462 ARACHNIDA ACARINA CHAP. 



as to oviposition in the case of a South African Tick, AmUy- 

 omma hebraeum. 



The respiratory organs, if present, are always in the form of 

 tracheae. These are usually long and convoluted, but not 

 branching. The spiral structure is difficult to make out in these 

 animals, and in the Oribatidae at least, instead of the external 

 sheath being fortified with a spiral filament of chitin, there is a 

 very delicate enveloping membrane with an apparently unbroken 

 chitinous lining, which can, however, by suitable treatment, be 

 resolved into a ribbon-like spiral band. 1 The position of the 

 stigmata is very variable, and is utilised to indicate the main 

 groups into which the Mites have been divided. 



The Oribatidae possess two curious cephalothoracic organs 

 which were for a long time considered respiratory. These are in the 

 form of two bodies, like modified hairs, which protrude from sockets 

 on the dorsal surface of the cephalothoracic shield. Michael " has 

 shown that these have no connection with the tracheae, and he 

 regards them as sensory organs possibly olfactory. They are 

 generally referred to as the " pseudo-stigrnatic " organs. 



In the Oribatidae, at all events, well-developed coxal glands 

 are present. In many Mites, especially the Ixodoidea or Ticks, 

 the salivary glands are large and conspicuous. 



Metamorphosis. All Mites undergo a metamorphosis, vary- 

 ing in completeness in the different groups. Altogether six 

 stages can be recognised, though they are seldom or never all 

 exhibited in the development of a single species. These are 

 ovum, deutovum, larva, nymph, hypopial stage, and imago. 



THE OVUM. All Mites lay eggs. It is frequently stated 

 that the Oribatidae are viviparous exceptions, but though some 

 of them are perhaps ovoviviparous, most deposit eggs like the 

 rest of the Order. A phenomenon which has probably helped 

 to foster this erroneous view is the occasional emergence from 

 the dead body of the mother of fully-formed larvae. Towards 

 winter it is not unusual for the mother to die at a time when 

 her abdomen contains a few ripe .eggs, and these are able to 

 complete their development internally. 



THE DEUTOVUM. 3 In a few cases (Atax, Damaeus) a stage has 



1 Michael, British Oribatidae (Ray Soc.), i., 1883, p. 176. 2 Loc. cit. p. 168. 

 3 Claparede, Z. wiss. Zool. xviii., 1868, p. 455. Michael, British Oribatidae, i., 

 1883, p. 73, writes it " Deutovium. " 



