180 NATURAL HISTORY. 



characteristic is the amalgamation of the abdomen, which shows no signs of segments, with the 

 cephalothorax, to form a single mass. The second pair of maxillary palpi, developed into a leg- 

 like form, act as legs, and are counted as the first of the four pairs of legs with which the normal 

 adnit Mite is furnished. The mouth is constructed either fcr biting or for sucking. But the reader 

 will see, even from the few descriptions and figures of animals of this order that we can here offer 

 to him, that it is almost impossible to draw up a character of this order which shall strictly include all 

 its members. 



As already stated, the segments of the body are fused into a single mass, whence the name 

 Monomerosomata has been applied to the order. Only in a few species a tranverse impressed line 

 marks off the head, and, in a still smaller number, there is a similar indication of the hinder limit 

 of the thorax. The chelicerae, which represent the antennae, are here, as in most of the preceding 

 forms, the principal organs of the mouth. In the biting species they are permanently prominent, 

 and terminated either by a claw or by a small nipper ; in the suctorial forms, they acquire the 

 form of hooks, needles, or minute saw-like organs, and are then protrusible from and retractile 

 within a sort of sheath formed by the first pair of maxillae, in conjunction with which they 

 form a sucker. The palpi of the second maxillae (labium) are, as already stated, developed into 

 acting legs, and, including these, the mature Acarine has usually four pairs of limbs. 



With regard to their internal structure, these animals are rather simple. The intestinal canal 

 is short, running from the mouth to the anal opening, which, in most of them, is situated upon the 

 lower surface, at some distance from the apex of the abdomen. In some cases it is almost a simple 

 tube, but generally there is a more or less distinct stomachal part, from each side of which three 

 blind tubes are given off. Except in the Trombidia, which have the intestine partly surrounded by 

 a bunch of minute glandular bodies, there is no trace of the liver-like organ which attains such a 

 development in the higher Arachnida ; but the walls of the blind stomachal tubes are generally 

 glandular, and may take the place of a liver. 



As above stated, the Acarinaare regarded as Arachnida with tracheal respiration, but in many, 

 especially parasitic forms, no organs of respiration have yet been discovei'ed, although from other 

 characters presented by the creatures there can be no doubt that they are rightly placed in this present 

 order. When respiratory organs have been detected they consist entirely of very delicate tracheae, 

 sometimes even destitute of the spiral thi-ead which is characteristic of insect tracheae, branching in a 

 tuft from a main stem on each side. These main stems communicate with the stigmata, through 

 which the air has access to the interior of the body, and these are generally only two in number, 

 placed one on each side of the body, and situated either at the base of the chelicerse or in one of 

 the hinder pairs of legs. The circulation of the blood appears to take place in the body-spaces, 

 and no dorsal vessel has yet been discovered. The central nervous system, as might be expected from 

 the general structure of the body, is much concentrated, consisting, in fact, of a single great 

 ganglionic mass, traversed by the oesophagus, arid giving off nerves in all directions. The Acarina 

 are of separate sexes, and the internal sexual organs are sometimes rather complex. They open in 

 the ventral surface, often far forward. Nearly all lay eggs, but the species of the family Oribatidae 

 produce living young. In most cases the young quit the egg under a form more or less different from 

 that of their parents, and in attaining to the latter many of them pass through transformations which 

 may be regarded as, to some extent, analogous to those of insects. The main difference consists in 

 the absence of one pair of legs, which does not make its appearance until after a change of skin, 

 and frequently a resting or pupal stage, in which the immature animal is generally parasitic in 

 its habits. 



The Acarina, which are all of small size, and many of them of microscopic minuteness, 

 are, as might be expected, of universal diffusion over the face of the globe, and their distribution in 

 any given country is equally universal, while the functions they perform in nature fulfil nearly every 

 office that creatures so small are capable of. Some inhabit the water, and even the sea has its 

 Acarine inhabitants : others, the great majority, live on land or on plants of various kinds. Many 

 are parasitic both upon and beneath the surface of other animals ; others are predaceous, seizing and 

 devouring such little creatures as they are able to overcome. Some again feed upon living vegetable 

 matters, and many of these give rise to gall-like deformations of the parts of plants that they 



