Palpigradi is an order of very small arachnids commonly known as microwhip scorpion or palpigrades.
Description
Palpigrades belong to the
arachnid
Arachnids are arthropods in the Class (biology), class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, camel spiders, Amblypygi, wh ...
class. They are the sister group to
Solifugae
Solifugae is an Order (biology), order of Arachnid, arachnids known variously as solifuges, sun spiders, camel spiders, and wind scorpions. The order includes more than 1,000 described species in about 147 genus, genera. Despite the common names, ...
, no more than in length, and averaging . They have a thin, pale, segmented
integument
In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, Exoskeleton, shell, germ or Peel (fruit), rind.
Etymology
The term is derived from ''integumentum'', which is Latin for "a coverin ...
, and a segmented abdomen that terminates in a whip-like flagellum. This is made up of 15 segment-like parts, or "articles", and may make up as much as half the animal's length. Each article of the flagellum bears
bristle
A bristle is a stiff hair or feather (natural or artificial), either on an animal, such as a pig, a plant, or on a tool such as a brush or broom.
Synthetic types
Synthetic materials such as nylon are also used to make bristles in items such as b ...
s, giving the whole flagellum the appearance of a bottle brush. The carapace is divided into two plates between the third and fourth leg pair of legs. They have no eyes.
As in some other arachnids, the first pair of legs is modified to serve as sensory organs, and are held clear of the ground while walking. Often, however, palpigrades use their
pedipalp
Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among Chelicerata, chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to ...
s for locomotion, so that the animal appears to be walking on five pairs of legs. But they do not swing in phase with the walking legs, and are mostly used as legs in rough terrain. Both the nine-segmented pedipalps and the four pairs of legs end in three claws each. The first pair of legs are 11-segmented, the second and third pairs seven-segmented and the fourth pair eight-segmented.
The family Prokoeneniidae have three pairs of lung-sacs on the fourth, fifth and sixth abdominal segments, although these are not true
book lung
A book lung is a type of respiration organ used for atmospheric gas-exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open, ventral-abdominal, air-filled cavity (atrium) and co ...
s as there is no trace of the characteristic leaflike lamellae which defines book lungs. Family Eukoeneniidae have no respiratory organs at all and breathe directly through the cuticle.
Their
exoskeleton
An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...
is very weakly sclerotized compared to other arachnids, which is the reason why fossils are so rare, and go no further back than 99 million years ago in Burmese Amber.
Ecology and behavior
Species of Palpigradi live interstitially in wet
tropical
The tropics are the regions of Earth surrounding the equator, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...
and
subtropical
The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones immediately to the Northern Hemisphere, north and Southern Hemisphere, south of the tropics. Geographically part of the Ge ...
soils. A few species have been found in shallow coral sands and on tropical beaches. In Europe, they have been found in caves and underground spaces. There is one
endemic species
Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...
on the
island of Malta
Malta is an island in Southern Europe. It is the largest and most populous of the three major islands that constitute the Maltese Archipelago and the country of Malta. The island is situated in the Mediterranean Sea directly south of Italy and ...
, in the
Mediterranean Sea
The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean basin and almost completely enclosed by land: on the east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern Eur ...
, which exists only in one specific cave. They need a damp environment to survive, and they always hide from light, so they are commonly found in the moist earth under buried stones and rocks. They can be found on every continent, except in Arctic and Antarctic regions. Terrestrial Palpigradi have
hydrophobic
In chemistry, hydrophobicity is the chemical property of a molecule (called a hydrophobe) that is seemingly repelled from a mass of water. In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, thu ...
cuticle
A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...
s, but littoral (beach-dwelling) species are able to pass through the water surface easily.
Very little is known about palpigrade behavior. They are generally believed to be predators like their larger relatives, feeding on minuscule animals in their habitat. However, their chelicerae have been described as "more like a comb or brush than the forceps of a predator", and the species ''
Eukoenenia spelaea
''Eukoenenia'' is a genus of Eukoeneniidae, Eukoeneniid palpigrades, also known as microwhip scorpions, which was first described by Carl Julius Bernhard Börner in 1901.
Species
, the World Palpigradi Catalog accepted the following eighty-f ...
'' has been shown to feed on
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
("blue-green algae"). Their mating habits are unknown, except that they lay only a few relatively large eggs at a time.
Classification
Palpigradi is split into two families, differentiated by the presence of ventral sacs on
sternite
The sternum (: sterna) is the ventral portion of a segment of an arthropod thorax or abdomen.
In insects, the sterna are usually single, large sclerites, and external. However, they can sometimes be divided in two or more, in which case the su ...
s IV–VI in
Prokoeneniidae
Prokoeneniidae is a family of microscorpions in the order Palpigradi
Palpigradi is an order of very small arachnids commonly known as microwhip scorpion or palpigrades.
Description
Palpigrades belong to the arachnid class. They are the siste ...
, and their absence in
Eukoeneniidae
Palpigradi is an order of very small arachnids commonly known as microwhip scorpion or palpigrades.
Description
Palpigrades belong to the arachnid class. They are the sister group to Solifugae, no more than in length, and averaging . They hav ...
.
Two fossil palpigrade species have been described. The first one is from the Onyx Marble of
Arizona
Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...
, which is probably of
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Electrokoenenia yaksha''), belonging to the family Eukoeneniidae, is known from
Cretaceous
The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
(
Cenomanian
The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...
)
Burmese amber
Burmese amber, also known as Burmite or Kachin amber, is amber from the Hukawng Valley in northern Myanmar. The amber is dated to around 100 million years ago, during the latest Albian to earliest Cenomanian ages of the mid-Cretaceous period. Th ...
from northern
Myanmar
Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and has ...
. Older publications refer to a fossil palpigrade (or palpigrade-like animal) from the
Jurassic
The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
of the
Solnhofen limestone
The Solnhofen Limestone or Solnhofen Plattenkalk, formally known as the Altmühltal Formation, is a Jurassic Konservat-Lagerstätte that preserves a rare assemblage of fossilized organisms, including highly detailed imprints of soft bodied organi ...
in Germany, but this has now been shown to be a misidentified fossil insect.