146 RECORDS OF THE AUSTRALIAN MUSEUM. 



which have developed a remarkable nymphal form, termed 

 technically Hypop^is. This nymph has a number of tiny sucking 

 discs on the ventral surface at the posterior extremity, which 

 enables it to adhere to some other animal, and so be carried 

 from place to place^ until it finds a suitable locality. Extreme 

 cold or excessive moisture has little effect upon the species. 

 Some species are of myrmecophagus habits, and these are of very 

 restricted range. 



In respect of geographical distribution, many of the species of 

 this order are almost, if not quite, world-wide. This is only 

 natural, seeing that some of these animals are parasitic on man, 

 domestic animals — both great and small — and economic and 

 ornamental plants. Admitting this, it follows, therefore, that 

 the study of the Acarina is of paramount importance, and hence 

 worthy of careful and systematic study. Many terrestrial 

 animals in their wild state — mammals, birds, reptiles, insects, a;* 

 well as fresh-water and pelagic forms, have Acarid parasites, some 

 of which ai-e harmful and some beneficial ; others are parasites 

 upon parasites. Again, some Acarina, so far as we aie at present 

 able to judge, have no economic importance whatever. 



The life of an Acarid may be divided into four distinct stages : 

 the k^'^'g, larva, nymph and adult. The greater majority of 

 Acarians are oviparous ; a few are vivi-, or ovo-viviparous, and 

 those of one genus, Pedimiloides, Targioni-Tozzetti, are recorded 

 as bringing forth adult males and females. One species of this 

 genus, F. alastoris, Frogg., occurs near Sydney. 



Banks summarises the general plan of development as follows : 

 " The egg is usually deposited by the female. Often within this 

 egg, while the embryo is developing, an inner membrane is formed 

 whicli encloses the young mile ; this stage is the ' deutovum.' 

 The outer shell may be cracked so as to show this membrane, or 

 it may be wholly discai'ded. The larva at birth has but six legs. 

 It feeds for a while, then passes into a resting stage which in 

 time discloses the eight-legged nymph. The added pair of legs is 

 the fourth, at least usually. During the nymphal period the 

 mite may moult one or more times and change its appearance, 

 but is always destitute of true genital orifices. At the end of 

 the nymphal stage it passes again into a quiescent condition, and 

 in due time moults into the adult mite. During tfeese resting 

 stages much of the internal anatomy of the mite may undergo 

 histolysis, each new stage being rebuilt from the disintegrated 

 tissues of the preceding stage. The genital organs are, however, 

 not effected by these histolytic processes."* 



■> Banks— Prof. U. States Nat. Mus., xxviii., 1904, pp. 7-S. 



