ENCYSTMENT OF TARDIGRADA. 843 



Encystment OF certain Acarina. 



The account published by Michael in 1901 (23) of the encystment of certain Mites, 

 of the family Tyroglyphidse, indicates such a close correspondence with the process as 

 observed in M. dispar, that it is thought desirable to give a pretty full account of it here. 



The Tyroglyphidse (or Cheese Mites, etc.), like most other Acarina, undergo a 

 distinct metamorphosis, passing through the stages of larva and nymph before reaching 

 the adult condition. They cast the skin three times before becoming adult, once in 

 the larval state and twice as nymphs. In the course of growth they do not greatly alter 

 in form, so that the species at the different ages is generally easily recognised ; but the 

 full complement of limbs is only acquired when the larval skin is thrown off, and the 

 young nymph more nearly resembles the larva, while the old nymph (after the first 

 nymphal moult) is more like the imago. 



It has long been known that many Tyroglyphidse have an immature condition in 

 which there are special adaptations to assist distribution, and earlier authors founded 

 on these immature forms the genera Hypopus and Homopus. These hypopial nymphs 

 are active, but are said to be able to exist for a long time without food, and to survive 

 great heat and drought. 



Seeking for the hypopial nymphs of certain species of the genus Glycophagus, 

 Michael discovered the rudimentary hypopi, of which the encystment so resembles 

 that of M. dispar. 



Michael (23), on p. 168, etc., tells how he found inert nymphs of Glycophagus 

 domesticus, which had the cuticle thicker, whiter, and more opaque, the skin of the 

 legs empty. These " cases," which are simply nymphal skins under peculiar conditions, 

 contained each a protoplasmic mass, of the general form of a Hypopus, but without 

 trace of legs, mouth, or other external organs. It was covered by a transparent, 

 colourless, and almost structureless cuticle, and was rounded behind, and bluntly 

 pointed in front. 



Michael saw immature G. domesticus emerge from the cases " which did not split 

 irregularly like ordinary nymphal skins, but usually opened by the posterior end of 

 the case, which had been concave, being pushed out so as to become convex ; and 

 separating from the lateral and ventral parts of the case, but remaining attached to the 

 dorsal. The cases, although open, were not entirely empty ; I found that each contained 

 a cast skin." 



The nymphs which emerged from the cases became inert in about a week, and 

 a few days later adults emerged. " It was thus ascertained that the cases were a 

 penultimate-nymphal stage, i.e. the nymph which emerged from the case became adult 

 at the first ecdysis." 



A sketch copy of Michael's figure of the case containing the rudimentary hypopus 

 is given in Plate II. fig. 18a, and fig. 186 shows the hinged lid by which the nymph 

 emerged. 



