Arthropoda: Myriapoda

Alisa L. Newton1,2 and Elise E.B. LaDouceur3

1 Wildlife Conservation Society, Bronx, NY, USA

2 Disney’s Animals, Science and Environment, Orlando, FL, USA

3 Joint Pathology Center, Silver Spring, MD, USA

10.1 Introduction

Phylum Myriapoda consists of 13 000 species divided into four classes: Chilopoda (centipedes), Diplopoda (millipedes), Symphyla (garden centipede, pseudocentipede), and Pauropoda (millipede‐like arthropods). All four classes are terrestrial. Their range varies from temperate to tropical, moist forest floor to grasslands, and semi‐arid to arid habitats. Symphylans and pauropodans are typically microscopic and live in the soil. Most chilopods are nocturnal and some are intertidal (Ruppert et al. 2004). All are dioecious and fertilization is internal with sperm transfer indirectly via spermatophores.

Diet varies among phyla. Chilopods are predatory and venomous, consuming predominantly small arthropods but also earthworms, snails, and nematodes. Larger scolopendropmorphs can consume small vertebrates including amphibians, snakes, lizards, birds, and mice. Diplopods, symphylans, and pauropods live primarily on leaf litter and soil debris and are important environmental detritivores, enhancing decomposition of dead plant material. A few do consume live vegetation and one family (siphonophoridae) have a modified gnathochilarium to pierce live plants and feed on juices. One species (Nematophora) has a carnivorous/omnivorous diet and feeds on harvestmen, hexapods, centipedes, and earthworms. Chilopods have defensive adaptations in addition to their poison forcipules (fangs). These include anal legs that can be used to pinch. They may also autotomize limbs to escape predators and replace lost legs with the next molt. Most scolopendromorphs and geophilomorphs have repugnatorial glands on the ventral aspect of each segment and some have coxal organs, both of which make defensive secretions.

Myriapods are used as experimental models of research in the genetic control of segmentation. Conservation risks are similar to other terrestrial species and include habitat destruction and collection of species for the pet trade. Like most terrestrial invertebrates, conservation programs and environmental impact assessments often pay little to no attention to the subterranean habitat. Millipedes live from one to 10 years depending on the species (Ruppert et al. 2004).

10.2 Gross Anatomy

Myriapods share a similar body plan. They have a head and an elongate body (trunk) composed of numerous individual segments and lack the defined thoracic or abdominal divisions present in hexapods. Each segment is composed of a dorsal tergite and ventral sternite that are connected laterally by paired pleurites. The cuticle is heavily calcified and typically each segment is either cylindrical or slightly flattened due to lateral projections from the tergites (paranotum). Segments bear one (Chilopoda, Symphyla, Pauropoda) or two (Diplopoda) pairs of legs depending on class. Each diplopod segment is derived from the fusion of two segments (diplosegment), hence the paired legs on each ring. Chilopods range from 4 to 6 cm in length, though they can be as small as 1 cm to as long as 30 cm (Scolopendra gigantea). They have between 15 and 191 pairs of legs and the pairs are always odd; no “centipede” truly has 100 legs. Diplopods range from 2 mm to 30 cm in length and have up to 710 legs. Symphylans are small, typically 2–10 mm long, with the exception of Hanseniella magna which reaches up to 30 mm in length. They typically have 15–24 segments, of which only 10 or 12 segments bear one set of legs. The last segment lacks legs but has a pair of spinerettes (cerci) (Minelli and Golovatch 2001). Most pauropods are microscopic, measuring only 0.5–0.7 mm long. The largest are only 1.9 mm long. The trunk typically bears 5–6 segments and only 8–11 pairs of legs on each (Minelli 2011).

Schematic illustration of a transverse section of a diplopod with the segments of the appendages labeled.

Figure 10.1 Schematic of a transverse section of a diplopod with the segments of the appendages labeled. H, heart; MG, midgut; MT, Malpighian tubules; PtF, parietal fat body; PvF, perivisceral fat body; S, spiracle; T, tracheae; VN, ventral nerve cord.

Each walking leg has eight identifiable segments; from anterior to posterior these are the coxa, trochanter, prefemur, femur, tibia, tarsus 1, tarsus 2, and claw (Figure 10.1). The appendages of the first segment are enlarged forcipules (maxillipeds, poison fangs) in centipedes. They are curved toward the ventral midline and have a terminal pointed fang that connects via a duct to the poison gland which is typically in the telopodite. The terminal pair of legs on the last trunk segment are also modified and not used in locomotion. These can have sensory, defensive, or aggressive adaptations, including pincers.

A feature of all myriapods is the genital opening. In symphylans, it is between the coxa of the fourth segment. In pauropods, the opening is between the second pair of legs (Minelli 2011). In centipedes, the last two segments are the pregenital and genital segments. The genital segment bears a telson. Either segment may have a pair of gonopods, which in female lithobiomorphs and scutigeromorphs are used to manipulate the eggs after laying. In diplopods, the first segment is legless and referred to as the collum. The second through fourth segments have only a single pair of appendages and are not considered diplosegments. In most taxa, one or both legs of the seventh ring are gonopods, modified to transfer sperm from the male to the female. The body terminates in the telson with a ventral anus present. Some of the posterior segments are similarly limbless.

