ACARINA. 



847 



third is the larva, which almost invariably has but three pairs of 

 legs (Fig. 558). The fourth is the nymph, which lasts from the 

 moult which leaves the larva with its complete complement of 

 four pair of legs, until the last moult which produces the adult. 



FIG. 558. -Larva of a Hydrachna. b Its pupa (nymph) ; Kf chelicera ; Kt pedipalpus ; 



Oc eyes ; B legs. 



The nymph is at first active, but this stage is succeeded by a longer 

 or shorter period of rest during which the tissues are resolved and 

 reconstituted as in a Dipterous insect. The fifth instar is the 

 adult which often differs markedly from the nymph ; so much is 

 this the case in Tyroglyphus, the cheese mite, that the young 

 nymphs were for long thought to belong to a different genus 

 (Hypopus), and the stage is still referred to as the hypopial stage. 

 During the hypopial stage the mite can resist comparative 

 drought and by clinging on to insects, etc., effect the distribution 

 of the members of the family. 



We have taken our classification in the main from Lankester * 

 who however does not differ very widely from Trouessart.f 

 There are seven sub-orders and thirteen families of very varying 

 extent. 



Sub-order 1. NOTOSTIGMATA. % 



Skeleton soft without sclerites,the abdomen has 10 segment.?, the anterior 

 four of which each bear a single pair of stigmata. 



Fam. 1. Opilioacaridae (Eucaridae). This family contains 1 genus 



* Q. J. Micr. Sci., xlviii, 1904, p. 1(55. 

 t Rev. Sci. Nat. Quest., ii, 1892, p. 20. 

 j With, Vid. Med. Copenhagen, 1904, pp. 137-192. 



