462 ARACHNIDA—ACARINA CHAP. 
as to oviposition in the case of a South African Tick, Ambly- 
omma hebraewm. 
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.’ 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, ike 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-stigmatic ” 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. 
Tur Deutovum.’—In a few cases (Atax, Damaeus) a stage has 
' 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. 78, writes it ‘‘ Deutovium.”’ 
