Arachnida

“ This concentration and obliteration of somites, often accompanied by dislocation of important segmental structures (such as appendages and nerve-ganglia) , may lead to highly developed specialization (individuation, H. Spencer) , as in the Araneae and Opiliones, and, on the other hand, may terminate in simplification and degeneration, as in the Acari.}
The most important general change which has affected the structure of the nomomeristic Arachnida in the course of their historic development is the transition from an aquatic to a terrestrial life. ”

“ Naturally, therefore, we compare the arachnids with the Crustacea rather than with the Insecta. The immediate progenitors of the Arachnida appear to have possessed a head with four pairs of limbs, a thorax with three segments, and an abdomen with thirteen segments and' a telson, only six of which can be clearly shown by comparative morphology to have carried appendicular gills. ”

“ In the numerous degenerate forms simplification occurs by obliteration of the demarcations of somites and the fusion of body-regions, together with a gradual suppression of the lamelliferous respiratory organs and the substitution for them of tracheae, which, in their turn, in the smaller and most reduced members of the group, may also disappear. The Eu-arachnida are divided into two grades with reference to the condition of the respiratory organs as adapted to aquatic or terrestrial life. ”

“ It may in this connection be observed that some authors derive the book-lungs or book-leaf tracheæ of Arachnida from the gills of Limulus; hence if those of Arachnida arose from quite different and more specialized organs than dermal glands, it is not impossible that the tracheæ of Peripatus, Myriopods, and insects arose de novo, and then we need not look for any primitive structures in worms from which they arose. ”

“ But most important of the evidences presented by the trilobites of affinity with Limulus, and therefore with the Arachnida, is the tendency less marked in some, strongly carried out in others, to form a pygidial or telsonic shield—a fusion of the posterior somites of the body, which is precisely identical in character with the metasomatic carapace of Limulus. ”

“ The fact that the single pair of prae-oral appendages of trilobites, known only as yet in one genus, is in that particular case a pair of uni-ramose antennae—does not render the association of trilobites and Arachnids improbable. Although the prae-oral pair of appendages in the higher Arachnida is usually chelate, it is not always so; in spiders it is not so; nor in many Acari. ”

“ In the larger and more complex Arachnida the number of segments is fixed and constant, and though possibly no adult member of the group, owing to the suppression of one or more segments during the ontogeny, ever shows the full number at any one time, the body can be analysed into twenty-one segments. ”

“ The five classes of Articulates differ from each other in the number of legs they possess in the adult form, as follows: Hexapoda, 6 legs; Arachnida, 8 legs; Crustacea, 10–14 legs; Myriapoda, more than 14 legs; Vermes, none. This system holds for the adult form only, because some mites (Arachnida) when young have only 6 legs, and many true insects in the larva state either have no legs at all, or have additional abdominal legs which are not jointed, but membranous, and are lost in the perfect or adult state. ”

“ The Arachnida are so varied in structure that it is not easy to give characteristics common to them all, and to any general statement there are bound to be exceptions, but for practical purposes it may be said that while an insect, when mature, has only six legs, and a pair of feelers or antennae of quite different structure, Arachnids have normally eight legs, and their feeling organs are not antennae but leg-like “pedipalps.” ”

“ The proportionately enormous chelae (chelicerae) of the first pair of appendages are not provided with poison glands; their bite is not venomous.
Galeodes has been made the means of a comparison between the structure of the Arachnida and Hexapod insects by Haeckel and other writers, and it was at one time suggested that there was a genetic affinity between the two groups—through Galeodes, or extinct forms similar to it. ”

“ VI. Arachnid parasites, Arachnida (part of) .—The great class of articulated, limb-jointed, or, more strictly, arthropodous animals, includes a variety of parasites. The mites, true ticks, and such like creatures, belong to this group. Some few of them are entozoal in habit, others are only partially so, whilst the majority are entirely ectozoal. Of the two great sections of Arachnida, namely, Pulmonaria and Trachearia, the latter alone contains strictly parasitic forms. ”

“ PANTOPODA.—Nomomeristic Arachnids, in which the somites corresponding to mesosoma and metasoma have entirely aborted. The seventh, and sometimes the eighth, leg-bearing somite is present and has its leg-like appendages fully developed. Monomeniscous eyes with a double (really triple) cell-layer formed by invagination, as in the Eu-arachnida, are present The Pantopoda stand in the same relation to Limulus and Scorpio that Cyamus holds to the thoracostracous Crustacea. ”

“ Remarks on the Araneae.—The Spiders are the most numerous and diversified group of the Arachnida; about 2000 species are known. No noteworthy fossil spiders are known; the best-preserved are in amber of Oligocene age. Protolycosa and Arthrolycosa occur in the Carboniferous. Morphologically, the spiders are remarkable for the concentration and specialization of their structure, which is accompanied with high physiological efficiency. ”

“ The first segment of the mesosoma of Scorpio and Limulus thus remains the first segment, and can be identified as such throughout the Eu-arachnida, carrying as it always does the genital apertures. But it is necessary to remember, in the light of recent discoveries, that the sixth prosomatic pair of appendages is carried on the seventh somite of the whole series, there being two prosthomeres or somites in front of the mouth, the first carrying the eyes, the second the chelicerae ”

“ ARACHNIDA, the zoological name given in 1815 by Lamarck (Gr. ἁράχνη, a spider) to a class which he instituted for the reception of the spiders, scorpions and mites, previously classified by Linnaeus in the order Aptera of his great group Insecta. Lamarck at the same time founded the class Crustacea for the lobsters, crabs and water-fleas, also until then included in the order Aptera of Linnaeus. Lamarck included the Thysanura and the Myriapoda in his class Arachnida. The Insecta of Linnaeus was a group exactly equivalent to the Arthropoda founded a hundred years later by Siebold and Stannius. ”

