34;abdominal organs" of ectoderm, characteristic of Onychophora and retained in Symphyla and Pauropoda among arthropods. In one important respect Onychophora differ from adult arthropods, namely, in the wide division of the nerve cords.
ARTHROPOD INTERRELATIONSHIPS
Some writers have suggested a separate origin of the millipedes from the primitive Onychophora, but the mandibular arthropods are too clearly a monophyletic group not to have had a single origin. Although all arthropods are thought to have originated in an aquatic stock, we have no evidence of how and when the terrestrial myriad pods and insects emerged from the water.
BODY SEGMENTATION
With the acquisition of flight organs, they left behind their crawling relatives and assumed the leading role in arthropod evolution. They have overpopulated the earth with their offspring, who have come into competition with the offspring of.
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SCLEROTIZATION AND SCLERITES
Calcium sclerotization occurs mainly in crustaceans and diplopoda, probably in combination with protein sclerotization; portein sclerotization is characteristic of the insects. Then finally there is the question of how sclerites were laid down in the first place in accordance with the animal's mechanical needs.
SCLEROTIZATION OF THE BODY SEGMENTS
Apparently there must exist a prior differentiation in the epidermis that determines the pattern of sclerotization. If there is really a replacement of the skeleton of one body region by expansion of that of another, there should be a corresponding expansion of the epidermis.
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2 ARTHROPOD MECHANISM — SNODGRASS IQThe carapace of Cumacea usually covers the first three, but the carapace of Cumacea usually covers the first three, but sometimes more, of the thoracic segments. Ventrally, its wide lateral wings are connected to the lower edges of the laterals by means of submarginal flanges.
NO. 2 ARTHROPOD MECHANISMS — SNODGRASS 21 Sphaeropoeus the thickened mesal margins of the spiracle plates stand
This evidence of a subcoxal origin of the pleural plates, as presented by Heymons and by Roonwal, does not necessarily prove, as Heymons admits, the idea that the subcoxal elements were once the functional basal segments of the legs, although Heymons claims that the embryological evidence is in favor of the theory. The pleural sclerotization of a winged thoracic segment of adult insects generally covers most of the lateral wall of the segment. Characteristically, it is marked by a vertical or oblique median groove that internally forms a strengthening ridge from the coxatothewing, ending below in a coxalarticular process and above in a fixation wing (WP) that supports the base of the wing.
The pleuron of the wingless prothorax, or that of a future alate segment of the nymph (Fig. 8D) or larva (F, G), bears a coxal articular process, but lacks the peculiar features associated with the wing. However, the pleural sclerotization of the thorax in adult insects undergoes different modifications in the different orders, and may be divided into subsidiary parts, or reduced if the wings are small or suppressed.
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INTERSEGMENTAL MECHANISMS
Chilopoda.—The centipedes differ from most other arthropods in that they make lateral undulating movements of the body when running around objects that block their paths. The pleural regions of Scolopendra (G) are mostly occupied by sclerites surrounding the bases of the coxae. The whole body is soft and flexible; a dead specimen can be stretched slightly by a flattening of the segmental plates.
The leg movements and accompanying body swings by chilopods are fully described and illustrated by Manton (1952b). The typical longitudinal abdominal muscles of insects (fig. 11 A, rmcl) retract the segments in the usual way.
NO. 2 ARTHROPOD MECHANISMS — SNODGRASS 3I tion is produced by a reversed muscle on each side (dlmcl) arising on
Although there is only a relatively small amount of movement between successive segments, the julids can bend themselves into a spiral due to the large. The ball-and-socket nature of the intersegmental joints also allows a small amount of transverse rotation of the segments relative to each other. Crustacea.- The main body movements of the malacostracan Crustacea are dorsoventral flexions of the abdomen on the thorax, and of the abdominal segments on each other.
Movements of the segments on each other are therefore limited to flexion and extension in a vertical plane. Because of this intersegmental structure, the animal is able to propel itself backwards in the water by sudden, powerful downward and forward strokes of the broad-tailed fan, which shrimps and lobsters also do.
NO. 2 ARTHROPOD MECHANISMS — SNODGRASS 35 segments are strongly arched and overlap each other from before
FOLDING AND ROLLING ARTHROPODS
When the ventral and the dorsoventral muscles contract simultaneously, the bottom of the body is both shortened and lifted, resulting in a ventral flexion of the body, or in the role of those species that bend into a ball. Several other adaptive modifications of the skeleton are shown by Gruner to correlate with role. This plate is the tar gum of the thirteenth body segment, counting the column (G, i) as the first.
The housing of the head and the column does not depend so much on the degree of rolling. Instead of rolling, which is accompanied by the separation of the dorsal parts of the dorsal plates, the mite becomes a ball by collapsing.
TAGMOSIS
Gruner (1953) fully described the structure, body musculature and rolling mechanism of Glomeria, and Manton (1954) comprehensively analyzed the rolling mechanism of Sphaerotherium and Glom-. The dorsal skeleton of one of these mites (Fig. 141), as described by Grandjean (1932), is divided into five plates. The first is the thyroid lobe, the aspis, which covers the gnathosome, the genital area, and the legs.
So while the trilobite, the isopod and the diplopod adopt the spherical shape by folding or rolling the body onto itself, the mite achieves the same thing by compressing the body into a ball.
