"fly embryo. His books on evolutionary developmental biology (evo-devo) include ""Imaginal Discs: The Genetic and Cellular Logic of Pattern Formation"" (2002), ""Quirks of Human Anatomy: An Evo-Devo Look at the Human Body"" (2009), and ""Deep Homology? Uncanny Similarities of Humans and Flies Uncovered by Evo-Devo"" (2017). In Held's view, ""Quirks"", ""Snake"", and ""Deep Homology"" form a trilogy on evo-devo. ""How the Snake Lost Its Legs"" is ostensibly organised into six chapters, but in effect into three parts. The first chapter serves as an introduction and overview. The next four chapters provide what Held calls ""the meatier aspects of evo-devo"" with"
"have been lost or become vestigial through evolutionary adaptation, adult insects have six legs, one pair attached to each of the three segments of the thorax. They do also have paired appendages on some other segments, in particular, mouthparts, Antennae and cerci, all of which are suspected to be derived from paired legs on each segment of some common ancestor. Some larval insects do however have extra walking legs on their abdominal segments; these extra legs are called prolegs. They are found most frequently on the larvae of moths and sawflies. Prolegs do not have the same structure as modern"
"move on many more legs. Terrestrial crustaceans may have a fair number – woodlice having fourteen legs. Also, as previously mentioned, some insect larvae such as caterpillars and sawfly larvae have up to five (caterpillars) or nine (sawflies) additional fleshy prolegs in addition to the six legs normal for insects. Some species of invertebrate have even more legs, the unusual velvet worm having stubby legs under the length of its body, with around several dozen pairs of legs. Centipedes have one pair of legs per body segment, with typically around 50 legs, but some species have over 200. The terrestrial"
"pair in males may be reduced to tiny hooks or stubs, while in others the first pair may be enlarged. Insects and their relatives are hexapods, having six legs, connected to the thorax, each with five components. In order from the body they are the coxa, trochanter, femur, tibia, and tarsus. Each is a single segment, except the tarsus which can be from three to seven segments, each referred to as a ""tarsomere"". A representative insect leg, such as that of a housefly or cockroach, has the following parts, in sequence from most proximal to most distal: Associated with the"
"The most diverse group of animals on earth, the insects, are included in a larger taxon known as hexapods, most of which are hexapedal, walking and standing on six legs. Exceptions among the insects include praying mantises and water scorpions, which are quadrupeds with their front two legs modified for grasping, some butterflies such as the Lycaenidae (blues and hairstreaks) which use only four legs, and some kinds of insect larvae that may have no legs (e.g., maggots), or additional prolegs (e.g., caterpillars). Spiders and many of their relatives move on eight legs – they are octopedal. However, some creatures"
"of legs. The meso- and metathoracic segments may each have a pair of wings, depending on the insect. The abdomen consists of eleven segments, though in a few species of insects, these segments may be fused together or reduced in size. The abdomen also contains most of the digestive, respiratory, excretory and reproductive internal structures. Considerable variation and many adaptations in the body parts of insects occur, especially wings, legs, antenna and mouthparts. The head is enclosed in a hard, heavily sclerotized, unsegmented, exoskeletal head capsule, or epicranium, which contains most of the sensing organs, including the antennae, ocellus or"
"In Solifugae, the palps are quite leg-like, so that these animals appear to have ten legs. The larvae of mites and Ricinulei have only six legs; a fourth pair usually appears when they moult into nymphs. However, mites are variable: as well as eight, there are adult mites with six or even four legs. Arachnids are further distinguished from insects by the fact they do not have antennae or wings. Their body is organized into two tagmata, called the prosoma, or cephalothorax, and the opisthosoma, or abdomen. The cephalothorax is derived from the fusion of the cephalon (head) and the"
"each of which bears a single pair of legs. As is typical of arthropods adapted to life on land, each leg has only a single walking branch composed of five segments, without the gill branches found in some other arthropods and with gill on the abdominal segments of some immature aquatic insects. In most insects the second and third thoracic segments also support wings. It has been suggested that these may be homologous to the gill branches of crustaceans, or they may have developed from extensions of the segments themselves. The abdomen follow epimorphic development, where all segments are already"
"all extant arachnids are terrestrial, living mainly on land. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species. Almost all adult arachnids have eight legs, and arachnids may be easily distinguished from insects by this fact, since insects have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and sensory perception. The first pair, the chelicerae, serve in feeding and defense. The next pair of appendages, the pedipalps, have been adapted for feeding, locomotion, and/or reproductive functions."
