1902 Encyclopedia > Arachnida > Arachnida - External Organisation

(Part 2)


Having thus briefly explained the most obvious relationships of the Arachnida, let us now proceed to a more detailed, though still general, diagnosis of this group. And first, in respect to their external organization, Arachnids are articulate animals, with eight articulated legs, each in general consisting of seven joints; the head and thorax are soldered together into one piece (cephalo-thorax), from which with few exceptions the legs invariably spring. In one group of the Aracnida (Solpugidea) the cephalo-thorax is separately segemented, as it is also in one family of the Thelyphonidea (Tartarides); the rest of the body forms also one more or less homogeneous piece (abdomen) The abdomen is always more or less closely attached to the cephalo-thorax; in some instance it is soldered to the thorax, and forms with it one undivided piece, in other cases it is only joined to it by a narrow pedicle. At times, also, the abdomen constitutes simple unsegmented portion, at other times a segmented or annulate portion of the body, occasionally, as in the true scorpions, prolonged into a segmented tail: in one small but very distinct group (Thelyphonidea), a the abdomen is prolonged into either a simple button, a short jointed, or a more or less long setiform tail; in another large group (Araneidea) the abdomen terminates with organs (spinners and spinnerets) for spinning threads. The eyes (when present) are simple, always sessile, and placed on the fore part of the cephalo-thorax; they seem to represent the simple ocelli of the Insecta; the large compound eyes of that order having here no representation, the caput of Arachids appearing to begin at a point posterior to that which bears the antennae and compound eyes of insects. In Arachnida the number of eyes varies from two or twelve. In front of and articulated beneath the fore part of the cephalo-thorax, and moving in different planes in different groups, are two independent, variously modified, organs for seizing and compressing the insects or other substances on which Arachnids in general prey; these organs are often called mandibles, but more generally, and very appropriately, falces; these are considered by some systematists to be the true homologues of the antennae in insects, and to have been derived from those portions of the insect organization by long and unceasing modification; when, however, the caput of an Arachid (scorpion or spider, for instance) is compared with that of a coleopterous or hymenopterous insect, in which the mandibles are well developed, there seems far more reason to conclude that the antennoe of the insect have no homologue at all in the Arachnid (Claparede, l.c. post), but that the falces of the latter are the true representatives of the mandibles of the former. Behind the falces (and also used in manducation) are two other large movable portions of structure called maxillae; these vary in shape and size, and from in fact lateral and hinder boundaries to the mouth, as well as an apparatus for comminuting and squeezing the food substances; from each of these maxillae, on the outer side, springs a palpus of four or five joints, varying in structure and use. Between the maxillae most Arachids have also a fixed piece (labium) of various form, completing the hinder limits of the mouth organs, and in some (perhaps in most) there is, within the parts already named, another portion (tongue), not yet sufficiently observed either in regard to its form or use, but probably acting so as to hinder choking. In some Arachnidans these different parts of the mouth are soldered together and form a tolerably simple sucking apparatus, analogous to the mouth of some insects (Hemiptera, &c.). As observed above, Arachnids are not, in a proper sense, subject to metamorphosis: in most of them there is little real change after they leave the egg; though, in some of the lowest forms, indeed, there are after-changes by moultings of the skin, which approach the incomplete metamorphoses of some insects.

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Arachnida - Table of Contents

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