The head has one pair of antennae. In millipedes, antenna are divided into eight segments with four chemoreceptive cones at the tip of the eighth segment. Centipedes and millipedes have simple lateral eyes. Symphylans and pauropods have no eyes. All myriapods lack median ocelli. The head is composed of multiple complex segments. In chilopods, this includes a dorsal cephalic shield, labrum (upper lip), paired mandibles, and a lower lip formed from either the first or second maxillae. The coxa and sternite of the forcipule form a plate‐like coxosternite that covers the ventral surface of the head. In diplopods, the head consists of a domed dorsal shield and ventrally two pairs of mandibles and the gnathochilarium, which may represent the fused first and second maxillae. The broad flattened plate is attached to the posterior ventral surface of the head. It has distal sensory palps that form the floor and posterior wall of the preoral cavity. Six short sensory structures (apical pegs covered with chemoreceptive sensilla) are also present. Along the medial edges of the mandibles there are teeth and a rasping surface.

The digestive system begins with a preoral cavity formed by the various maxillary and mandibular plates. This connects to a soft hypopharynx and extends as a single long tube along the middle of the coelom. Segments of the intestinal tract consist of a foregut (pharynx and esophagus), midgut, and hindgut (Figure 10.2). The foregut makes up approximately two‐thirds of the gut. The hindgut is extremely short and the midgut forms the majority of the remaining tract. The heart lies on the midline dorsally within the coelomic cavity (Figure 10.3). It extends the length of the body immediately dorsal to the intestinal tract. There are pairs of ostia present at each segment and in some species arteriole branches are present. The nervous system is not strongly cephalized. The syncerebrum, circumenteric connectives and small subesophageal ganglion are present and visible. A ventral nerve cord is apparent along the ventral coelom. There are paired ganglia in each segment. The openings to the respiratory tract (spiracles) are typically paired and are present in each segment, though the distribution pattern varies with each taxon and some segments may be lacking. In all but scutigeromophs, the spiracles are present in the pleural membrane immediately anterior to the coxa of the walking legs.

Schematic illustration of a longitudinal section of a myriapod.

Figure 10.2 Schematic of a longitudinal section of a myriapod. Br, brain; FG, foregut; H, heart; HG, hindgut; MG, midgut; MT, Malpighian tubules; PtF, parietal fat body; PvF, perivisceral fat body; T, tracheae; VN, ventral nerve cord.

Photo depicts the transverse histologic section of a giant African millipede at the level of the head.

Figure 10.3 Transverse histologic section of a giant African millipede (Archispirostreptus gigas) at the level of the head. Br, brain; FG, foregut; Gl, glands; H, heart; MT, Malpighian tubules; PtF, parietal fat body; SM, skeletal muscle; T, tracheae.

10.3 Histology

10.3.1 Body Wall/Musculoskeletal System

Table 10.1 provides a list of organs for histologic examination in myriapods. Myriapods require a humid environment due to the permeability of their cuticle which is not waterproof and lacks the lipid/wax characteristics of spiders and insects. The lipids present appear to repel water from the outside, rather than preventing water loss internally. The integument, particularly in the tergites, is heavily mineralized and impregnated with calcium salts. The surface is generally smooth but some taxa have ridges, tubercles, spines, or isolated bristles.

Repugnatorial glands are present in all diplopod taxa other than bristly millipedes, usually one pair of glands per diplosegment. Openings are located on the sides of the tergites or the margins of the paranota. Glands consist of a secretory sac that empties to a duct leading to an external pore. Secretory composition depends on species but may include aldehydes, quinones, phenols, iodine, chlorine, and hydrogen cyanine (Ruppert et al. 2004).

10.3.2 Integument

The cuticle rests on an epidermis (also called hypodermis), which is subtended by a basement membrane. It is generally composed of three layers: the epicuticle, exocuticle, and endocuticle (Figure 10.4). The outer epicuticle is very thin (approximately 1 μm). Internal to this is the exocuticle, which is hard and golden to brown colored. The thickness of the exocuticle varies; it is typically much thicker than the epicuticle but much thinner than the endocuticle. The thickest layer is the internal endocuticle, which is layered and eosinophilic. Multiple thin canals may extend through the cuticle to the surface (e.g., for secretions of epidermal glands) (Figure 10.5) (Minelli 1990). Flexible portions of the cuticle called arthrodial membranes are present at articulations of adjacent cuticle plates. The cuticle in these regions has loss of distinct layering and is thin (Figure 10.6).

10.3.3 Parietal Fat Body

There are two fat bodies of myriapods: one surrounds the midgut and regional viscera and is called the perivisceral fat body, and the other is immediately below the epidermis and is called the parietal fat body. Fat body cells (sometimes called adipocytes) are arranged in cords or aggregates. These cells are polygonal and have moderate anisocytosis. The nuclei are round. The cytoplasm is highly vacuolated and may contain granules or spherules (Figure 10.7). Sudan III stain has been used to demonstrate fat in the cytoplasm of these cells (Fontanetti et al. 2004).