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In cultivars that first develop into larvae, differentiation of the thorax may be delayed until the pupal stage. The structure of the embryo therefore does not necessarily reproduce the shape or appearance of a primitive insect; It is only thanks to its fundamental organization that the embryo can be taken to repeat evolutionary development. The large size of the head of the early arthropod embryo is probably due to the premature growth that hastens the development of the brain and optic lobes, just as the head of a vertebrate embryo is disproportionate to the size of the body , and does not mean that early vertebrates were animals with large heads.
If the embryo leaves the egg at a very early stage of development, as in most crustaceans, it must immediately be equipped to lead an independent life. From these few examples of head structure, it is clear that there is no standardized arthropod head with a uniform composition.
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THE SEGMENTAL APPENDAGES
In the evolution of the arthropods, however, it was quickly found that fewer appendages could equally well serve for locomotion, and that the others could be usefully converted into organs for grasping, feeding, swimming, mating, egg-laying, and numerous other purposes . To their polypodance ancestors, modern arthropods thus owe their equipment with a wide range of anatomical tools, and the fact that they have thus become the most diversified animals on earth today. Arthropod appendages, regardless of their adult form, all have the same origin in the embryo, namely from paired, lateroventral, bud-like lobes of the body segments (Figs. 15A, 17A).
The embryonic development of the arthropod appendages appears to be a case where recapitulation can safely be invoked in support of the idea that the embryonic origin and growth of the appendages recapitulates their phylogenetic history. The legs of Onychophora likewise begin in the embryos as simple buds (Fig. 17G), but since the onychophoran ancestors did not acquire a sclerotization of the cuticle, their legs of necessity remained short, thick, and unsegmented.
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These smaller parts of the limbs have been counted by some writers as "segments," thus confusing true segmentation and segmental nomenclature. For example, the subparts of the tarsus are tarsomeres, but taxonomists usually refer to them as "tarsal segments".
NO. 2 ARTHROPOD MECHANISMS SNODGRASS 47 ends of the adjoining segments are specifically articulated on each
In some cases the coxa is fixedly attached to the body, and the second segment then becomes the functional basis of the limb. The eight apparent segments of the trilobite leg (Fig. 19A) are not of equal length, with the exception of the very small apical pretarsus with three claws. Typically, eight segments are distinguished in the legs of a modern pycnogonid (C). After the coxa follow two short trochanteral segments (iTr, 2Tr), a long femur (Fm), a patella (Pat), a slender tibia (Tb), a subdivided tarsus (Tar) of two tarsomeres, and a three-clawed pretarsus .
These same eight segments are present in some legs of solpugid arachnids (D) and in Acarina, although in the latter the second trochanter is much reduced. Malacostracan Crustacea (F) and millipedes (G, H, I) have legs with seven segments, with two segments in the trochanter region.
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In pycnogonids, arachnids, and insects, a pair of accessory lateral claws arise from the base of the dactylopodite, thus forming a three-clawed foot or a two-clawed foot if the middle dactyl is suppressed. The typical pretarsus of adult insects is biclawed, but in some apterygote insects the median dactyl is preserved. In general, the body of the insect's pretarsus is constructed as an adhesive lobe or pad, the arolium, which projects between or below the claws, enabling the insect to walk on vertical surfaces.
A similar adhesive layer is present on the legs of ticks and on the legs of the pselaphognathous diplopod Polyxenus (the latter described by Manton, 1956). The continued existence of dorsal musculature in hemriapods and insects is difficult to explain functionally, but there must be a phylogenetic relationship between these two groups of mandibulates.
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THE INSECT WINGS
An idea once proposed is that the wings being flat folds of the body wall sufficiently provided with tracheae, are derived from the tracheal gills, but this theory involves a circuitous path of wing development. According to another theory, that of Goldschmidt (1945), based on wing deformations in the Drosophila fly, the wings are homodynamos with legs. Raw (1956) says of the parapodia "it is improbable that between the ancestral polychaete and the Insecta they ever ceased to be appendages moved by muscles." It should be noted, however, that the musculature of the worm moves the parapodia back and forth, and that the up and down movement of the wings of an insect.
If the first rigid paranotal lobes have become flexible at their bases, they can be flapped up and down by action of pectoral muscles, thus enabling the hovering insect to keep itself longer in the air. However, even this simple wing movement involved adaptive modifications of the thoracic skeleton, and some adaptation in the musculature.
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Thus, the muscles attached to the mesad of the fulcrum became wing elevators (B), and those on the side of it became wing depressors (C). Most of the wing muscles are thick cylinders of fibers attached to the wing bases by pre-closed cuptendons. The pleural ridges are greatly strengthened to withstand the double pull of the muscles at the wing bases.
24.—Development of postnotal plates and phragmata in the evolution of the indirect type of insect wing mechanism. In some of the higher insects there are a number of small muscles attached to the axillary sclerites of the wing base.
NO. 2 ARTHROPOD MECHANISMS — SNODGRASS 67 which the wing of most insects with an indirect motor mechanism
2 LORDPOD MECHANISMS — SNODGRASS 67which is the wing of most insects with an indirect motor mechanism. mobility of the thoracic segments on each other. Most insects move the two wings on each side in unison, and generally there appears to be a tendency to reduce the size of the hind wings. This has resulted in an economy of the motor apparatus by permitting a reduction of the metathorax and its.
There is little doubt that the ancestors of the Diptera were four-winged insects, and there must have been some advantage in the change to the two-winged condition by reducing the hind wings. Finally, there are the so-called veins of the wings, which, being reinforcing ribs, must be important in the wing mechanism.
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The early embryonic stages of Peripatopsis, and some general considerations concerning the morphology and phylogeny of the Arthropoda.
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