ARACHNE

ARACHNE, in fabulous history, a young maid of Lydia, said to have been the inventress of spinning. She is fabled to have been so skilful in this art as to challenge Minerva at it, who tore her work, and struck her; which disgrace driving her to despair, she hanged herself. Minerva, from compassion, brought her to life, and transformed her into a spider, which still employs itself in spinning.

FROM arachne, a spider, and videre, resemblance, a class of invertebrated animals, formerly regarded as true insects, and as such classed by Linnæus in his order Aptera, under the generic names of Phalangium, Aranea, Acarus, and Scorpio. The Arachnides were first formed into a distinct class by M. Lamarck, whose arrangement of the Linnæan Aptera is admitted by Latreille to approach the most nearly to the natural order.

The third primary division in the system of Cuvier, that of the articulated animals (animalia articulata), contains four great classes, viz. ANNELIDES, CRUSTACEA, ARACHNIDES, and INSECTA. We have already stated our intention (see ANIMAL KINGDOM) to treat of the first of these classes under the title of Helminthology, of the last under Entomology, and of the two intermediate classes in the order of their alphabetical occurrence. We shall therefore now proceed with the history and classification of Arachnides.

The naturalists who preceded Lamarck appear to have confounded this class either with the true Insecta, or with the crustaceous tribes, such as crabs and lobsters. It was in the course of his public lectures (in 1800) that the last-named observer instituted for their reception a separate class, under the title which they now bear.

Cuvier, in one of his earliest works (Tableau Élémentaire de l'Hist. Nat. des Animaux, 1798), continued, with his contemporaries, to class the Arachnides with insects, of which he regarded them as forming the third principal division, preceded only by the Crustacea and Myriapoda; thus far departing from and improving the old Linnæan system, by assigning them a higher place in his general arrangement, but not yet admitting them to the honour of a separate class.

Lamarck published the first edition of his Système des Animaux sans Vertèbres in the year 1801, and he there explains the reasons which induced him to form the separation previously advocated in his lectures.

M. Latreille did not admit the classico separation of the Arachnides, either in his Histoire Naturelle de Crustacés et des Insectes (1802-5), or in his later work, the Génér. Crustacorum et Insectorum (1806-7), but merely placed them at the head of the class of insects. In a subsequent volume, however (Considérations Générales sur l'Ordre Naturel des Crustacés, &c. 1810), he practically admits the propriety of M. Lamarck's views; and in it we find the Arachnides forming a separate class, and constituted of the same materials as those used by his predecessor. In that portion of Baron Cuvier's Règne Animal (vol. iii. 1817) of which M. Latreille is the author, we find the same arrangement followed, with this exception, that in the last-named work the Myriapoda and Chelopoda are removed from the Arachnides and placed at the head of the class Insecta—a modification likewise adhered to both in the Familles Naturelles du Règne Animal (1825), and in the recent edition of the Règne Animal itself (1829), which contains the latest view of M. Latreille. By these, in as far as concerns our general principles of arrangement, we shall be chiefly guided in the course of this article; but we conceive it due to M. Lamarck to present our

readers, in the first place, with a short statement of his opinions.¹

He defines the Arachnides as follows: Oviparous animals, at all times provided with articulated legs,—not subject to metamorphoses, nor ever acquiring new kinds of organs. Respiration tracheal or branchial: stigmatiform openings for the entrance of the air. A rudimentary heart and circulation in the greater proportion of species. Copulationes plures per vitam in plurimis.²

It is known that no vertebrated animal provided with feet has ever more than four; and that among such as are invertebrated, those which in a state of complete development are provided with feet, have never less than six. Amongst invertebrated animals, insects have essentially the smallest number of feet; for the various orders and families of that class, when arrived at their final development, have never more than six. They are hence called hexapods by some modern writers. It is otherwise, however, with the Crustacea and Arachnides, the greater proportion of which have more than six feet. Certain species, in their earliest state, have no more than that number; but their other feet appear as they advance in age. A few are hexapod, or six-footed, during the entire term of their existence; but in addition to those characters which determine the class to which they belong, various other relations connect them with their congeners, and show that they are not genuine insects.

Among those articulated animals which possess no system of circulation, insects alone undergo genuine metamorphoses; and none of the Arachnides are subject to such changes. Now, as all Arachnides are essentially distinct from the Crustacea, and as they differ from insects in the important characteristic just mentioned, it follows, according to M. Lamarck, that they constitute an assemblage of beings which ought not to be held in separation, however diversified they may be in various points of their organization.

The most remarkable circumstances in the structure of Arachnides are the following: That while many of these creatures are obviously provided with a circulating system, others present no vestige of that system; that the former breathe by means of branchial cells, while the latter respire through tracheæ; and that certain tribes are provided with antennæ, and others are entirely destitute of any such organs. These apparent non-conformities of structure result from this, that throughout the extended course of their class, the organization of the Arachnides undergoes rapid and remarkable changes; and if in the course of our observations we were to attend not only to the differences of external or internal parts, but to the progress of nature in the order of her productions, we would sooner perceive the propriety of that succession or change of organization even among animals really allied to each other by the great majority of their relations; and it would not have been deemed necessary to remove to another class such of the Arachnides as were antennated, because it would have been then more clearly perceived how incompatible was such a change with the assignment of their natural and appropriate place in the system.

