MITES 1009 



membrane becoming the external covering in the space left. The 

 eggs of the so-called stone-mite (Petrobia lapidum) are discoidal and 

 sculptured ; they occasionally appear in countless numbers over a 

 large space of ground in a single night, making the place look 

 whitewashed ; they have been mistaken for fungi and called Crate- 

 rium pyriforme ; they are good microscopical objects. The larvae 

 of all Acarina, except PJiytoptus and possibly Dermanyssus, are 

 hexapod ; the fourth pair of legs is absent. The nymphal stage is 

 usually the principal period of growth ; occasionally, however, it is 

 wanting. The nymph is an active chrysalis, as in the Orthoptera ; it 

 usually undergoes several ecdyses. In many species of the Oribatidce 

 the whole skin is not cast, but splits round the edge of the body, 

 and the dorso-abdominal portion remains attached to the new skin ; 

 often it has a row of elegant spines or hairs round its edge ; thus 

 after two or three ecdyses these spines form concentric rings on the 

 notogaster (Plate XXI, fig. 2). In the Trombidiidce, Tyroglyphi, &c. 

 the nymphs usually greatly resemble the adults ; in the Oribatidce 

 they are often totally different, and every intermediate stage occurs. 

 The change from nymph to adult is usually preceded by an inert 

 period. 



The number and variety of the families, and the differences in 

 the external form and internal anatomy, are so great and so endless 

 that it is impossible here to do more than indicate a few leading 

 features and refer to a few examples of interest. The caput is, of 

 course, fused with the thorax, but sometimes a constriction at the 

 base of the rostrum gives a false appearance of there being a distinct 

 head. The trophi are extremely different in the respective families, 

 or even genera. In the more highly organised of the Gamasidce 

 almost all the parts which exist in the most elaborate insect-mouths 

 except the labial palpi may be found ; they are well described by 

 M. Megnin. 1 A large oral tube is formed by the ankylosed 

 maxillae and probably upper lip and lingua. Up the centre of this 

 tube the mandibles pass freely ; they are very long and chelate ; 

 the first joint is simply cylindrical ; the second similar, but having 

 the fixed chela at its distal end ; the third is the movable chela. 

 They are capable of being projected far beyond the body, or of being 

 withdrawn wholly within it, the muscles which withdraw them 

 often arising from quite the posterior end of the body. These man- 

 dibles are different in the two sexes, and those of the male often 

 have most remarkable appendages. One of the best examples is 

 that of Gamasus terribilis, a species found in moles' nests by Mr. 

 Michael. Professor Canestrini, of Padua, also has figured some very 

 singular forms. In the Oribatidce, Tetranychus, the Sarcoptidce, 

 &c. the mandibles are also chelate, but of two joints only, shorter, 

 more powerful, and not capable of such great protrusion. In the 

 Hydrachnidce, Trombidiince, &c. the mandible is not chelate, but 

 the terminal joint shuts back like a clasp-knife, as in the poison- 

 fangs of spiders. Other forms of mandible are found. The maxillae 

 are large toothed crushing organs in the Oribatidce ; they are very 



1 Journ. de I'Anat. et de la PhysioL Kobin, May 1876. 



3 T 



