1902 Encyclopedia > Arachnida > Arachnida - Order I: Acaridea - Introduction; External Structure

Arachnida
(Part 5)



(5) ARACHNIDA - ORDER I: ACARIDEA - INTRODUCTION; EXTERNAL STRUCTURE

Order I. - ACARIDEA.

The inclusion in this order of such aberrant forms as Demodex, Pentastoma, and Tardigrada, somewhat destroys its homogeneity; undoubtedly they indicate a passage from the Annelides (Anarthropoda) to the Arthropoda, but whether it would not be more consistent with a strict scientist method to separate the two latter, at least, from the Acaridea, and make each the type of a distinct order is a point for the future consideration of systematists. At present the balance of opinion appears to be for including them as the lowest transitional forms of the lowest order of Arachnids. With regard to the position of Tardigrada, see Packard's Guide to the Study of Insects, p. 668, where Claparede is quoted from his Studien an Acariden; also Introduction to the Classification of Animals, Huxley, pp. 123-124, where the position of Pentastomides is also considered. See likewise on this, Cobbold's Entozoa, part iii. p. 393-402, where reference is made to Leuckart's Bau-und Entrwicklungsgeschichte der Pentastomiden.

Owing to the excessively small size of most of the Acaridea, and their obscure mode of existence, much of their economy and internal structure is yet unknown. The following is an endeavour to present their characters in somewhat greater detail than in the short abstract given above.





EXTERNAL STRUCTURE. - In their general from the Acaridea are more or less round, oblong, or oval; the integument is some is soft, in others coriaceous, and some are quite hard, like a Coleopterous insect.

The cephalo-thorax and abdomen are consolidated into one piece devoid of articulations; not only is the head, as in other Arachnids, soldered to the thorax, but the portion (cephalo-thorax) formed by this union is generally joined invisibly to the abdomen, which is consequently merged in the general covering of the body. In the family Bdellides, however, and also in some other instances, the junctional points of the different parts of the body are visible enough. The legs are eight in number in the mature acarid, except in Pentastoma, which in a state of maturity has none (fig. 1) - the four abnormal-lloking legs visible in the young state disappearing in the adult (fig. 2). Many other Acaridea have six legs only until the last moult, when maturity is attained, one or two, even in the adult state, being said to have but four. Each leg consists in general of seven joints, and the tarsi end, in some, with two movable books; in others the tarsus is dilated at its extremity as if for feeling with, while in some of those whose habits are aquatic it is expanded; in short, the structure of this part is very various, and adapted to the habits of the different general. Duges distinguishes the different kinds of legs as follows: - 1. Palpatorii; 2. Gressorii; 3. Remigantes; 4. Cursorii; 5. Textorii; 6. Carunculati.

The parts of the mouth consist of two movable pieces, or falces, in front of which is another piece (labium); on each side of the labium is a strong piece (maxilla), and from the outer side of each of the maxillae springs a palpus of f our or five joints. The different forms and structure of these parts give good characters for the subordinate divisions of the Acaridea. The mouth parts in the species of Acaridea present greater variety than is to be found in any other order, no doubt owing to the greater variation in their mode of existence. In some instances the falces, maxillae, and labium form, by their union, a sort of tube or proboscis fitted for piercing, holding on to, and sucking the juices of their prey; when not so united, the falces are terminated variously by a didactyle claw, somewhat like the claw of crab, or by a movable fang (sometimes two fold) as in the true spiders (Araneidea), or they consist of "two long styles, which by moving backwards and forwards alternately, perforate the substance of their prey."

The palpi of Acaridians are also variously formed, and, like th legs, have been minutely and well distinguished by Duges, who (l.c.) divides them into seven kinds. - 1. Rapaces, armed with hooks for seizing hold with. 2. Anchorarii, found in aquatic species, and used for retaining their position by means of terminating points. 3. Fusiformes, without any claw on the last joint. 4. Filiformes, as their name implies, filiform, or not at all tumid. 5. Antenniformes, chiefly differing from the last in their greater length.6. Valvoeformes, flattered, or excavated, or sheathed in form. 7. Adnati, or united throughout the greater part of their length to the labium and always slightly developed.

The eyes is very many Acaridians are entirely wanting. When present they vary in number, being generally two, four, or six, and are placed on the cephalo-thorax; sometimes, as in the family Hydrachnides, they apparently consist of mere spots of pigment beneath the cuticle; in some other cases there is but one eye, which is composed of a varying number of small facets.






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