1902 Encyclopedia > Arachnida > Arachnida - Order III: Phalangidea - Introduction; External Structure

Arachnida
(Part 9)



(9) ARACHNIDA - ORDER III: PHALANGIDEA - INTRODUCTION; EXTERNAL STRUCTURE

Order III. - PHALANGIDEA.

The Phalangidea present a very well defined and compact group, though the transition, in one plane, from the Acaridea, through the families Cyphophthalmides and Trogulides, is tolerably apparent, while in another plane they show evident affinity to the Pycnogonides. The following are more detailed characters of the order.

EXTERNAL STRUCTURE. - The body is of a more or less round, oval, or oblong, and sometimes quadrangular form; the integument is either hard and horny, or coriaceous, and generally destitute of hairs, but often tuberculous and armed with spines.

The cephalo-thorax and abdomen are united throughout their whole breadth, the junction frequently shown by a slight constriction, but always visible from a strongly-marked groove or ridge; the upper side of the cephalo-thorax, and sometimes of the abdomen, is frequently furnished with spines, sharp teeth, or tubercles. The abdomen is articulate, or segmented in a modified form, from several more or less strongly developed transverse foldings in the epidermis.

The mouth organs are rather complex (fig. 11), consisting of three pairs of maxillae, above which is a tongue (languette); and below the maxillae is a membranous lip (labium). From the first pair of these maxillae there issue two five-jointed palpi, the intermediate joints of which often exhibit processes or branches, the last or digital joints ending with a claw. The other two pairs of maxillae support the basal joints of the two first pairs of legs. The presence of these supernumerary, or crural, maxillae strongly supports the idea long since advanced by Savigny and others, that the palpi are but modified legs (see, however, note 1, p. 272 supra); the same idea being also raised in regard to the Thelyphonidea and Solpugidea by the use as palpi of the first pair of legs, which are in fact completely palpiform, and differ markedly from the rest in their structure. Above these parts, directly in front of the upper extremity of the cephalo-thorax, are two falces, each of which consists of two, or (in Cyphophthalmides, G. Joseph) three joints; The second joint terminates with a didactyle claw or pair of pincers, formed by a movable claw acting in opposition to a fixed one.

The falces vary greatly in their size and development. The terminal joint is sometimes articulated to the basal one at the extremity of its longer axis or at its end; at other times it is attached by its shorter axis, or more or less near the middle. In some species they are furnished with processes or horny prolongations, differing in size and form in different species; this is, however, only a modification of the last mentioned mode of articulation.

The legs are, in most species, very long and slender; in some groups they are, however, shorter and stronger, while in others they are of extreme length and tenuity. They are eight in number; the seven normal joints, of which each is composed, may be more properly described as consisting of five invariable and two variable ones; the sixth (or metatarsus) being divided into several immovable divisions, and the seventh (or tarsus) subdivided into a greater or less number of minute movable articulations, the terminal one furnished with one or two fine claws. The legs are often armed (some, at least, of their joints) with strong spines and spiny processes; this is particularly the case in the family Gonyleptides. The first joint (coax) of each leg is immovable, being fixed to the side and under part of the cephalo-thorax. Between the fixed (or basal) joints of the legs is an oblong, or at times somewhat obtusely triangular, sternal platre, divided at its base from the abdomen by a transverse groove; the anterior extremity is free, and beneath it is concealed (according to the sex) the penis of the male, or the ovipositor of the female; in some species of Phalangides these organs are of great length and varied in form; this sternal plate forms the lower side of te cephalo-thorax. The eyes, two or eight in number, are generally of large size, and situated on the sides of a common eminence on the upper side of the cephalo-thorax; this eminence is often armed with spines and tubercles.






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