In some Mites there is no intromittent organ, and Michael[[360]] has described some remarkable cases in which the chelicerae are used in the fertilisation of the female, a spermatophore, or bag containing spermatozoa, being removed by them from the male opening and deposited in that of the female. The most remarkable instance is that of Gamasus terribilis, the movable joint of whose chelicera is perforated by a foramen through which the spermatophore is, so to speak, blown and carried as a bi-lobed bag, united by the narrow stalk which passes through the foramen, to the female aperture.

The ovaries are fused in the middle line, and connected by oviducts with the tube (vagina or uterus) which passes to the exterior. There is often an ovipositor.

Professor Gené of Turin[[361]] described, in 1844, some remarkable phenomena in connection with the reproduction of Ticks. The male Ixodes introduced his rostrum into the female aperture, two small white fusiform bodies emerging right and left from the labium at the moment of introduction. On retraction they had disappeared. When the female laid eggs, a bi-lobed vesicle was protruded from beneath the anterior border of the scutum and grasped the egg delivered to it by the ovipositor, appearing to manipulate it for some minutes. Then the vesicle was withdrawn, and the egg was left on the rostrum, and deposited by it in front of the animal. When the vesicle was punctured, and so rendered useless, the unmanipulated eggs quickly shrivelled and dried up.

Lounsbury[[362]] has recently confirmed Professor Gené’s observation as to oviposition in the case of a South African Tick, Amblyomma 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.[[363]] 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[[364]] 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, varying 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.[[365]]—In a few cases (Atax, Damaeus) a stage has been observed in which the outer envelope of the egg becomes brown and hard, and splits longitudinally, so as to allow the thin inner membrane to become visible through the fissure. More room is thus obtained for the developing larva, which is, moreover, protected, over most of its surface, by a hard shell. The deutovum stage may occur either within the body of the mother, or after the egg has been laid.