“ The internal reproductive organs have the ringed arrangement generally observed in the Arachnida. The two testes, which are sometimes bi-lobed, are connected by a median structure which may serve as a vesicula seminalis, and there are two vasa deferentia which proceed to the intromittent organ, which is sometimes situated quite in the anterior part of the ventral surface, but at others towards its centre. The male Mite is often provided with a pair of suckers towards the posterior end of the abdomen, and sometimes accessory clasping organs are present. ”

“ HARVEST-BUG, a name erroneously applied to the hexapod larval condition of a mite, not one of the Insecta, but belonging to the division Acaridea of the class Arachnida. It is very small, of a brick-red colour, and swarms both among wild vegetation and cultivated plants, especially near the sea, attaching itself, however, on the first opportunity to the bodies of human beings, hares, dogs, cats, and other mammals, and even insects. ”

“ Toiling with what appeared to be a furious haste were hundreds of the arachnida at respinning damaged traps torn and (illegible text) ed in the recent passage of the night, revolting-looking creatures they were with bodies as large as a silver dollar and a span of the legs as big as a daddy-long-legs but with much more powerful limbs. ”

“ ARACHNIDA EMBOLOBRANCHIATA (CONTINUED) —ACARINA—HARVEST-BUGS—PARASITIC MITES—TICKS—SPINNING MITES—STRUCTURE—METAMORPHOSIS—CLASSIFICATION Order IX. Acarina (Acari, Acaridea) . Arachnids with unsegmented, [349] non-pediculated abdomen. Respiration by tracheae, or by the general surface of the body. Month parts suctorial, but frequently capable of biting or piercing. Metamorphosis always observable. ”

“ The resemblance between the Chilopoda and the Diplopoda is principally superficial and due to the elongation and vermiform shape of the body, which in both is composed of a number of similar or subsimilar somites not differentiated as are those of Insecta, existing Arachnida and most Crustacea, into series or “tagmata” of varying function. Until 1893 no one doubted the correctness of the assumption that the Chilopoda and Diplopoda were orders of a class Myriapoda of the same systematic status as the Arachnida or Hexapoda. ”

“ Among the Annulosa, the carboniferous insects are in several cases referable to existing genera, as are the Arachnida, the highest group of which, the scorpions, is represented in the coal by a genus differing from its living congeners only in the disposition of its eyes. ”

“ It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question) , really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes. ”

“ Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequently acts in Arthropoda as an organ of nitrogenous excretion in the absence of any special excretory tubules, and that the production of such caeca from its surface in separate lines of descent does not involve any elaborate or unlikely process of growth. In other words, the Malpighian tubes of the terrestrial Arachnida are homoplastic with those of Hexapoda and Myriapoda, and not homogenetic with them. ”

“ The class Arachnida, which contains the scorpions, spiders, mites and their allies, connects the Crustacea with the Insects; and some naturalists include within it the eurypterids and king crabs, classified in this book with the Crustacea. All live on land and breathe air except a small group of allies (Pycnogonida) which are marine, and may be found on the rocks, and clinging to wharf piles, etc., on our coasts as well as elsewhere; they appear to be all legs, and are known to New England fishermen as "no-body crabs." ”

“ Arachnida and some other species are again widely separated. The majority of the young on leaving the egg are quite 579 unlike the parent, and only acquires their definite form after undergoing a series of changes. The earliest stage, which has been called the Nauplius, already referred to in connection with the barnacle, is a minute body showing no trace of segmentation, and provided with a single eye, and three pairs of swimming appendages, which become the two pairs of antennæ and the mandibles of the adult. ”

“ From activity in seizing, tearing, and devouring their food, and from their pugnacity, crabs are interesting inmates of the aquarium. They also moult, or cast off their shells; not at regular seasons, but when the demand for more room requires it.
Class II.—Arachnida are closely related to the crustaceans, having, like them, the body divided into two sections—cephalo-thorax and abdomen. To the former are attached four pairs of legs, but the abdomen has no appendages for locomotion. There are about 5,000 species, produced from eggs, and undergoing no metamorphoses in their development. ”

“ Others are found in different parts of the globe, and twenty-four species have been described. Several new American Ixodes have been noticed lately by Mr. Packard on the stag, the monax marmot, the Lepus palustris, &c. These arachnida live at first in freedom in the bushes, but after fecundation the female attacks the first mammal which she finds in her way, and establishes herself upon it; dogs become infested with it by running in and out among the brushwood. ”

“ But embryological evidence enables us to postulate with confidence still more remote ancestors in which the head carried well developed compound eyes and five pairs of appendages, while it may be supposed that all the abdominal segments, except the anal, bore limbs. In these very ancient arthropods, all the limbs, except the feelers, had ambulatory and branchial branches; and one important feature in the evolution of the Arachnida must have been the division of labour between the anterior and posterior limbs, the former becoming specialized for locomotion, the latter for breathing. ”

“ Leach (1814) , whose great service it was to dissociate the Edriophthalmata and the Myriapoda from the Latreillian medley, left the group Arachnida as we still have it (save for the inclusion of the Dipterous Insect Nycteribia) , and divided the group (with the same exception) into four Orders of which the Podosomata, i.e. the Pycnogonida, are one. ”

“ The Solifugae are, in some respects, the most primitive of the tracheate Arachnida. Their general appearance is very spider-like, and by the old writers they are uniformly alluded to as spiders. The segmented body and the absence of spinning organs, however, make them readily distinguishable on careful inspection. They are for the most part nocturnal creatures, though some seem to rove about by day, and are even called “Sun-spiders” by the Spaniards. The night-loving species are attracted by light. They are, as a rule, exceedingly hairy. ”