ACARIASES. 39 



larvae. These hexapod larva? undergo a change of 

 skin several times ; at last they reach the four-legged 

 stage, the nymph or pupa. Another moult brings 

 about the development of the genital organs, the 

 acarus then becoming the imago. This imago again 

 moults, and if a female, becomes the true egg-pro- 

 ducino- or ovioerous female. 



Amongst the great variety of Mites we find many 

 ways of living ; the majority are parasitic, some 

 permanent, some partial parasites. Many are ex- 

 ternal guests ; others, however, are internal. Their 

 food consists of either epidermal products and natural 

 excretions of the skin, or blood, whilst many exist 

 under the skin, forming sometimes serious cutaneous 

 affections, such as ' Scaly Leg ' in Fowls and 

 • Scab ' in Cattle. 



The diseases produced by these various Mites are 

 known under the name of Acariases. 



There are two distinct modes of attack in Fowls as 

 well as in other animals : firstly, those that suck the 

 blood onlv of the host ; secondly, those that multiply 

 rapidly, and excavate passages under the skin, giving 

 rise to deep-seated cutaneous affections. These 

 latter are known as Scabies or Psoric diseases. 



There are, then, Psoric and non-Psoric Acariases. 



We will treat of the former first. 



A. Psoric Acariases. 



The Psoric diseases in birds are produced by a 

 group of Mites known as the SarcoptitUe. These are 