Table 10.1 Organs for histologic evaluation in Merostomata.a

Organ system Organs
Body wall/musculoskeletal Cuticle, epidermis/dermis, arthrodial membrane, skeletal muscle, repugnatorial glands
Digestive Alimentary canal Preoral cavity, foregut (pharynx, esophagus), midgut, hindgut, rectum

Storage and exocrine glands Salivary glands, fat body
Malpighian tubules, maxillary nephridia (saccate nephridia)
Heart, hemocoel, ostia, cephalic aorta
Immune Hemocytes
Spiracle, atrium (tracheal pouch), tracheae
Brain, periesophageal ring, subesophageal ganglion, ventral nerve cord
Reproductive Male Testis, gonopore (paired)

Female Ovary, oviduct, gonopore (single)
Special senses/organs
Eye, Tömösváry organ, antennae

a Alternative names for organs are provided parenthetically, in italics.

Photo depicts the detail of the layers of the cuticle of a giant African millipede including the thin epicuticle, exocuticle, and distinct layering of the thick endocuticle. The epidermis (Epd) is below the endocuticle and is subtended by a basement membrane (arrows).

Figure 10.4 Detail of the layers of the cuticle of a giant African millipede (Archispirostreptus gigas) including the thin epicuticle (Ep), exocuticle (Ex), and distinct layering of the thick endocuticle (En). The epidermis (Epd) is below the endocuticle and is subtended by a basement membrane (arrows). SM, skeletal muscle. 400×. HE.

10.3.4 Skeletal Muscle

Myriapods have numerous skeletal muscles in the body and limbs. Skeletal muscle is typical as seen in other groups and is striated. Skeletal muscles can attach directly to the internal surface of the cuticle, or via tendons that attach to the connective tissue deep to the cuticle (Figure 10.8). The cuticle has multiple apodemes (inward growths or projections of the cuticle) to which skeletal muscles also attach. Similar to arachnids, extension of some joints is facilitated through hydrostatic pressure of the hemolymph, rather than specific extensor muscles. This hydrostatic pressure is antagonized by skeletal muscle contraction of flexor muscles (Minelli 1990).

10.3.5 Digestive System

The digestive system is composed of a foregut (pharynx and esophagus), midgut, and hindgut. Digestive enzymes are provided by buccal, mandibular, maxillary, and vesicular glands associated with the preoral cavity and pharynx (Figure 10.9). Salivary glands open into the preoral cavity in millipedes.

In chilopods, prey items are held by the second maxillae and the forcipules and are manipulated by the mandibles and first maxillae. Some centipedes masticate prey, others pull the prey into the gut using the pharynx. Geophilomorphs have small teeth and partially digest their prey in the preoral cavity prior to ingestion.

Photo depicts the body wall of a giant desert centipede. The epidermis (Epd) is a simple layer of columnar cells interspersed with occasional glandular, polygonal cells (arrowheads). The epidermis rests on a basement membrane (BM) and is covered by a multilayered cuticle (C). Multiple channels (arrows) traverse the cuticle representing ducts for secretions of the glandular cells.

Figure 10.5 Body wall of a giant desert centipede (Scolopendra heros). The epidermis (Epd) is a simple layer of columnar cells interspersed with occasional glandular, polygonal cells (arrowheads). The epidermis rests on a basement membrane (BM) and is covered by a multilayered cuticle (C). Multiple channels (arrows) traverse the cuticle representing ducts for secretions of the glandular cells. SM, skeletal muscle. 400×. HE.

Photo depicts the arthrodial membrane between two tergites in a giant African millipede. The arthrodial membrane (arrowhead) is thinner than the surrounding cuticle with loss of distinct layering.

Figure 10.6 The arthrodial membrane between two tergites in a giant African millipede (Archispirostreptus gigas). The arthrodial membrane (arrowhead) is thinner than the surrounding cuticle with loss of distinct layering. The epidermis (Epd) is artifactually separated from the cuticle. 200×. HE.

Digestion and absorption occur in the midgut. The midgut creates a peritrophic membrane to surround food, similar to hexapods and crustaceans. Surrounding the midgut is tissue referred to as the perivisceral fat body that appears similar to chloragogen tissue of annelids. The cells store glycogen and may play a role in excretion by storing/sequestering toxins. This fat body also stores lipids, protein, and uric acid.

In pauropods, the digestive tract consists of a preoral cavity with paired salivary glands. This transitions to the ectodermal foregut which extends to the third segment. The midgut is entodermal and amuscular with a diameter typically twice of the foregut. It begins at the fourth segment, extends to the 10th segment and may be convoluted or straight. At the 10th segment it narrows and becomes a short winding tube before the rectum. The rectum is characterized as ovoid, with an outer circumferential muscle layer and inner longitudinal muscle layer (Minelli 2011).

Photo depicts the parietal fat body in a giant African millipede. Large cells with a central or an eccentric, small, round nucleus and heavily vacuolated cytoplasm are separated into lobules by a fine fibrovascular stroma.

Figure 10.7 The parietal fat body in a giant African millipede (Archispirostreptus gigas

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Nov 28, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on Arthropoda

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