The class of Arachnides, as established by Lamarck in his Cours, contained five or six small families, which, though each possessed of particular characters of distinction, it would have been difficult to separate from their common frame-work, without considerable inconvenience to whatever other class one or more of such families might have been removed. If, for example, the antennated Arachnides should be classed among insects, we then destroy the most simple and satisfactory definition which can be given of the last-named class, while we are forced to assign to the animals with which we unite them a position in the general series unsuitable to their real nature and attributes. If we transport the tracheal Arachnides to the class Insecta, in order to enable us to define the latter class by the exclusive character of respiring by tracheæ, then insects can no longer be said to be uniformly distinguished by the possession of antennæ, and the genus Phalangium, &c. would be separated from the class which contains the spiders. For these reasons Lamarck is of opinion that the division which contains his Arachnides, whether antennated or non-antennated, ought to be preserved in its integrity, if we wish to avoid the impropriety of associating with insects other forms of animal life which nature has distinguished by different characters.¹

A class may be constituted of natural materials, and contained within suitable limits, and nevertheless present among its various families animals of very different forms and construction. During every period of its existence, a butterfly presents an aspect very dissimilar to that of a scarabæus, yet both belong to the true Insecta. When strong general affinities (analogies d'ensemble) prevail, those special diversities of structure which are occasionally observable are insufficient to authorize a separation of classes. The genera Aranea, Phalangium, and Galeodes, are very nearly related to each other, although the first respire by obvious branchial cells, while the others enjoy a tracheal respiration.

The non-antennated Arachnides are in general provided with eight feet; and the Acarides and Pycnogonides conduct naturally to the Phalangides, such as the genus phalangium and others. Now, if these Acarides belong essentially to the Arachnides, can the parasitical genera, such as Pediculus and Ricinus, which lead towards them so evidently, be regarded as belonging to another class? The transition is so gradual and prepared, that the Acarides, provided for the most part with eight feet, like the other non-antennated Arachnides, yet present us with many genera (such as Astoma, Leptus, Caris) which have never more than six. Lamarck maintains the necessity of preserving the class Arachnides according to the limits which he has himself assigned to it, because its being so preserved relieves the class of insects from species with which that class has little or no connection. Without citing the impossibility of assigning a suitable position among insects to tribes like the Parasites, the Thysanoura, and the Myriapoda, the strongest objection to the reunion of these animals with insects is, that it will alter the general and truly natural characters assigned to the latter, viz. that of presenting, after exclusion from the egg, a particular state of larva, singularly varied according to the different orders in the forms and parts of the animal, and that of finally exhibiting an imago or perfect state very different from the larva, and distinguished by six articulated legs, two reticulated eyes, and a pair of antennæ.

Arachnides, on the contrary, although in many instances antennated, and, like all other living beings, undergoing

successive developments of parts from the period of their birth, yet offer not the condition of a larva distinct from that of the perfect state: they preserve, not of course the dimensions, but the form and proportions of the parts with which they were originally produced; and if a few of them acquire additional parts in the course of their development, these are not of a new kind, but merely supernumerary to such as previously existed; for example, a pair of feet, or another segment to the abdomen. But this kind of development is different from that called metamorphosis in insects. All of these acquire either a new form, or parts of structure of a different kind from those which they possessed when first produced from the egg; and their state of larva, manifestly distinct from that of the perfect insect, is never equivocal.

Thus the Arachnides of Lamarck, distinguished in a general point of view from true insects by the want of metamorphoses, yet breathing in air, are remarkable for the singular gradations which their organization presents us when we compare the different families with each other. In fact, these animals exhibit, in their totality, different groups, which offer among themselves such great dissimilarities of organization, as might almost furnish the materials of as many classes; but such a proceeding would injure the simplicity of systematic arrangement, and would be unsuitable, in as far as these great divisions might still be connected together by characters of general application, the same, in truth, as those by which the class of Arachnides is itself distinguished.

Although certain Arachnides possess organs adapted to a circulating system, they do not on that account belong to the crustaceous class. They are chiefly distinguished from that class by their respiratory organs, which, whether tracheal or branchial, are always placed in the interior of the body, whilst in the Crustacea they are external. In the former, also, the opening which admits the fluid to be respired is stigmatiform; but it is not so in the latter.