"to suck up the liquefied insides of their prey. However, they are able to eat their own silk to recycle proteins needed in the production of new spider webs. Some spiders, such as the dewdrop spiders (""Argyrodes""), even eat the silk of other spider species. Spiders typically have eight walking legs (insects have six). They do not have antennae; the pair of appendages in front of the legs are the pedipalps (or just palps). Spiders' legs are made up of seven segments. Starting from the body end, these are the coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The tip"
"adult insect legs, and there has been a great deal of debate as to whether they are homologous with them. Current evidence suggests that they are indeed homologous up to a very primitive stage in their embryological development, but that their emergence in modern insects was not homologous between the Lepidoptera and Symphyta. Such concepts are pervasive in current interpretations of phylogeny. In general the legs of larval insects, particularly in the Endopterygota, vary more than in the adults. As mentioned, some have prolegs as well as ""true"" thoracic legs. Some have no externally visible legs at all (though they"
"Metathorax The metathorax is the posterior of the three segments in the thorax of an insect, and bears the third pair of legs. Its principal sclerites (exoskeletal plates) are the metanotum (dorsal), the metasternum (ventral), and the metapleuron (lateral) on each side. The metathorax is the segment that bears the hindwings in most winged insects, though sometimes these may be reduced or modified, as in Diptera, in which they are reduced to form halteres, or flightless beetles (Coleoptera), in which they may be completely absent even though forewings are still present. All adult insects possess legs on the metathorax. In"
"Prothorax The prothorax is the foremost of the three segments in the thorax of an insect, and bears the first pair of legs. Its principal sclerites (exoskeletal plates) are the pronotum (dorsal), the prosternum (ventral), and the propleuron (lateral) on each side. The prothorax never bears wings in extant insects, though some fossil groups possessed wing-like projections. All adult insects possess legs on the prothorax, though in a few groups (e.g., the butterfly family Nymphalidae) the forelegs are greatly reduced. In many groups of insects, the pronotum is reduced in size, but in a few it is hypertrophied, such as"
"species. Individuals can range from 0.3 mm (fairyflies) to 30 cm across (great owlet moth); have no eyes or many; well-developed wings or none; and legs modified for running, jumping, swimming, or even digging. These modifications allow insects to occupy almost every ecological niche on the planet, except the deep ocean and the Antarctic. This article describes the basic insect body and some of the major variations of the different body parts; in the process it defines many of the technical terms used to describe insect bodies. Insects, like all arthropods, have no interior skeleton; instead, they have an exoskeleton,"
"Insect Insects or Insecta (from Latin ') are hexapod invertebrates and the largest group within the arthropod phylum. Definitions and circumscriptions vary; usually, insects comprise a class within the Arthropoda. As used here, the term Insecta is synonymous with Ectognatha. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; they include more than a million described species and represent more than half of all known living organisms. The total number of extant species is estimated at between"
"been debated, prolegs are not widely regarded as true legs, derived from the primitive uniramous limbs. Certainly in their morphology they are not jointed, and so lack the five segments (coxa, trochanter, femur, tibia, tarsus) of thoracic insect legs. Prolegs do have limited musculature, but much of their movement is hydraulically powered. Proleg A proleg is a small, fleshy, stub structure found on the ventral surface of the abdomen of most larval forms of insects of the order Lepidoptera, though they can also be found on other larval insects such as sawflies and a few types of flies. In all"
"of legs per segment. Around 12,000 species have been described, which may represent less than a tenth of the true global millipede diversity. The name ""millipede"" is a compound word formed from the Latin roots ""millia"" (""thousand"") and ""pes"" (gen. ""pedis"") (""foot""), although millipedes typically have between 36 and 400 legs. One species, ""Illacme plenipes"", has the greatest number of legs of any animal, with 750. Pill millipedes are much shorter, and are capable of rolling up into a ball, like pillbugs. About 200 species of them are known worldwide. They resemble centipedes but are smaller and translucent. Many spend"
