works of a dam (Burrows etal., 1994; Black, 1997). There was obvious risk of damage to the clay walls of the creek, where the millipedes were suspected to lay their eggs. Animals were present in numbers in and near the water from autumn to late spring, but moved to other habitats during the summer, probably burrowing into damp soil or the creek bed. They were observed leaving the water in large numbers during early winter to mate and lay eggs in cracks in the banks. They apparently breathe cutaneously, and both adult and juvenile stadia have been found totally submerged; long-term submergence is considered a normal part of their behavior (Burrows etal., 1994; Adis etal., 1998). The specimens were later identified as Aporodesminus wallacei Silvestri, 1904 (Pyrgodesmidae), and a well-documented re-description of the species was published (Adis etal., 1998). A. wallacei has also been reported from St. Helena (southern Atlantic Ocean), Tahiti, and the Hawaiian Islands. It is not known to be native anywhere in its range but belongs to the pyrgodesmid millipede fauna of Australia (Mesibov, 2012).

The modified mouthparts in A. wallacei exhibit hypertrophied, hair-shaped teeth of pectinate lamellae and reduced masticating parts (Adis etal., 1998: Figs 5-8), and the species show highly adapted features of the tegument allowing long-term resistance to water submersion (see discussion above on the cerotegument). Having both mouthparts adapted for food uptake underwater and a cerotegument for plastron respiration, A. wallacei is one of the best semiaquatic millipede examples we have, with the others distributed in deep caves.

Millipedes in Subterranean Habitats of Europe

Caves and subterranean habitats more generally are shelters for many different lineages of myriapods, particularly diplopods. Most of the cave-dwelling and strongly adapted troglobionts differ from their epigean relatives in showing striking morphological and physiological features (Shear, 1969; Mauriès, 1994, 2004; Culver and Shear, 2012). Among these adaptations to cave life, mouthparts are commonly convergently modified in Julidae, Blaniulidae, Polydesmidae, etc. in different geographic areas (Enghoff, 1985). The cave environment is sometimes considered equivalent ecologically to an island, and their insularity combined with general habitat conditions in subterranean environments appear to be highly significant components of millipede diversity and adaptation (Enghoff, 1993). Therefore, cave-dwelling millipedes are obviously adapted to hygrophily and most certainly comprise the best candidates to possess aquatic or at least semiaquatic ways of life among terrestrial myriapods.

This semiaquatic troglobiont is a stenothermal species that enters cold subterranean water bodies voluntarily as part of its natural semiaquatic behavior, which contrasts with the behavior of its relative S. hirsutipes. Taken together, the listed adaptations enable specimens to enter subterranean water bodies periodically during the day or and to remain submerged for up to four weeks in laboratory conditions; the former demonstrates the species’ tolerance and the latter its submersion resistance (Adis etal., 1997).

In Papua, New Guinea, the hygrophilous montane troglobite Selminosoma chapmani Hoffman, 1978 (Polydesmida, Paradoxosomitadae) is also highly adapted both to ecological conditions in caves in general and to total immersion in water in particular (Hoffman, 1977; Messner etal., 1996). Moreover, several other cases are strongly suspected from China or Southwest Asia where millipedes have been observed entering water without apparent difficulties (Deharveng and Bedos, 2000).