THE GAMASIDJE. 183 



In this mrasitic condition, deriving nourishment from the fluids of their hosts, they gradually increase 

 in size, and at length, after a period of quiescence, undergo the change into the adult form within 

 the skin which has covered them as larvse. In this mature state they are generally handsome little 

 creatures, glorying in bright colours, especially red, and frequently adorned with black or brown 

 markings. As parasites they may be found commonly upon the larger Water Beetles, the Water 

 Scorpion (Nepa\ and especially the species of Gerris which run upon the surface of the water. Some 

 species appear to become parasitic in Molluscs when adult, and one of these (Hydrachna concharum) 

 has been supposed to cause the formation of pearls in fresh-water mussels by the irritation that it 

 excites in the mantle. 



One or two species which appear to belong to the present family have been found swimming in 

 the sea, and form the genus Pontarachna. Examples have been met with on both sides of the 

 Atlantic arid in the Mediterranean. Besides these, several species of marine Mites of more doubtful 

 relationships have been obtained on English and other coasts, partly by dredging and partly by the 

 investigation of rock-pools on the shore, and these may be mentioned here, although most of them 

 appear to be more allied to the following family. Special attention was first called to them by 

 Mr. Gosse, who met with examples at Ilfracombe, and the late Mr. Andrew Murray was inclined 

 to place them in a separate family, which he names HALACARID^:, from the name (Halacarus) 

 given by Mr. Gosse to one of his genera. As general structural characters of his proposed family, 

 Mr. Murray says that they have either a stiff or a more or less rigid cuirassed skin, and their legs 

 springing from the outer margin of the body. 



FAMILY IV. ORIBATIDvE, OR BEETLE MITES. 



This family consists chiefly of ovate or globular Mites covered with a hard and shining skin, 

 so hard and shining in many cases as to remind one of a Beetle. They have retractile pincer-like 

 chelicei'se ; and the first joint of their short four-jointed palpi large, and converted into a masticating 

 organ. They have no eyes, and the tarsi are terminated either by one or three claws. These are all 

 terrestrial Mites, generally of small size, although some of them attain the bulk of an ordinary pin's 

 head. They appear to be vegetable feeders, although this is not quite certain, as they occur in moss 

 and under the bark of trees, where minute animals abound ; but some species have been observed 

 to bore into rotten wood, and apparently feed upon it. Their colour is generally dark brown, 

 or nearly black, and the cephalothorax is often dilated at the sides, and sometimes provided with 

 a pair of cup-shaped stigmata. In the young state there would appear to be an approximation 

 between these Mites and the Acaridse, as Professor Claparede found associated with a black species 

 (Hoplophora contractilis) a soft white Mite, like a Cheese Mite, and it 

 seemed clear that this was a stage in the development of the black 

 Oribatid. 



FAMILY V. GAMASID^E. 



This family includes a great number of small, eyeless, horny-looking 

 Mites, which may be found free upon the ground and in moss, but are 

 more frequently parasitic in their habits, living especially upon the surface 

 of terrestrial insects of various kinds. They have nipper-like chelicerse, 

 free, nearly equal-jointed palpi, and legs generally similar in size and form, 

 covered with hairs, and terminated with a pair of claws and a large pad. 

 The skin is sometimes firm throughout, sometimes only in parts, the rest 

 being soft and flexible as in the Acaridae. 



Although parasitic in their mode of life, the Gamasida? do not attach 



themselves to their victims by any permanent suctorial apparatus, but QAMASUS COLEOPTRATORUM. 

 remain free and able to crawl about at pleasure, except that in one common 



species on Beetles, called Uropoda vegetans, the Mite fixes itself to the surface of its host by means 

 of a sort of cord, the ends of which are attached to the Beetle and to the under surface of the 

 Mite. This cord, which is neither horny nor tubular, would seem to be formed by the excrements of 

 the Mite. A common and characteristic species is the Gamasns coleoptratoriim, which also infests 

 Beetles, and is one of those species in which part of the upper surface is not horny. The insects on 