"most groups of insects, the metanotum is reduced relative to the mesonotum. In the suborder Apocrita of the Hymenoptera, the first abdominal segment is fused to the metathorax, and is then called the propodeum. Metathorax The metathorax is the posterior of the three segments in the thorax of an insect, and bears the third pair of legs. Its principal sclerites (exoskeletal plates) are the metanotum (dorsal), the metasternum (ventral), and the metapleuron (lateral) on each side. The metathorax is the segment that bears the hindwings in most winged insects, though sometimes these may be reduced or modified, as in Diptera,"
"legs are without joints, while the feet have four to eight claws each. The cuticle contains chitin and protein and is moulted periodically. The first three pairs of legs are directed downward along the sides, and are the primary means of locomotion, while the fourth pair is directed backward on the last segment of the trunk and is used primarily for grasping the substrate. Tardigrades lack several Hox genes and a large intermediate region of the body axis. In insects, this corresponds to the entire thorax and the abdomen. Practically the whole body, except for the last pair of legs,"
"walking and running type insects respectively, usually have well-developed femora and tibiae on all legs, whereas jumping (saltatorial) insects such as grasshoppers have disproportionately developed metafemora and metatibiae. In aquatic beetles (Coleoptera) and bugs (Hemiptera), the tibiae and/or tarsi of one or more pairs of legs usually are modified for swimming (natatorial) with fringes of long, slender hairs. Many ground-dwelling insects, such as mole crickets (Orthoptera: Gryllotalpidae), nymphal cicadas (Hemiptera: Cicadidae), and scarab beetles (Scarabaeidae), have the tibiae of the forelegs (protibiae) enlarged and modified for digging (fossorial), whereas the forelegs of some predatory insects, such as mantispid lacewings (Neuroptera)"
"most extreme in the highly derived Muscomorpha infraorder. Some species of flies are exceptional in that they are secondarily flightless. The only other order of insects bearing a single pair of true, functional wings, in addition to any form of halteres, are the Strepsiptera. In contrast to the flies, the Strepsiptera bear their halteres on the mesothorax and their flight wings on the metathorax. Each of the fly's six legs has a typical insect structure of coxa, trochanter, femur, tibia and tarsus, with the tarsus in most instances being subdivided into five tarsomeres. At the tip of the limb is"
"hard plate called the pronotum, which is only the front part of the thorax; the back part of the thorax is concealed by the beetle's wings. This further segmentation is usually best seen on the abdomen. The multisegmented legs end in two to five small segments called tarsi. Like many other insect orders, beetles have claws, usually one pair, on the end of the last tarsal segment of each leg. While most beetles use their legs for walking, legs have been variously adapted for other uses. Aquatic beetles including the Dytiscidae (diving beetles), Haliplidae, and many species of Hydrophilidae, the"
"has seven segments: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The first, or anterior, of the five pairs of leg-like appendages are not ""actual"" legs, but pedipalps, and they have only five segments each. The pedipalps of the Solifugae function partly as sense organs similar to insects' antennae, and partly in locomotion, feeding, and fighting. In normal locomotion, they do not quite touch the ground, but are held out to detect obstacles and prey; in that attitude, they look particularly like an extra pair of legs or perhaps arms. Reflecting the great dependence of the Solifugae on their tactile"
"remainder of the body remains more or less immobile. Mantises have two spiked, grasping forelegs (""raptorial legs"") in which prey items are caught and held securely. In most insect legs, including the posterior four legs of a mantis, the coxa and trochanter combine as an inconspicuous base of the leg; in the raptorial legs, however, the coxa and trochanter combine to form a segment about as long as the femur, which is a spiky part of the grasping apparatus (see illustration). Located at the base of the femur is a set of discoidal spines, usually four in number, but ranging"
"has three pairs of segmented legs, one pair each for the three segments that compose the thorax and one or two pairs of wings. The abdomen is composed of eleven segments, some of which may be fused and houses the digestive, respiratory, excretory and reproductive systems. There is considerable variation between species and many adaptations to the body parts, especially wings, legs, antennae and mouthparts. Spiders a class of arachnids have four pairs of legs; a body of two segments—a cephalothorax and an abdomen. Spiders have no wings and no antennae. They have mouthparts called chelicerae which are often connected"