Even the structure of the eyes affords an index to the Arachnides. All true insects have two eyes, with plane facets, presenting the appearance of a very delicate network; but in the Arachnides the eyes are smooth, whether isolated, as among the greater number, or grouped as in spiders, and form a small mass, of which the surface is granular or sub-granular, and not in facets. Arachnides form a class superior to insects, because many of them are more highly organized, and all of them are more nearly allied to the Crustacea than genuine insects. It does not follow from this that all the individuals of that class are superior to the most perfect insects, or that they have derived their existence, according to the French phraseology, by transition from these last-named creatures, or by an advance or progression in the scale of organization. For in the progress of animal life from the lower to the higher groups, the Arachnides appear to commence almost at the same epoch as the genuine insects, and present a dichotomous or double ramification, corresponding in its level with that which bears the entire class of insects. About that same point of the animal kingdom there seems in fact to be three distinct streams or courses of life, immediately succeeding the Epizoa, viz. that of the apterous insects (Pulex), which leads successively to all the other forms of insect life; that of the antennated parasitical Arachnides (Pediculus and Ricinus), which leads to the Acarides and other non-antennated Arachnides; and that of the wandering (ragabunda) antennated Arachnides (Thysanoura and Myriapoda), from which the crustaceous tribes may be said to spring.²

Thus, of those great divisions which appear to derive their origin almost from the same point in the scale, the first is formed of an enormous series of animated beings, characterized by the strongly contrasted aspects of larva and perfect insect; and the others are true Arachnides, and consequently present no such marked distinction between the conditions of the young and old. In the most perfect of the arachnidean class, such as spiders and scorpions, Cuvier has demonstrated a muscular and dorsal heart, subject to sensible systole and diastole; and on the abdomen he discovered several stigmatic openings (from two to eight), which lead to an equal number of particular purse-shaped cavities, each of which contained a great many small and very delicate laminae or plates. These isolated cavities, and the plates with which they are filled, are without doubt the respiratory organs of these Arachnides. Cuvier regards them in the light of lungs; while Lamarck himself is inclined to view them rather as branchial cavities analogous to those observable in the leech and earth-worm—the property of branchia being, in the first place, the power of becoming habituated to the respiration of air (water being their usual medium), whilst the true lung respires air alone; and, in the second place (and this character applies of course a fortiori to lungs also), never to exist except in animals which are possessed of a circulating system.¹

Finally, from the dorsal heart already mentioned two large vessels derive their origin, and are then ramified over the membrane of the respiratory cavities. Cuvier regards these vessels as pulmonary, the one acting as an artery, the other as a vein—and other vessels spring from the same dorsal trunk, and distribute themselves to various parts of the body. In these animals there is even a liver, composed of four pairs of glandular clusters, which discharge their fluid at four different points of the intestine.

It is among the Arachnides that we find the first establishment of organs for the circulation of the animal fluids; and in the same class we perceive, as it were, the termination of tracheal respiration by ramified tracheæ, with a view to the substitution of the branchial system, which, though in itself admitting of considerable variation, is always characterized by its more local restriction. Among the Arachnides also we find the commencement of the principal of the conglomerated glands.

The respiratory sacks, which Cuvier pointed out in the spiders and scorpions, were also detected by M. Latreille in the genus Phryna, in such a manner as to connect, by that dominating feature in their structure, the Arachnides pedipalpes and the Arachnides fileuses of that author. Although among the Phalangides the respiratory sacks have not yet become perceptible, yet the aeriferous tracheæ have changed their character, and no longer consist of a double cord with a series of plexus, but are merely branched or ramified. The same order prevails among the Acarides, and results mainly from the reduced number and altered position of the stigmata. Among the antennated Arachnides, in which the stigmata are more numerous, and in general lateral, the tracheal cords, like those of insects, have as many plexus as stigmata; and such Arachnides, in fact, approach the most nearly to the class of insects. Thus tracheal respiration changes by degrees its nature and mode of action, as the stigmata undergo an alteration in regard to their number and position; and, becoming more and more reduced and restricted, it prepares the way, as it were, for branchial respiration, which only shows itself effectively along with the

establishment of a circulating system. It results from these considerations that, in spite of the difference of organization observable in different families of Arachnides, these families are nevertheless related and united by natural affinities, which it is impossible to mistake, and would be improper to separate, and which appear to place them at an almost equal distance from Insects and Crustacea. In their aspect, however, they in general resemble somewhat more nearly the latter class. For example, the cancerides or crabs represent in some respects, by their short bodies, and heads confounded with the thorax, the usual form of spiders; while the cray-fish and Thalassineæ (Cancer anomalus, Leach) recall the figure of the scorpion.

The greater proportion of Arachnides dwell on the land, a few inhabit the waters, and a certain number are parasitical on the bodies of other animals, of which they suck the juices. In general they are carnivorous, and live on blood, or at least on animal substances; a very limited number existing on vegetable matter. Many are furnished with mandibles, which perform the functions of a trunk or sucker; and others are provided with an isolated or separate trunk, frequently accompanied both by mandibles and palpi. They are for the most part solitary animals, of gloomy habits, and forbidding aspect; they court concealment, and avoid exposure to strong light. The bite of many species is dangerous, in consequence of a poisonous or irritating fluid frequently instilled into the wound. The offensive organ in the scorpionides is placed at the extremity of the abdomen.

The following is a tabular view of M. Lamarck's system of arrangement.