"is similar in each caste, but the soldiers have larger and heavier legs. The structure of the legs is consistent with other insects: the parts of a leg include a coxa, trochanter, femur, tibia and the tarsus. The number of tibial spurs on an individual's leg varies. Some species of termite have an arolium, located between the claws, which is present in species that climb on smooth surfaces but is absent in most termites. Unlike in ants, the hind-wings and fore-wings are of equal length. Most of the time, the alates are poor flyers; their technique is to launch themselves"
"legs. In the absence of segmentation, the positioning of the eyes, limbs and gonopore on the idiosoma provide the only locational guidance. Larval ticks hatch with six legs, acquiring the other two after a blood meal and molting into the nymph stage. In the nymphal and adult stages, ticks have eight legs, each of which has seven segments and is tipped with a pair of claws. The legs are sometimes ornamented and usually bear sensory or tactile hairs. In addition to being used for locomotion, the tarsus of leg I contains a unique sensory structure, Haller's organ, which can detect"
"the reduction and loss of them in other species. The modifications enable the insects to exploit a variety of food materials. Insect mouthparts and antennae are considered homologues of insect legs. Parallel developments are seen in some arachnids: The anterior pair of legs may be modified as analogues of antennae, particularly in whip scorpions, which walk on six legs. These developments provide support for the theory that complex modifications often arise by duplication of components, with the duplicates modified in different directions. Similar to the pentadactyl limb in mammals, the earliest dinosaurs split into two distinct orders—the ""saurischia"" and ""ornithischia""."
"surface such as the table top or chair seat. Many taxa are characterized by the number of legs: A leg is a structure of gross anatomy, meaning that it is large enough to be seen unaided. The components depend on the animal. In humans and other mammals, a leg includes the bones, muscles, tendons, ligaments, blood vessels, nerves, and skin. In insects, the leg includes most of these things, except that insects have an exoskeleton that replaces the function of both the bones and the skin. Sometimes the end of the leg, or foot, is considered part of the leg;"
"Mesothorax The mesothorax is the middle of the three segments in the thorax of an insect, and bears the second pair of legs. Its principal sclerites (exoskeletal plates) are the mesonotum (dorsal), the mesosternum (ventral), and the mesopleuron (lateral) on each side. The mesothorax is the segment that bears the forewings in all winged insects, though sometimes these may be reduced or modified, as in beetles (Coleoptera) or Dermaptera, in which they are sclerotized to form the elytra (""wing covers""), and the Strepsiptera, in which they are reduced to form halteres. All adult insects possess legs on the mesothorax. In"
"and has been studied extensively in cockroaches and ants. The legs are used in alternate triangles touching the ground. For the first step, the middle right leg and the front and rear left legs are in contact with the ground and move the insect forward, while the front and rear right leg and the middle left leg are lifted and moved forward to a new position. When they touch the ground to form a new stable triangle the other legs can be lifted and brought forward in turn and so on. The purest form of the tripedal gait is seen"
"used in flight. All four wings have branching longitudinal veins, and multiple cross-veins. The three pairs of legs are sturdy, with large coxae and five claws each. They are attached to each of the three thoracic segments. The front legs are the shortest and the hind legs the longest, providing the main propulsive power when the insect runs. The spines on the legs were earlier considered to be sensory, but observations of the insect's gait on sand and wire meshes have demonstrated that they help in locomotion on difficult terrain. The structures have been used as inspiration for robotic legs."
"of the insect leg, is usually the longest and strongest part of the limb, but it varies in size from the huge hind femur of leaping Orthoptera to a very small segment such as is present in many larval forms. The volume of the femur is generally correlated with the size of the tibial muscles contained within it, but it is sometimes enlarged and modified in shape for other purposes than that of accommodating the tibial muscles. The tibia is characteristically a slender segment in adult insects, only a little shorter than the femur or the combined femur and trochanter."